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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<h3><b><a href="index.html">cf 3.0.6</a></b></h3>
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<li class="toctree-l1"><a class="reference internal" href="introduction.html"><strong>Introduction</strong></a></li>
<li class="toctree-l1"><a class="reference internal" href="cf_data_model.html"><strong>CF data model</strong></a></li>
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<li class="toctree-l1 current"><a class="current reference internal" href="#"><strong>Tutorial</strong></a></li>
<li class="toctree-l1"><a class="reference internal" href="analysis.html"><strong>Analysis</strong></a></li>
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<span id="id1"></span><h1><strong>Tutorial</strong><a class="headerlink" href="#tutorial" title="Permalink to this headline">¶</a></h1>
<hr class="docutils" />
<p>Version 3.0.6 for version 1.7 of the CF conventions.</p>
<p>All of the Python code in this tutorial is available in an executable
script (<a class="reference download internal" download="" href="_downloads/e41de433ea2882dda43ee38c20662258/tutorial.py"><code class="xref download docutils literal notranslate"><span class="pre">download</span></code></a>, 36kB).</p>
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<p class="first admonition-title">Note</p>
<p><strong>This version of cf is for Python 3 only</strong> and there are
<a class="reference internal" href="2_to_3_changes.html#two-to-three-changes"><span class="std std-ref">incompatible differences between versions 2.x and 3.x</span></a> of cf.</p>
<p>Scripts written for version 2.x but running under version
3.x should either work as expected, or provide informative
error messages on the new API usage. However, it is advised
that the outputs of older scripts be checked when running
with Python 3 versions of the cf library.</p>
<p class="last">For version 2.x documentation, see the <a class="reference internal" href="releases.html#older-releases"><span class="std std-ref">older releases</span></a> page.</p>
</div>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#sample-datasets" id="id338"><strong>Sample datasets</strong></a></li>
<li><a class="reference internal" href="#import" id="id339"><strong>Import</strong></a><ul>
<li><a class="reference internal" href="#cf-version" id="id340">CF version</a></li>
</ul>
</li>
<li><a class="reference internal" href="#field-construct" id="id341"><strong>Field construct</strong></a></li>
<li><a class="reference internal" href="#reading-field-constructs-from-datasets" id="id342"><strong>Reading field constructs from datasets</strong></a><ul>
<li><a class="reference internal" href="#cf-compliance" id="id343">CF-compliance</a></li>
</ul>
</li>
<li><a class="reference internal" href="#inspection" id="id344"><strong>Inspection</strong></a><ul>
<li><a class="reference internal" href="#minimal-detail" id="id345">Minimal detail</a></li>
<li><a class="reference internal" href="#medium-detail" id="id346">Medium detail</a></li>
<li><a class="reference internal" href="#full-detail" id="id347">Full detail</a></li>
<li><a class="reference internal" href="#file-inspection-with-cfa" id="id348">File inspection with cfa</a></li>
</ul>
</li>
<li><a class="reference internal" href="#visualization" id="id349"><strong>Visualization</strong></a></li>
<li><a class="reference internal" href="#field-lists" id="id350"><strong>Field lists</strong></a></li>
<li><a class="reference internal" href="#properties" id="id351"><strong>Properties</strong></a><ul>
<li><a class="reference internal" href="#field-identities" id="id352">Field identities</a></li>
</ul>
</li>
<li><a class="reference internal" href="#metadata-constructs" id="id353"><strong>Metadata constructs</strong></a></li>
<li><a class="reference internal" href="#data" id="id354"><strong>Data</strong></a><ul>
<li><a class="reference internal" href="#data-axes" id="id355">Data axes</a></li>
<li><a class="reference internal" href="#date-time" id="id356">Date-time</a></li>
<li><a class="reference internal" href="#manipulating-dimensions" id="id357">Manipulating dimensions</a></li>
<li><a class="reference internal" href="#data-mask" id="id358">Data mask</a></li>
</ul>
</li>
<li><a class="reference internal" href="#subspacing-by-index" id="id359"><strong>Subspacing by index</strong></a></li>
<li><a class="reference internal" href="#assignment-by-index" id="id360"><strong>Assignment by index</strong></a><ul>
<li><a class="reference internal" href="#masked-values" id="id361">Masked values</a></li>
<li><a class="reference internal" href="#assignment-from-other-field-constructs" id="id362">Assignment from other field constructs</a></li>
</ul>
</li>
<li><a class="reference internal" href="#units" id="id363"><strong>Units</strong></a><ul>
<li><a class="reference internal" href="#calendar" id="id364">Calendar</a></li>
</ul>
</li>
<li><a class="reference internal" href="#filtering-metadata-constructs" id="id365"><strong>Filtering metadata constructs</strong></a><ul>
<li><a class="reference internal" href="#construct-identities" id="id366">Construct identities</a></li>
</ul>
</li>
<li><a class="reference internal" href="#metadata-construct-access" id="id367"><strong>Metadata construct access</strong></a><ul>
<li><a class="reference internal" href="#metadata-construct-properties" id="id368">Metadata construct properties</a></li>
<li><a class="reference internal" href="#metadata-construct-data" id="id369">Metadata construct data</a></li>
</ul>
</li>
<li><a class="reference internal" href="#time" id="id370"><strong>Time</strong></a><ul>
<li><a class="reference internal" href="#time-duration" id="id371">Time duration</a></li>
<li><a class="reference internal" href="#time-duration-constructors" id="id372">Time duration constructors</a></li>
</ul>
</li>
<li><a class="reference internal" href="#domain" id="id373"><strong>Domain</strong></a></li>
<li><a class="reference internal" href="#metadata-construct-types" id="id374"><strong>Metadata construct types</strong></a><ul>
<li><a class="reference internal" href="#domain-axes" id="id375">Domain axes</a></li>
<li><a class="reference internal" href="#coordinates" id="id376">Coordinates</a><ul>
<li><a class="reference internal" href="#bounds" id="id377">Bounds</a></li>
</ul>
</li>
<li><a class="reference internal" href="#domain-ancillaries" id="id378">Domain ancillaries</a></li>
<li><a class="reference internal" href="#coordinate-systems" id="id379">Coordinate systems</a></li>
<li><a class="reference internal" href="#cell-methods" id="id380">Cell methods</a></li>
<li><a class="reference internal" href="#field-ancillaries" id="id381">Field ancillaries</a></li>
</ul>
</li>
<li><a class="reference internal" href="#cyclic-domain-axes" id="id382"><strong>Cyclic domain axes</strong></a><ul>
<li><a class="reference internal" href="#rolling-a-cyclic-axis" id="id383">Rolling a cyclic axis</a></li>
<li><a class="reference internal" href="#anchoring-a-cyclic-axis" id="id384">Anchoring a cyclic axis</a></li>
</ul>
</li>
<li><a class="reference internal" href="#subspacing-by-metadata" id="id385"><strong>Subspacing by metadata</strong></a><ul>
<li><a class="reference internal" href="#susbaces-in-time" id="id386">Susbaces in time</a></li>
<li><a class="reference internal" href="#subspace-mode" id="id387">Subspace mode</a></li>
<li><a class="reference internal" href="#multiple-dimensions" id="id388">Multiple dimensions</a></li>
<li><a class="reference internal" href="#assignment-by-metadata" id="id389">Assignment by metadata</a></li>
</ul>
</li>
<li><a class="reference internal" href="#sorting-and-selecting-from-field-lists" id="id390"><strong>Sorting and selecting from field lists</strong></a><ul>
<li><a class="reference internal" href="#testing-criteria-on-a-field-construct" id="id391">Testing criteria on a field construct</a></li>
</ul>
</li>
<li><a class="reference internal" href="#encapsulating-conditions" id="id392"><strong>Encapsulating conditions</strong></a><ul>
<li><a class="reference internal" href="#compound-conditions" id="id393">Compound conditions</a></li>
<li><a class="reference internal" href="#condition-constructors" id="id394">Condition constructors</a></li>
</ul>
</li>
<li><a class="reference internal" href="#assignment-by-condition" id="id395"><strong>Assignment by condition</strong></a></li>
<li><a class="reference internal" href="#field-creation" id="id396"><strong>Field creation</strong></a><ul>
<li><a class="reference internal" href="#manual-creation" id="id397">Manual creation</a></li>
<li><a class="reference internal" href="#command-modification" id="id398">Command modification</a></li>
<li><a class="reference internal" href="#creating-data-from-an-array-on-disk" id="id399">Creating data from an array on disk</a></li>
<li><a class="reference internal" href="#creation-by-conversion" id="id400">Creation by conversion</a></li>
<li><a class="reference internal" href="#creation-by-reading" id="id401">Creation by reading</a></li>
<li><a class="reference internal" href="#creation-with-cfa" id="id402">Creation with cfa</a></li>
</ul>
</li>
<li><a class="reference internal" href="#copying" id="id403"><strong>Copying</strong></a><ul>
<li><a class="reference internal" href="#field-list-copying" id="id404">Field list copying</a></li>
</ul>
</li>
<li><a class="reference internal" href="#equality" id="id405"><strong>Equality</strong></a><ul>
<li><a class="reference internal" href="#field-list-equality" id="id406">Field list equality</a></li>
</ul>
</li>
<li><a class="reference internal" href="#netcdf-interface" id="id407"><strong>NetCDF interface</strong></a></li>
<li><a class="reference internal" href="#writing-to-a-netcdf-dataset" id="id408"><strong>Writing to a netCDF dataset</strong></a><ul>
<li><a class="reference internal" href="#global-attributes" id="id409">Global attributes</a></li>
<li><a class="reference internal" href="#conventions" id="id410">Conventions</a></li>
<li><a class="reference internal" href="#scalar-coordinate-variables" id="id411">Scalar coordinate variables</a></li>
</ul>
</li>
<li><a class="reference internal" href="#external-variables" id="id412"><strong>External variables</strong></a><ul>
<li><a class="reference internal" href="#external-files-with-cfa" id="id413">External files with cfa</a></li>
</ul>
</li>
<li><a class="reference internal" href="#aggregation" id="id414"><strong>Aggregation</strong></a></li>
<li><a class="reference internal" href="#compression" id="id415"><strong>Compression</strong></a><ul>
<li><a class="reference internal" href="#discrete-sampling-geometries" id="id416">Discrete sampling geometries</a></li>
<li><a class="reference internal" href="#gathering" id="id417">Gathering</a></li>
</ul>
</li>
<li><a class="reference internal" href="#pp-and-um-fields-files" id="id418"><strong>PP and UM fields files</strong></a><ul>
<li><a class="reference internal" href="#converting-pp-and-um-fields-files-to-netcdf-files" id="id419">Converting PP and UM fields files to netCDF files</a></li>
<li><a class="reference internal" href="#mapping-of-pp-header-items-to-field-constructs" id="id420">Mapping of PP header items to field constructs</a></li>
<li><a class="reference internal" href="#stash-code-to-standard-name-mappings" id="id421">STASH code to standard name mappings</a></li>
</ul>
</li>
<li><a class="reference internal" href="#statistical-collapses" id="id422"><strong>Statistical collapses</strong></a><ul>
<li><a class="reference internal" href="#collapse-methods" id="id423">Collapse methods</a></li>
<li><a class="reference internal" href="#data-type-and-missing-data" id="id424">Data type and missing data</a></li>
<li><a class="reference internal" href="#collapse-weights" id="id425">Collapse weights</a></li>
<li><a class="reference internal" href="#multiple-collapses" id="id426">Multiple collapses</a></li>
<li><a class="reference internal" href="#grouped-collapses" id="id427">Grouped collapses</a></li>
<li><a class="reference internal" href="#climatological-statistics" id="id428">Climatological statistics</a></li>
</ul>
</li>
<li><a class="reference internal" href="#other-statistical-operations" id="id429"><strong>Other statistical operations</strong></a><ul>
<li><a class="reference internal" href="#cumulative-sums" id="id430">Cumulative sums</a></li>
<li><a class="reference internal" href="#histograms" id="id431">Histograms</a></li>
<li><a class="reference internal" href="#binning-operations" id="id432">Binning operations</a></li>
<li><a class="reference internal" href="#percentiles" id="id433">Percentiles</a></li>
</ul>
</li>
<li><a class="reference internal" href="#regridding" id="id434"><strong>Regridding</strong></a><ul>
<li><a class="reference internal" href="#spherical-regridding" id="id435">Spherical regridding</a></li>
<li><a class="reference internal" href="#cartesian-regridding" id="id436">Cartesian regridding</a></li>
<li><a class="reference internal" href="#regridding-masked-data" id="id437">Regridding masked data</a></li>
<li><a class="reference internal" href="#vertical-regridding" id="id438">Vertical regridding</a></li>
</ul>
</li>
<li><a class="reference internal" href="#mathematical-operations" id="id439"><strong>Mathematical operations</strong></a><ul>
<li><a class="reference internal" href="#arithmetical-operations" id="id440">Arithmetical operations</a></li>
<li><a class="reference internal" href="#unary-operations" id="id441">Unary operations</a></li>
<li><a class="reference internal" href="#relational-operations" id="id442">Relational operations</a></li>
<li><a class="reference internal" href="#arithmetical-and-relational-operations-with-insufficient-metadata" id="id443">Arithmetical and relational operations with insufficient metadata</a></li>
<li><a class="reference internal" href="#trigonometrical-functions" id="id444">Trigonometrical functions</a></li>
<li><a class="reference internal" href="#exponential-and-logarithmic-functions" id="id445">Exponential and logarithmic functions</a></li>
<li><a class="reference internal" href="#rounding-and-truncation" id="id446">Rounding and truncation</a></li>
<li><a class="reference internal" href="#convolution-filters" id="id447">Convolution filters</a></li>
<li><a class="reference internal" href="#derivatives" id="id448">Derivatives</a></li>
<li><a class="reference internal" href="#relative-vorticity" id="id449">Relative vorticity</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="sample-datasets">
<span id="id2"></span><h2><a class="toc-backref" href="#id338"><strong>Sample datasets</strong></a><a class="headerlink" href="#sample-datasets" title="Permalink to this headline">¶</a></h2>
<p>This tutorial uses a number of small sample datasets, all of which can
be found in the zip file <code class="docutils literal notranslate"><span class="pre">cf_tutorial_files.zip</span></code>
(<a class="reference download internal" download="" href="_downloads/4cd32e1c6bdf28fb61e15ffab2a8d84e/cf_tutorial_files.zip"><code class="xref download docutils literal notranslate"><span class="pre">download</span></code></a>,
164kB):</p>
<div class="literal-block-wrapper docutils container" id="id101">
<div class="code-block-caption"><span class="caption-text"><em>Unpack the sample datasets.</em></span><a class="headerlink" href="#id101" title="Permalink to this code">¶</a></div>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$ unzip -q cf_tutorial_files.zip
$ ls -1
air_temperature.nc
cf_tutorial_files.zip
contiguous.nc
external.nc
file2.nc
file.nc
gathered.nc
parent.nc
precipitation_flux.nc
timeseries.nc
umfile.pp
vertical.nc
wind_components.nc
</pre></div>
</div>
</div>
<p>The tutorial examples assume that the Python session is being run from
the directory that contains the zip file and its unpacked contents,
and no other files.</p>
<p>The tutorial files may be also found in the <a class="reference external" href="https://github.com/NCAS-CMS/cf-python/tree/master/docs/_downloads">downloads directory</a>
of the on-line code repository.</p>
<hr class="docutils" />
</div>
<div class="section" id="import">
<span id="id3"></span><h2><a class="toc-backref" href="#id339"><strong>Import</strong></a><a class="headerlink" href="#import" title="Permalink to this headline">¶</a></h2>
<p>The cf package is imported as follows:</p>
<div class="literal-block-wrapper docutils container" id="id102">
<div class="code-block-caption"><span class="caption-text"><em>Import the cf package.</em></span><a class="headerlink" href="#id102" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">cf</span>
</pre></div>
</div>
</div>
<div class="section" id="cf-version">
<span id="id4"></span><h3><a class="toc-backref" href="#id340">CF version</a><a class="headerlink" href="#cf-version" title="Permalink to this headline">¶</a></h3>
<p>The version of the <a class="reference external" href="http://cfconventions.org">CF conventions</a> and
the <a class="reference internal" href="cf_data_model.html#cf-data-model"><span class="std std-ref">CF data model</span></a> being used may be found with
the <a class="reference internal" href="function/cf.CF.html#cf.CF" title="cf.CF"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.CF</span></code></a> function:</p>
<div class="literal-block-wrapper docutils container" id="id103">
<div class="code-block-caption"><span class="caption-text"><em>Retrieve the version of the CF conventions.</em></span><a class="headerlink" href="#id103" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">cf</span><span class="o">.</span><span class="n">CF</span><span class="p">()</span>
<span class="go">'1.7'</span>
</pre></div>
</div>
</div>
<p>This indicates which version of the CF conventions are represented by
this release of the cf package, and therefore the version can not be
changed.</p>
<p>Note, however, that datasets of different CF versions may be
<a class="reference internal" href="#reading-datasets"><span class="std std-ref">read</span></a> from, or <a class="reference internal" href="#writing-to-a-netcdf-dataset"><span class="std std-ref">written</span></a> to netCDF.</p>
</div>
</div>
<hr class="docutils" />
<div class="section" id="field-construct">
<h2><a class="toc-backref" href="#id341"><strong>Field construct</strong></a><a class="headerlink" href="#field-construct" title="Permalink to this headline">¶</a></h2>
<p>The central construct (i.e. element) to CF is the <a class="reference internal" href="cf_data_model.html#cf-data-model"><span class="std std-ref">field
construct</span></a>. The field construct, that corresponds to a
CF-netCDF data variable, includes all of the metadata to describe it:</p>
<blockquote>
<div><ul class="simple">
<li>descriptive properties that apply to field construct as a whole
(e.g. the standard name),</li>
<li>a data array, and</li>
<li>“metadata constructs” that describe the locations of each cell
of the data array, and the physical nature of each cell’s datum.</li>
</ul>
</div></blockquote>
<p>A field construct is stored in a <a class="reference internal" href="classes/cf.Field.html#cf.Field" title="cf.Field"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.Field</span></code></a> instance, and henceforth
the phrase “field construct” will be assumed to mean “<a class="reference internal" href="classes/cf.Field.html#cf.Field" title="cf.Field"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.Field</span></code></a>
instance”.</p>
<hr class="docutils" />
</div>
<div class="section" id="reading-field-constructs-from-datasets">
<span id="reading-datasets"></span><h2><a class="toc-backref" href="#id342"><strong>Reading field constructs from datasets</strong></a><a class="headerlink" href="#reading-field-constructs-from-datasets" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="function/cf.read.html#cf.read" title="cf.read"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.read</span></code></a> function reads files from disk, or from an <a class="reference external" href="https://www.opendap.org/">OPeNDAP</a> URLs <a class="footnote-reference" href="#dap" id="id5">[1]</a>, and returns the contents in
a <a class="reference internal" href="classes/cf.FieldList.html#cf.FieldList" title="cf.FieldList"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.FieldList</span></code></a> instance that contains zero or more <a class="reference internal" href="classes/cf.Field.html#cf.Field" title="cf.Field"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.Field</span></code></a>
instances, each of which represents a field construct. Henceforth, the
phrase “field list” will be assumed to mean a <a class="reference internal" href="classes/cf.FieldList.html#cf.FieldList" title="cf.FieldList"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.FieldList</span></code></a> instance.</p>
<p>A <a class="reference internal" href="#field-lists"><span class="std std-ref">field list</span></a> is very much like a Python <a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#list" title="(in Python v3.8)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">list</span></code></a>,
with the addition of extra methods that operate on its field construct
elements.</p>
<p>The following file type can be read:</p>
<ul class="simple">
<li>All formats of netCDF3 and netCDF4 files (including <a class="reference external" href="http://www.met.reading.ac.uk/~david/cfa/0.4/index.html">CFA-netCDF</a> files)
can be read, containing datasets for any version of CF up to and
including CF-1.7.</li>
</ul>
<ul class="simple">
<li>Files in <a class="reference external" href="https://www.unidata.ucar.edu/software/netcdf/netcdf/CDL-Syntax.html">CDL format</a>,
with or without the data array values.</li>
</ul>
<ul class="simple">
<li><a class="reference internal" href="#pp-and-um-fields-files"><span class="std std-ref">PP and UM fields files</span></a>, whose
contents are mapped into field constructs.</li>
</ul>
<p>For example, to read the file <code class="docutils literal notranslate"><span class="pre">file.nc</span></code>, which contains two field
constructs:</p>
<div class="literal-block-wrapper docutils container" id="id104">
<div class="code-block-caption"><span class="caption-text"><em>Read file.nc and show that the result is a two-element
field list.</em></span><a class="headerlink" href="#id104" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'file.nc'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="go"><class 'cf.fieldlist.FieldList'></span>
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="go">2</span>
</pre></div>
</div>
</div>
<p>Descriptive properties are always read into memory, but <a class="reference external" href="https://en.wikipedia.org/wiki/Lazy_loading">lazy loading</a> is employed for all
data arrays, which means that no data is read into memory until the
data is required for inspection or to modify the array contents. This
maximises the number of field constructs that may be read within a
session, and makes the read operation fast.</p>
<p>Multiple files may be read in one command using <a class="reference external" href="https://en.wikipedia.org/wiki/Glob_(programming)#Syntax">UNIX wild card
characters</a>, or a
sequence of file names (each element of which may also contain wild
cards). Shell environment variables are also permitted.</p>
<div class="literal-block-wrapper docutils container" id="id105">
<div class="code-block-caption"><span class="caption-text"><em>Read the ten sample netCDF files, noting that they
contain more than ten field constructs.</em></span><a class="headerlink" href="#id105" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">y</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'*.nc'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
<span class="go">14</span>
</pre></div>
</div>
</div>
<div class="literal-block-wrapper docutils container" id="id106">
<div class="code-block-caption"><span class="caption-text"><em>Read two particular files, noting that they contain more
than two field constructs.</em></span><a class="headerlink" href="#id106" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">z</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">([</span><span class="s1">'file.nc'</span><span class="p">,</span> <span class="s1">'precipitation_flux.nc'</span><span class="p">])</span>
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">z</span><span class="p">)</span>
<span class="go">3</span>
</pre></div>
</div>
</div>
<p>All of the datasets in one more directories may also be read by
replacing any file name with a directory name. An attempt will be made
to read all files in the directory, which will result in an error if
any have a non-supported format. Non-supported files may be ignored
with the <em>ignore_read_error</em> keyword.</p>
<div class="literal-block-wrapper docutils container" id="id107">
<div class="code-block-caption"><span class="caption-text"><em>Read all of the files in the current working directory.</em></span><a class="headerlink" href="#id107" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">y</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'$PWD'</span><span class="p">)</span> <span class="c1"># Raises Exception</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="o">...</span>
<span class="gr">Exception</span>: <span class="n">Can't determine format of file cf_tutorial_files.zip</span>
<span class="gp">>>> </span><span class="n">y</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'$PWD'</span><span class="p">,</span> <span class="n">ignore_read_error</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
<span class="go">15</span>
</pre></div>
</div>
</div>
<p>In all cases, the default behaviour is to aggregate the contents of
all input datasets into as few field constructs as possible, and it is
these aggregated field constructs are returned by <a class="reference internal" href="function/cf.read.html#cf.read" title="cf.read"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.read</span></code></a>. See the
section on <a class="reference internal" href="#aggregation"><span class="std std-ref">aggregation</span></a> for full details.</p>
<p>The <a class="reference internal" href="function/cf.read.html#cf.read" title="cf.read"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.read</span></code></a> function has optional parameters to</p>
<ul class="simple">
<li>allow the user to provide files that contain <a class="reference internal" href="#external-variables"><span class="std std-ref">external
variables</span></a>;</li>
<li>request <a class="reference internal" href="#creation-by-reading"><span class="std std-ref">extra field constructs to be created from “metadata”
netCDF variables</span></a>, i.e. those that are
referenced from CF-netCDF data variables, but which are not regarded
by default as data variables in their own right;</li>
<li>display information and warnings about the mapping of the netCDF
file contents to CF data model constructs;</li>
<li>remove from, or include, size one dimensions on the field
constructs’ data;</li>
<li>configure the <a class="reference internal" href="#aggregation"><span class="std std-ref">field construct aggregation process</span></a>;</li>
<li>configure the reading of directories to allow sub-directories to be
read recursively, and to allow directories which resolve to symbolic
links; and</li>
<li>configure parameters for <a class="reference internal" href="#pp-and-um-fields-files"><span class="std std-ref">reading PP and UM fields files</span></a>.</li>
</ul>
<div class="section" id="cf-compliance">
<span id="id6"></span><h3><a class="toc-backref" href="#id343">CF-compliance</a><a class="headerlink" href="#cf-compliance" title="Permalink to this headline">¶</a></h3>
<p>If the dataset is partially CF-compliant to the extent that it is not
possible to unambiguously map an element of the netCDF dataset to an
element of the CF data model, then a field construct is still
returned, but may be incomplete. This is so that datasets which are
partially conformant may nonetheless be modified in memory and written
to new datasets. Such “structural” non-compliance would occur, for
example, if the “coordinates” attribute of a CF-netCDF data variable
refers to another variable that does not exist, or refers to a
variable that spans a netCDF dimension that does not apply to the data
variable. Other types of non-compliance are not checked, such whether
or not controlled vocabularies have been adhered to. The structural
compliance of the dataset may be checked with the
<a class="reference internal" href="method/cf.Field.dataset_compliance.html#cf.Field.dataset_compliance" title="cf.Field.dataset_compliance"><code class="xref py py-obj docutils literal notranslate"><span class="pre">dataset_compliance</span></code></a> method of the field construct, as
well as optionally displayed when the dataset is read.</p>
<hr class="docutils" />
</div>
</div>
<div class="section" id="inspection">
<span id="id7"></span><h2><a class="toc-backref" href="#id344"><strong>Inspection</strong></a><a class="headerlink" href="#inspection" title="Permalink to this headline">¶</a></h2>
<p>The contents of a field construct may be inspected at three different
levels of detail.</p>
<div class="section" id="minimal-detail">
<span id="id8"></span><h3><a class="toc-backref" href="#id345">Minimal detail</a><a class="headerlink" href="#minimal-detail" title="Permalink to this headline">¶</a></h3>
<p>The built-in <a class="reference external" href="https://docs.python.org/3/library/functions.html#repr" title="(in Python v3.8)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">repr</span></code></a> function, invoked on a variable by calling that variable
alone at the interpreter prompt, returns a short, one-line description:</p>
<div class="literal-block-wrapper docutils container" id="id108">
<div class="code-block-caption"><span class="caption-text"><em>Inspect the contents of the two field constructs from
the dataset and create a Python variable for each of
them.</em></span><a class="headerlink" href="#id108" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'file.nc'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">x</span>
<span class="go">[<CF Field: specific_humidity(latitude(5), longitude(8)) 1>,</span>
<span class="go"> <CF Field: air_temperature(atmosphere_hybrid_height_coordinate(1), grid_latitude(10), grid_longitude(9)) K>]</span>
<span class="gp">>>> </span><span class="n">q</span> <span class="o">=</span> <span class="n">x</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">t</span> <span class="o">=</span> <span class="n">x</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">q</span>
<span class="go"><CF Field: specific_humidity(latitude(5), longitude(8)) 1></span>
</pre></div>
</div>
</div>
<p>This gives the identity of the field construct
(e.g. “specific_humidity”), the identities and sizes of the dimensions
spanned by the data array (“latitude” and “longitude” with sizes 5 and
8 respectively) and the units of the data (“1”, i.e. dimensionless).</p>
</div>
<div class="section" id="medium-detail">
<span id="id9"></span><h3><a class="toc-backref" href="#id346">Medium detail</a><a class="headerlink" href="#medium-detail" title="Permalink to this headline">¶</a></h3>
<p>The built-in <a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#str" title="(in Python v3.8)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">str</span></code></a> function, invoked by a <a class="reference external" href="https://docs.python.org/3/library/functions.html#print" title="(in Python v3.8)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">print</span></code></a> call on a field construct,
returns similar information as the one-line output, along with short
descriptions of the metadata constructs, which include the first and last
values of their data arrays:</p>
<div class="literal-block-wrapper docutils container" id="id109">
<div class="code-block-caption"><span class="caption-text"><em>Inspect the contents of the two field constructs with
medium detail.</em></span><a class="headerlink" href="#id109" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="o">>>></span> <span class="k">print</span><span class="p">(</span><span class="n">q</span><span class="p">)</span>
<span class="n">Field</span><span class="p">:</span> <span class="n">specific_humidity</span> <span class="p">(</span><span class="n">ncvar</span><span class="o">%</span><span class="n">q</span><span class="p">)</span>
<span class="o">----------------------------------</span>
<span class="n">Data</span> <span class="p">:</span> <span class="n">specific_humidity</span><span class="p">(</span><span class="n">latitude</span><span class="p">(</span><span class="mi">5</span><span class="p">),</span> <span class="n">longitude</span><span class="p">(</span><span class="mi">8</span><span class="p">))</span> <span class="mi">1</span>
<span class="n">Cell</span> <span class="n">methods</span> <span class="p">:</span> <span class="n">area</span><span class="p">:</span> <span class="n">mean</span>
<span class="n">Dimension</span> <span class="n">coords</span><span class="p">:</span> <span class="n">latitude</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mf">75.0</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">75.0</span><span class="p">]</span> <span class="n">degrees_north</span>
<span class="p">:</span> <span class="n">longitude</span><span class="p">(</span><span class="mi">8</span><span class="p">)</span> <span class="o">=</span> <span class="p">[</span><span class="mf">22.5</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">337.5</span><span class="p">]</span> <span class="n">degrees_east</span>
<span class="p">:</span> <span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2019</span><span class="o">-</span><span class="mo">01</span><span class="o">-</span><span class="mo">01</span> <span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">]</span>
<span class="o">>>></span> <span class="k">print</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="n">Field</span><span class="p">:</span> <span class="n">air_temperature</span> <span class="p">(</span><span class="n">ncvar</span><span class="o">%</span><span class="n">ta</span><span class="p">)</span>
<span class="o">---------------------------------</span>
<span class="n">Data</span> <span class="p">:</span> <span class="n">air_temperature</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="n">K</span>
<span class="n">Cell</span> <span class="n">methods</span> <span class="p">:</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">):</span> <span class="n">mean</span> <span class="n">where</span> <span class="n">land</span> <span class="p">(</span><span class="n">interval</span><span class="p">:</span> <span class="mf">0.1</span> <span class="n">degrees</span><span class="p">)</span> <span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">):</span> <span class="n">maximum</span>
<span class="n">Field</span> <span class="n">ancils</span> <span class="p">:</span> <span class="n">air_temperature</span> <span class="n">standard_error</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">0.76</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">0.32</span><span class="p">]]</span> <span class="n">K</span>
<span class="n">Dimension</span> <span class="n">coords</span><span class="p">:</span> <span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">=</span> <span class="p">[</span><span class="mf">1.5</span><span class="p">]</span>
<span class="p">:</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span> <span class="o">=</span> <span class="p">[</span><span class="mf">2.2</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.76</span><span class="p">]</span> <span class="n">degrees</span>
<span class="p">:</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">)</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mf">4.7</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.18</span><span class="p">]</span> <span class="n">degrees</span>
<span class="p">:</span> <span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2019</span><span class="o">-</span><span class="mo">01</span><span class="o">-</span><span class="mo">01</span> <span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">]</span>
<span class="n">Auxiliary</span> <span class="n">coords</span><span class="p">:</span> <span class="n">latitude</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">53.941</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">50.225</span><span class="p">]]</span> <span class="n">degrees_N</span>
<span class="p">:</span> <span class="n">longitude</span><span class="p">(</span><span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">),</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">2.004</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">8.156</span><span class="p">]]</span> <span class="n">degrees_E</span>
<span class="p">:</span> <span class="n">long_name</span><span class="o">=</span><span class="n">Grid</span> <span class="n">latitude</span> <span class="n">name</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="o">--</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="sa">b</span><span class="s1">'kappa'</span><span class="p">]</span>
<span class="n">Cell</span> <span class="n">measures</span> <span class="p">:</span> <span class="n">measure</span><span class="p">:</span><span class="n">area</span><span class="p">(</span><span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">),</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">2391.9657</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">2392.6009</span><span class="p">]]</span> <span class="n">km2</span>
<span class="n">Coord</span> <span class="n">references</span><span class="p">:</span> <span class="n">grid_mapping_name</span><span class="p">:</span><span class="n">rotated_latitude_longitude</span>
<span class="p">:</span> <span class="n">standard_name</span><span class="p">:</span><span class="n">atmosphere_hybrid_height_coordinate</span>
<span class="n">Domain</span> <span class="n">ancils</span> <span class="p">:</span> <span class="n">ncvar</span><span class="o">%</span><span class="n">a</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mf">10.0</span><span class="p">]</span> <span class="n">m</span>
<span class="p">:</span> <span class="n">ncvar</span><span class="o">%</span><span class="n">b</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mf">20.0</span><span class="p">]</span>
<span class="p">:</span> <span class="n">surface_altitude</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">0.0</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">270.0</span><span class="p">]]</span> <span class="n">m</span>
</pre></div>
</div>
</div>
<p>Note that <a class="reference internal" href="#time"><span class="std std-ref">time values</span></a> are converted to date-times with
the <a class="reference external" href="https://unidata.github.io/cftime/">cftime package</a>.</p>
</div>
<div class="section" id="full-detail">
<span id="id10"></span><h3><a class="toc-backref" href="#id347">Full detail</a><a class="headerlink" href="#full-detail" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="method/cf.Field.dump.html#cf.Field.dump" title="cf.Field.dump"><code class="xref py py-obj docutils literal notranslate"><span class="pre">dump</span></code></a> method of the field construct gives all
properties of all constructs, including metadata constructs and their
components, and shows the first and last values of all data arrays:</p>
<div class="literal-block-wrapper docutils container" id="id110">
<div class="code-block-caption"><span class="caption-text"><em>Inspect the contents of the two field constructs with
full detail.</em></span><a class="headerlink" href="#id110" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="o">>>></span> <span class="n">q</span><span class="o">.</span><span class="n">dump</span><span class="p">()</span>
<span class="o">----------------------------------</span>
<span class="n">Field</span><span class="p">:</span> <span class="n">specific_humidity</span> <span class="p">(</span><span class="n">ncvar</span><span class="o">%</span><span class="n">q</span><span class="p">)</span>
<span class="o">----------------------------------</span>
<span class="n">Conventions</span> <span class="o">=</span> <span class="s1">'CF-1.7'</span>
<span class="n">project</span> <span class="o">=</span> <span class="s1">'research'</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'specific_humidity'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'1'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">latitude</span><span class="p">(</span><span class="mi">5</span><span class="p">),</span> <span class="n">longitude</span><span class="p">(</span><span class="mi">8</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">0.003</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">0.032</span><span class="p">]]</span> <span class="mi">1</span>
<span class="n">Cell</span> <span class="n">Method</span><span class="p">:</span> <span class="n">area</span><span class="p">:</span> <span class="n">mean</span>
<span class="n">Domain</span> <span class="n">Axis</span><span class="p">:</span> <span class="n">latitude</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="n">Domain</span> <span class="n">Axis</span><span class="p">:</span> <span class="n">longitude</span><span class="p">(</span><span class="mi">8</span><span class="p">)</span>
<span class="n">Domain</span> <span class="n">Axis</span><span class="p">:</span> <span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">Dimension</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">latitude</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'latitude'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'degrees_north'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">latitude</span><span class="p">(</span><span class="mi">5</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mf">75.0</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">75.0</span><span class="p">]</span> <span class="n">degrees_north</span>
<span class="n">Bounds</span><span class="p">:</span><span class="n">Data</span><span class="p">(</span><span class="n">latitude</span><span class="p">(</span><span class="mi">5</span><span class="p">),</span> <span class="mi">2</span><span class="p">)</span> <span class="o">=</span> <span class="p">[[</span><span class="o">-</span><span class="mf">90.0</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">90.0</span><span class="p">]]</span>
<span class="n">Dimension</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">longitude</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'longitude'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'degrees_east'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">longitude</span><span class="p">(</span><span class="mi">8</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mf">22.5</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">337.5</span><span class="p">]</span> <span class="n">degrees_east</span>
<span class="n">Bounds</span><span class="p">:</span><span class="n">Data</span><span class="p">(</span><span class="n">longitude</span><span class="p">(</span><span class="mi">8</span><span class="p">),</span> <span class="mi">2</span><span class="p">)</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">0.0</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">360.0</span><span class="p">]]</span>
<span class="n">Dimension</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">time</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'time'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'days since 2018-12-01'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2019</span><span class="o">-</span><span class="mo">01</span><span class="o">-</span><span class="mo">01</span> <span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">t</span><span class="o">.</span><span class="n">dump</span><span class="p">()</span>
<span class="o">---------------------------------</span>
<span class="n">Field</span><span class="p">:</span> <span class="n">air_temperature</span> <span class="p">(</span><span class="n">ncvar</span><span class="o">%</span><span class="n">ta</span><span class="p">)</span>
<span class="o">---------------------------------</span>
<span class="n">Conventions</span> <span class="o">=</span> <span class="s1">'CF-1.7'</span>
<span class="n">project</span> <span class="o">=</span> <span class="s1">'research'</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'air_temperature'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'K'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[[</span><span class="mf">0.0</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">89.0</span><span class="p">]]]</span> <span class="n">K</span>
<span class="n">Cell</span> <span class="n">Method</span><span class="p">:</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">):</span> <span class="n">mean</span> <span class="n">where</span> <span class="n">land</span> <span class="p">(</span><span class="n">interval</span><span class="p">:</span> <span class="mf">0.1</span> <span class="n">degrees</span><span class="p">)</span>
<span class="n">Cell</span> <span class="n">Method</span><span class="p">:</span> <span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">):</span> <span class="n">maximum</span>
<span class="n">Field</span> <span class="n">Ancillary</span><span class="p">:</span> <span class="n">air_temperature</span> <span class="n">standard_error</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'air_temperature standard_error'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'K'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">0.76</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">0.32</span><span class="p">]]</span> <span class="n">K</span>
<span class="n">Domain</span> <span class="n">Axis</span><span class="p">:</span> <span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">Domain</span> <span class="n">Axis</span><span class="p">:</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="n">Domain</span> <span class="n">Axis</span><span class="p">:</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">)</span>
<span class="n">Domain</span> <span class="n">Axis</span><span class="p">:</span> <span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">Dimension</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">atmosphere_hybrid_height_coordinate</span>
<span class="n">computed_standard_name</span> <span class="o">=</span> <span class="s1">'altitude'</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'atmosphere_hybrid_height_coordinate'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mf">1.5</span><span class="p">]</span>
<span class="n">Bounds</span><span class="p">:</span><span class="n">Data</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span> <span class="mi">2</span><span class="p">)</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">2.0</span><span class="p">]]</span>
<span class="n">Dimension</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">grid_latitude</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'grid_latitude'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'degrees'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mf">2.2</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.76</span><span class="p">]</span> <span class="n">degrees</span>
<span class="n">Bounds</span><span class="p">:</span><span class="n">Data</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="mi">2</span><span class="p">)</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">2.42</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.98</span><span class="p">]]</span>
<span class="n">Dimension</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">grid_longitude</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'grid_longitude'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'degrees'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mf">4.7</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.18</span><span class="p">]</span> <span class="n">degrees</span>
<span class="n">Bounds</span><span class="p">:</span><span class="n">Data</span><span class="p">(</span><span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">),</span> <span class="mi">2</span><span class="p">)</span> <span class="o">=</span> <span class="p">[[</span><span class="o">-</span><span class="mf">4.92</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="o">-</span><span class="mf">0.96</span><span class="p">]]</span>
<span class="n">Dimension</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">time</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'time'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'days since 2018-12-01'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">time</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2019</span><span class="o">-</span><span class="mo">01</span><span class="o">-</span><span class="mo">01</span> <span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">:</span><span class="mo">00</span><span class="p">]</span>
<span class="n">Auxiliary</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">latitude</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'latitude'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'degrees_N'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">53.941</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">50.225</span><span class="p">]]</span> <span class="n">degrees_N</span>
<span class="n">Auxiliary</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">longitude</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'longitude'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'degrees_E'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">),</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">2.004</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">8.156</span><span class="p">]]</span> <span class="n">degrees_E</span>
<span class="n">Auxiliary</span> <span class="n">coordinate</span><span class="p">:</span> <span class="n">long_name</span><span class="o">=</span><span class="n">Grid</span> <span class="n">latitude</span> <span class="n">name</span>
<span class="n">long_name</span> <span class="o">=</span> <span class="s1">'Grid latitude name'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="o">--</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="s1">'kappa'</span><span class="p">]</span>
<span class="n">Domain</span> <span class="n">ancillary</span><span class="p">:</span> <span class="n">ncvar</span><span class="o">%</span><span class="n">a</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'m'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mf">10.0</span><span class="p">]</span> <span class="n">m</span>
<span class="n">Bounds</span><span class="p">:</span><span class="n">Data</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span> <span class="mi">2</span><span class="p">)</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">5.0</span><span class="p">,</span> <span class="mf">15.0</span><span class="p">]]</span>
<span class="n">Domain</span> <span class="n">ancillary</span><span class="p">:</span> <span class="n">ncvar</span><span class="o">%</span><span class="n">b</span>
<span class="n">Data</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">=</span> <span class="p">[</span><span class="mf">20.0</span><span class="p">]</span>
<span class="n">Bounds</span><span class="p">:</span><span class="n">Data</span><span class="p">(</span><span class="n">atmosphere_hybrid_height_coordinate</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span> <span class="mi">2</span><span class="p">)</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">14.0</span><span class="p">,</span> <span class="mf">26.0</span><span class="p">]]</span>
<span class="n">Domain</span> <span class="n">ancillary</span><span class="p">:</span> <span class="n">surface_altitude</span>
<span class="n">standard_name</span> <span class="o">=</span> <span class="s1">'surface_altitude'</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'m'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">0.0</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">270.0</span><span class="p">]]</span> <span class="n">m</span>
<span class="n">Coordinate</span> <span class="n">reference</span><span class="p">:</span> <span class="n">atmosphere_hybrid_height_coordinate</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">computed_standard_name</span> <span class="o">=</span> <span class="n">altitude</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">standard_name</span> <span class="o">=</span> <span class="n">atmosphere_hybrid_height_coordinate</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">a</span> <span class="o">=</span> <span class="n">Domain</span> <span class="n">Ancillary</span><span class="p">:</span> <span class="n">ncvar</span><span class="o">%</span><span class="n">a</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">b</span> <span class="o">=</span> <span class="n">Domain</span> <span class="n">Ancillary</span><span class="p">:</span> <span class="n">ncvar</span><span class="o">%</span><span class="n">b</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">orog</span> <span class="o">=</span> <span class="n">Domain</span> <span class="n">Ancillary</span><span class="p">:</span> <span class="n">surface_altitude</span>
<span class="n">Datum</span><span class="p">:</span><span class="n">earth_radius</span> <span class="o">=</span> <span class="mi">6371007</span>
<span class="n">Dimension</span> <span class="n">Coordinate</span><span class="p">:</span> <span class="n">atmosphere_hybrid_height_coordinate</span>
<span class="n">Coordinate</span> <span class="n">reference</span><span class="p">:</span> <span class="n">rotated_latitude_longitude</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">grid_mapping_name</span> <span class="o">=</span> <span class="n">rotated_latitude_longitude</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">grid_north_pole_latitude</span> <span class="o">=</span> <span class="mf">38.0</span>
<span class="n">Coordinate</span> <span class="n">conversion</span><span class="p">:</span><span class="n">grid_north_pole_longitude</span> <span class="o">=</span> <span class="mf">190.0</span>
<span class="n">Datum</span><span class="p">:</span><span class="n">earth_radius</span> <span class="o">=</span> <span class="mi">6371007</span>
<span class="n">Dimension</span> <span class="n">Coordinate</span><span class="p">:</span> <span class="n">grid_longitude</span>
<span class="n">Dimension</span> <span class="n">Coordinate</span><span class="p">:</span> <span class="n">grid_latitude</span>
<span class="n">Auxiliary</span> <span class="n">Coordinate</span><span class="p">:</span> <span class="n">longitude</span>
<span class="n">Auxiliary</span> <span class="n">Coordinate</span><span class="p">:</span> <span class="n">latitude</span>
<span class="n">Cell</span> <span class="n">measure</span><span class="p">:</span> <span class="n">measure</span><span class="p">:</span><span class="n">area</span>
<span class="n">units</span> <span class="o">=</span> <span class="s1">'km2'</span>
<span class="n">Data</span><span class="p">(</span><span class="n">grid_longitude</span><span class="p">(</span><span class="mi">9</span><span class="p">),</span> <span class="n">grid_latitude</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="o">=</span> <span class="p">[[</span><span class="mf">2391.9657</span><span class="p">,</span> <span class="o">...</span><span class="p">,</span> <span class="mf">2392.6009</span><span class="p">]]</span> <span class="n">km2</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="file-inspection-with-cfa">
<span id="id11"></span><h3><a class="toc-backref" href="#id348">File inspection with cfa</a><a class="headerlink" href="#file-inspection-with-cfa" title="Permalink to this headline">¶</a></h3>
<p>The description for every field constructs found in datasets also be
generated from the command line, with minimal, medium or full detail,
by using the <code class="docutils literal notranslate"><span class="pre">cfa</span></code> tool, for example:</p>
<div class="literal-block-wrapper docutils container" id="id111">
<div class="code-block-caption"><span class="caption-text"><em>Use cfa on the command line to inspect the field
constructs contained in one or more datasets. The “-1”
option treats all input files collectively as a single CF
dataset, so that aggregation is attempted within and
between the input files.</em></span><a class="headerlink" href="#id111" title="Permalink to this code">¶</a></div>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$ cfa file.nc
CF Field: air_temperature<span class="o">(</span>atmosphere_hybrid_height_coordinate<span class="o">(</span><span class="m">1</span><span class="o">)</span>, grid_latitude<span class="o">(</span><span class="m">10</span><span class="o">)</span>, grid_longitude<span class="o">(</span><span class="m">9</span><span class="o">))</span> K
CF Field: specific_humidity<span class="o">(</span>latitude<span class="o">(</span><span class="m">5</span><span class="o">)</span>, longitude<span class="o">(</span><span class="m">8</span><span class="o">))</span> <span class="m">1</span>
$ cfa -1 *.nc
CF Field: specific_humidity<span class="o">(</span><span class="nv">cf_role</span><span class="o">=</span>timeseries_id<span class="o">(</span><span class="m">4</span><span class="o">)</span>, ncdim%timeseries<span class="o">(</span><span class="m">9</span><span class="o">))</span>
CF Field: cell_area<span class="o">(</span>ncdim%longitude<span class="o">(</span><span class="m">9</span><span class="o">)</span>, ncdim%latitude<span class="o">(</span><span class="m">10</span><span class="o">))</span> m2
CF Field: eastward_wind<span class="o">(</span>latitude<span class="o">(</span><span class="m">10</span><span class="o">)</span>, longitude<span class="o">(</span><span class="m">9</span><span class="o">))</span> m s-1
CF Field: specific_humidity<span class="o">(</span>latitude<span class="o">(</span><span class="m">5</span><span class="o">)</span>, longitude<span class="o">(</span><span class="m">8</span><span class="o">))</span> <span class="m">1</span>
CF Field: air_potential_temperature<span class="o">(</span>time<span class="o">(</span><span class="m">120</span><span class="o">)</span>, latitude<span class="o">(</span><span class="m">5</span><span class="o">)</span>, longitude<span class="o">(</span><span class="m">8</span><span class="o">))</span> K
CF Field: precipitation_flux<span class="o">(</span>time<span class="o">(</span><span class="m">1</span><span class="o">)</span>, latitude<span class="o">(</span><span class="m">64</span><span class="o">)</span>, longitude<span class="o">(</span><span class="m">128</span><span class="o">))</span> kg m-2 day-1
CF Field: precipitation_flux<span class="o">(</span>time<span class="o">(</span><span class="m">2</span><span class="o">)</span>, latitude<span class="o">(</span><span class="m">4</span><span class="o">)</span>, longitude<span class="o">(</span><span class="m">5</span><span class="o">))</span> kg m2 s-1
CF Field: air_temperature<span class="o">(</span>time<span class="o">(</span><span class="m">2</span><span class="o">)</span>, latitude<span class="o">(</span><span class="m">73</span><span class="o">)</span>, longitude<span class="o">(</span><span class="m">96</span><span class="o">))</span> K
CF Field: air_temperature<span class="o">(</span>atmosphere_hybrid_height_coordinate<span class="o">(</span><span class="m">1</span><span class="o">)</span>, grid_latitude<span class="o">(</span><span class="m">10</span><span class="o">)</span>, grid_longitude<span class="o">(</span><span class="m">9</span><span class="o">))</span> K
</pre></div>
</div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">cfa</span></code> may also be used to <a class="reference internal" href="#creation-with-cfa"><span class="std std-ref">write aggregated field constructs to
new datasets</span></a>, and may be used with <a class="reference internal" href="#external-variables-with-cfa"><span class="std std-ref">external
files</span></a>.</p>
</div>
</div>
<hr class="docutils" />
<div class="section" id="visualization">
<h2><a class="toc-backref" href="#id349"><strong>Visualization</strong></a><a class="headerlink" href="#visualization" title="Permalink to this headline">¶</a></h2>
<p>Powerful, flexible, and very simple to produce visualizations of field
constructs are available with the <a class="reference external" href="http://ajheaps.github.io/cf-plot">cf-plot package</a> (that needs to be installed
separately to cf).</p>
<div class="figure" id="id112">
<img alt="_images/cfplot_example.png" src="_images/cfplot_example.png" />
<p class="caption"><span class="caption-text"><em>Example output of cf-plot displaying a cf field construct.</em></span></p>
</div>
<p>See the <a class="reference external" href="http://ajheaps.github.io/cf-plot/gallery.html">cf-plot gallery</a> for the wide range
of plotting possibilities, with example code. These include, but are
not limited to:</p>
<ul class="simple">
<li>Cylindrical, polar stereographic and other plane projections</li>
<li>Latitude or longitude vs. height or pressure</li>
<li>Hovmuller</li>
<li>Vectors</li>
<li>Stipples</li>
<li>Multiple plots on a page</li>
<li>Colour scales</li>
<li>User defined axes</li>
<li>Rotated pole</li>
<li>Irregular grids</li>
<li>Trajectories</li>
<li>Line plots</li>
</ul>
<hr class="docutils" />
</div>
<div class="section" id="field-lists">
<span id="id12"></span><h2><a class="toc-backref" href="#id350"><strong>Field lists</strong></a><a class="headerlink" href="#field-lists" title="Permalink to this headline">¶</a></h2>
<p>A field list, contained in a <a class="reference internal" href="classes/cf.FieldList.html#cf.FieldList" title="cf.FieldList"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.FieldList</span></code></a> instance, is an ordered
sequence of field constructs. It supports all of the Python <a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#list" title="(in Python v3.8)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">list</span></code></a>
operations, such as indexing, iteration, and methods like
<a class="reference internal" href="method/cf.FieldList.append.html#cf.FieldList.append" title="cf.FieldList.append"><code class="xref py py-obj docutils literal notranslate"><span class="pre">append</span></code></a>.</p>
<div class="literal-block-wrapper docutils container" id="id113">
<div class="code-block-caption"><span class="caption-text"><em>List-like operations on field list instances.</em></span><a class="headerlink" href="#id113" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'file.nc'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">y</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'precipitation_flux.nc'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">x</span>
<span class="go">[<CF Field: specific_humidity(latitude(5), longitude(8)) 1>,</span>
<span class="go"> <CF Field: air_temperature(atmosphere_hybrid_height_coordinate(1), grid_latitude(10), grid_longitude(9)) K>]</span>
<span class="gp">>>> </span><span class="n">y</span>
<span class="go">[<CF Field: precipitation_flux(time(1), latitude(64), longitude(128)) kg m-2 day-1>]</span>
<span class="gp">>>> </span><span class="n">y</span><span class="o">.</span><span class="n">extend</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">y</span>
<span class="go">[<CF Field: precipitation_flux(time(1), latitude(64), longitude(128)) kg m-2 day-1>,</span>
<span class="go"> <CF Field: specific_humidity(latitude(5), longitude(8)) 1>,</span>
<span class="go"> <CF Field: air_temperature(atmosphere_hybrid_height_coordinate(1), grid_latitude(10), grid_longitude(9)) K>]</span>
<span class="gp">>>> </span><span class="n">y</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="go"><CF Field: air_temperature(atmosphere_hybrid_height_coordinate(1), grid_latitude(10), grid_longitude(9)) K></span>
<span class="gp">>>> </span><span class="n">y</span><span class="p">[::</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="go">[<CF Field: air_temperature(atmosphere_hybrid_height_coordinate(1), grid_latitude(10), grid_longitude(9)) K>,</span>
<span class="go"> <CF Field: specific_humidity(latitude(5), longitude(8)) 1>,</span>
<span class="go"> <CF Field: precipitation_flux(time(1), latitude(64), longitude(128)) kg m-2 day-1>]</span>
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
<span class="go">3</span>
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">y</span> <span class="o">+</span> <span class="n">y</span><span class="p">)</span>
<span class="go">6</span>
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">y</span> <span class="o">*</span> <span class="mi">4</span><span class="p">)</span>
<span class="go">12</span>
<span class="gp">>>> </span><span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="n">y</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">print</span><span class="p">(</span><span class="s1">'field:'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">f</span><span class="p">))</span>
<span class="gp">...</span>
<span class="go">field: <CF Field: precipitation_flux(time(1), latitude(64), longitude(128)) kg m-2 day-1></span>
<span class="go">field: <CF Field: specific_humidity(latitude(5), longitude(8)) 1></span>
<span class="go">field: <CF Field: air_temperature(atmosphere_hybrid_height_coordinate(1), grid_latitude(10), grid_longitude(9)) K></span>
</pre></div>
</div>
</div>
<p>The field list also has some additional methods for <a class="reference internal" href="#field-list-copying"><span class="std std-ref">copying</span></a>, <a class="reference internal" href="#field-list-equality"><span class="std std-ref">testing equality</span></a>,
<a class="reference internal" href="#sorting-and-selecting-from-field-lists"><span class="std std-ref">sorting and selection</span></a>.</p>
<hr class="docutils" />
</div>
<div class="section" id="properties">
<span id="id13"></span><h2><a class="toc-backref" href="#id351"><strong>Properties</strong></a><a class="headerlink" href="#properties" title="Permalink to this headline">¶</a></h2>
<p>Descriptive properties that apply to field construct as a whole may be
retrieved with the <a class="reference internal" href="method/cf.Field.properties.html#cf.Field.properties" title="cf.Field.properties"><code class="xref py py-obj docutils literal notranslate"><span class="pre">properties</span></code></a> method:</p>
<div class="literal-block-wrapper docutils container" id="id114">
<div class="code-block-caption"><span class="caption-text"><em>Retrieve all of the descriptive properties</em></span><a class="headerlink" href="#id114" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">q</span><span class="p">,</span> <span class="n">t</span> <span class="o">=</span> <span class="n">cf</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="s1">'file.nc'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">properties</span><span class="p">()</span>
<span class="go">{'Conventions': 'CF-1.7',</span>
<span class="go"> 'project': 'research',</span>
<span class="go"> 'standard_name': 'air_temperature',</span>
<span class="go"> 'units': 'K'}</span>
</pre></div>
</div>
</div>
<p>Individual properties may be accessed and modified with the
<a class="reference internal" href="method/cf.Field.del_property.html#cf.Field.del_property" title="cf.Field.del_property"><code class="xref py py-obj docutils literal notranslate"><span class="pre">del_property</span></code></a>, <a class="reference internal" href="method/cf.Field.get_property.html#cf.Field.get_property" title="cf.Field.get_property"><code class="xref py py-obj docutils literal notranslate"><span class="pre">get_property</span></code></a>, <a class="reference internal" href="method/cf.Field.has_property.html#cf.Field.has_property" title="cf.Field.has_property"><code class="xref py py-obj docutils literal notranslate"><span class="pre">has_property</span></code></a>,
and <a class="reference internal" href="method/cf.Field.set_property.html#cf.Field.set_property" title="cf.Field.set_property"><code class="xref py py-obj docutils literal notranslate"><span class="pre">set_property</span></code></a> methods:</p>
<div class="literal-block-wrapper docutils container" id="id115">
<div class="code-block-caption"><span class="caption-text"><em>Check is a property exists, retrieve its value, delete
it and then set it to a new value.</em></span><a class="headerlink" href="#id115" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">has_property</span><span class="p">(</span><span class="s1">'standard_name'</span><span class="p">)</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">get_property</span><span class="p">(</span><span class="s1">'standard_name'</span><span class="p">)</span>
<span class="go">'air_temperature'</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">del_property</span><span class="p">(</span><span class="s1">'standard_name'</span><span class="p">)</span>
<span class="go">'air_temperature'</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">get_property</span><span class="p">(</span><span class="s1">'standard_name'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'not set'</span><span class="p">)</span>
<span class="go">'not set'</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">set_property</span><span class="p">(</span><span class="s1">'standard_name'</span><span class="p">,</span> <span class="n">value</span><span class="o">=</span><span class="s1">'air_temperature'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">get_property</span><span class="p">(</span><span class="s1">'standard_name'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'not set'</span><span class="p">)</span>
<span class="go">'air_temperature'</span>
</pre></div>
</div>
</div>
<p>A collection of properties may be set at the same time with the
<a class="reference internal" href="method/cf.Field.set_properties.html#cf.Field.set_properties" title="cf.Field.set_properties"><code class="xref py py-obj docutils literal notranslate"><span class="pre">set_properties</span></code></a> method of the field construct, and all
properties may be completely removed with the
<a class="reference internal" href="method/cf.Field.clear_properties.html#cf.Field.clear_properties" title="cf.Field.clear_properties"><code class="xref py py-obj docutils literal notranslate"><span class="pre">clear_properties</span></code></a> method.</p>
<div class="literal-block-wrapper docutils container" id="id116">
<div class="code-block-caption"><span class="caption-text"><em>Update the properties with a collection, delete all of
the properties, and reinstate the original properties.</em></span><a class="headerlink" href="#id116" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">original</span> <span class="o">=</span> <span class="n">t</span><span class="o">.</span><span class="n">properties</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">original</span>
<span class="go">{'Conventions': 'CF-1.7',</span>
<span class="go"> 'project': 'research',</span>
<span class="go"> 'standard_name': 'air_temperature',</span>
<span class="go"> 'units': 'K'}</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">set_properties</span><span class="p">({</span><span class="s1">'foo'</span><span class="p">:</span> <span class="s1">'bar'</span><span class="p">,</span> <span class="s1">'units'</span><span class="p">:</span> <span class="s1">'K'</span><span class="p">})</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">properties</span><span class="p">()</span>
<span class="go">{'Conventions': 'CF-1.7',</span>
<span class="go"> 'foo': 'bar',</span>
<span class="go"> 'project': 'research',</span>
<span class="go"> 'standard_name': 'air_temperature',</span>
<span class="go"> 'units': 'K'}</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">clear_properties</span><span class="p">()</span>
<span class="go"> {'Conventions': 'CF-1.7',</span>
<span class="go"> 'foo': 'bar',</span>
<span class="go"> 'project': 'research',</span>
<span class="go"> 'standard_name': 'air_temperature',</span>
<span class="go"> 'units': 'K'}</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">properties</span><span class="p">()</span>
<span class="go">{'units': 'K'}</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">set_properties</span><span class="p">(</span><span class="n">original</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">properties</span><span class="p">()</span>
<span class="go">{'Conventions': 'CF-1.7',</span>
<span class="go"> 'project': 'research',</span>
<span class="go"> 'standard_name': 'air_temperature',</span>
<span class="go"> 'units': 'K'}</span>
</pre></div>
</div>
</div>
<p>Note that the “units” property persisted after the call to the
<a class="reference internal" href="method/cf.Field.clear_properties.html#cf.Field.clear_properties" title="cf.Field.clear_properties"><code class="xref py py-obj docutils literal notranslate"><span class="pre">clear_properties</span></code></a> method because is it deeply associated with
the field construct’s data, which still exists.</p>
<p>All of the methods related to the properties are listed <a class="reference internal" href="classes/cf.Field.html#field-properties"><span class="std std-ref">here</span></a>.</p>
<div class="section" id="field-identities">
<span id="id14"></span><h3><a class="toc-backref" href="#id352">Field identities</a><a class="headerlink" href="#field-identities" title="Permalink to this headline">¶</a></h3>
<p>A field construct identity is a string that describes the construct
and is based on the field construct’s properties. A canonical identity
is returned by the <a class="reference internal" href="method/cf.Field.identity.html#cf.Field.identity" title="cf.Field.identity"><code class="xref py py-obj docutils literal notranslate"><span class="pre">identity</span></code></a> method of the field construct,
and all possible identities are returned by the <a class="reference internal" href="method/cf.Field.identities.html#cf.Field.identities" title="cf.Field.identities"><code class="xref py py-obj docutils literal notranslate"><span class="pre">identities</span></code></a>
method.</p>
<p>A field construct’s identity may be any one of the following</p>
<ul class="simple">
<li>The value of the <a class="reference internal" href="attribute/cf.Field.standard_name.html#cf.Field.standard_name" title="cf.Field.standard_name"><code class="xref py py-obj docutils literal notranslate"><span class="pre">standard_name</span></code></a> property,
e.g. <code class="docutils literal notranslate"><span class="pre">'air_temperature'</span></code>,</li>
<li>The value of the <a class="reference internal" href="attribute/cf.Field.id.html#cf.Field.id" title="cf.Field.id"><code class="xref py py-obj docutils literal notranslate"><span class="pre">id</span></code></a> attribute, preceded by <code class="docutils literal notranslate"><span class="pre">'id%='</span></code>,</li>
<li>The value of any property, preceded by the property name and an
equals, e.g. <code class="docutils literal notranslate"><span class="pre">'long_name=Air</span> <span class="pre">Temperature'</span></code>, <code class="docutils literal notranslate"><span class="pre">'foo=bar'</span></code>, etc.,</li>
<li>The netCDF variable name, preceded by “ncvar%”, e.g. <code class="docutils literal notranslate"><span class="pre">'ncvar%tas'</span></code>
(see the <a class="reference internal" href="#netcdf-interface"><span class="std std-ref">netCDF interface</span></a>),</li>
</ul>
<div class="literal-block-wrapper docutils container" id="id117">
<div class="code-block-caption"><span class="caption-text"><em>Get the canonical identity, and all identities, of a
field construct.</em></span><a class="headerlink" href="#id117" title="Permalink to this code">¶</a></div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">identity</span><span class="p">()</span>
<span class="go">'air_temperature'</span>
<span class="gp">>>> </span><span class="n">t</span><span class="o">.</span><span class="n">identities</span><span class="p">()</span>
<span class="go">['air_temperature',</span>
<span class="go"> 'Conventions=CF-1.7',</span>
<span class="go"> 'project=research',</span>
<span class="go"> 'units=K',</span>
<span class="go"> 'standard_name=air_temperature',</span>
<span class="go"> 'ncvar%ta']</span>
</pre></div>
</div>
</div>
<hr class="docutils" />
</div>
</div>
<div class="section" id="metadata-constructs">
<span id="id15"></span><h2><a class="toc-backref" href="#id353"><strong>Metadata constructs</strong></a><a class="headerlink" href="#metadata-constructs" title="Permalink to this headline">¶</a></h2>
<p>The metadata constructs describe the field construct that contains
them. Each <a class="reference internal" href="cf_data_model.html#cf-data-model"><span class="std std-ref">CF data model metadata construct</span></a> has
a corresponding cf class:</p>
<table border="1" class="colwidths-auto docutils align-left">
<thead valign="bottom">
<tr class="row-odd"><th class="head">Class</th>
<th class="head">CF data model construct</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><a class="reference internal" href="classes/cf.DomainAxis.html#cf.DomainAxis" title="cf.DomainAxis"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.DomainAxis</span></code></a></td>
<td>Domain axis</td>
<td>Independent axes of the domain</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="classes/cf.DimensionCoordinate.html#cf.DimensionCoordinate" title="cf.DimensionCoordinate"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.DimensionCoordinate</span></code></a></td>
<td>Dimension coordinate</td>
<td>Domain cell locations</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="classes/cf.AuxiliaryCoordinate.html#cf.AuxiliaryCoordinate" title="cf.AuxiliaryCoordinate"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.AuxiliaryCoordinate</span></code></a></td>
<td>Auxiliary coordinate</td>
<td>Domain cell locations</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="classes/cf.CoordinateReference.html#cf.CoordinateReference" title="cf.CoordinateReference"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.CoordinateReference</span></code></a></td>
<td>Coordinate reference</td>
<td>Domain coordinate systems</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="classes/cf.DomainAncillary.html#cf.DomainAncillary" title="cf.DomainAncillary"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.DomainAncillary</span></code></a></td>
<td>Domain ancillary</td>
<td>Cell locations in alternative coordinate systems</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="classes/cf.CellMeasure.html#cf.CellMeasure" title="cf.CellMeasure"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.CellMeasure</span></code></a></td>
<td>Cell measure</td>
<td>Domain cell size or shape</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="classes/cf.FieldAncillary.html#cf.FieldAncillary" title="cf.FieldAncillary"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.FieldAncillary</span></code></a></td>
<td>Field ancillary</td>
<td>Ancillary metadata which vary within the domain</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="classes/cf.CellMethod.html#cf.CellMethod" title="cf.CellMethod"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.CellMethod</span></code></a></td>
<td>Cell method</td>
<td>Describes how data represent variation within cells</td>
</tr>
</tbody>
</table>
<p>Metadata constructs of a particular type can be retrieved with the
following attributes of the field construct:</p>
<table border="1" class="docutils">
<colgroup>
<col width="59%" />
<col width="41%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Attribute</th>
<th class="head">Metadata constructs</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><a class="reference internal" href="attribute/cf.Field.domain_axes.html#cf.Field.domain_axes" title="cf.Field.domain_axes"><code class="xref py py-obj docutils literal notranslate"><span class="pre">domain_axes</span></code></a></td>
<td>Domain axes</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="attribute/cf.Field.dimension_coordinates.html#cf.Field.dimension_coordinates" title="cf.Field.dimension_coordinates"><code class="xref py py-obj docutils literal notranslate"><span class="pre">dimension_coordinates</span></code></a></td>
<td>Dimension coordinates</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="attribute/cf.Field.auxiliary_coordinates.html#cf.Field.auxiliary_coordinates" title="cf.Field.auxiliary_coordinates"><code class="xref py py-obj docutils literal notranslate"><span class="pre">auxiliary_coordinates</span></code></a></td>
<td>Auxiliary coordinates</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="attribute/cf.Field.coordinate_references.html#cf.Field.coordinate_references" title="cf.Field.coordinate_references"><code class="xref py py-obj docutils literal notranslate"><span class="pre">coordinate_references</span></code></a></td>
<td>Coordinate references</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="attribute/cf.Field.domain_ancillaries.html#cf.Field.domain_ancillaries" title="cf.Field.domain_ancillaries"><code class="xref py py-obj docutils literal notranslate"><span class="pre">domain_ancillaries</span></code></a></td>
<td>Domain ancillaries</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="attribute/cf.Field.cell_measures.html#cf.Field.cell_measures" title="cf.Field.cell_measures"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cell_measures</span></code></a></td>
<td>Cell measures</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="attribute/cf.Field.field_ancillaries.html#cf.Field.field_ancillaries" title="cf.Field.field_ancillaries"><code class="xref py py-obj docutils literal notranslate"><span class="pre">field_ancillaries</span></code></a></td>
<td>Field ancillaries</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="attribute/cf.Field.cell_methods.html#cf.Field.cell_methods" title="cf.Field.cell_methods"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cell_methods</span></code></a></td>
<td>Cell methods</td>
</tr>
</tbody>
</table>
<p>Each of these attributes returns a <a class="reference internal" href="classes/cf.Constructs.html#cf.Constructs" title="cf.Constructs"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.Constructs</span></code></a> class instance that
maps metadata constructs to unique identifiers called “construct
keys”. A <a class="reference internal" href="classes/cf.Constructs.html#cf.Constructs" title="cf.Constructs"><code class="xref py py-obj docutils literal notranslate"><span class="pre">cf.Constructs</span></code></a> instance has methods for selecting constructs
that meet particular criteria (see
<a class="reference internal" href="#filtering-metadata-constructs"><span class="std std-ref">Filtering metadata constructs</span></a>). It also behaves like a
“read-only” Python dictionary, in that it has <a class="reference internal" href="method/cf.Constructs.items.html#cf.Constructs.items" title="cf.Constructs.items"><code class="xref py py-obj docutils literal notranslate"><span class="pre">items</span></code></a>,
<a class="reference internal" href="method/cf.Constructs.keys.html#cf.Constructs.keys" title="cf.Constructs.keys"><code class="xref py py-obj docutils literal notranslate"><span class="pre">keys</span></code></a> and <a class="reference internal" href="method/cf.Constructs.values.html#cf.Constructs.values" title="cf.Constructs.values"><code class="xref py py-obj docutils literal notranslate"><span class="pre">values</span></code></a> methods that work exactly
like their corresponding <a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#dict" title="(in Python v3.8)"><code class="xref py py-obj docutils literal notranslate"><span class="pre">dict</span></code></a> methods. It also has a
<a class="reference internal" href="method/cf.Constructs.get.html#cf.Constructs.get" title="cf.Constructs.get"><code class="xref py py-obj docutils literal notranslate"><span class="pre">get</span></code></a> method and indexing like a Python dictionary (see
<a class="reference internal" href="#metadata-construct-access"><span class="std std-ref">Metadata construct access</span></a> for details).</p>
<div class="literal-block-wrapper docutils container" id="id118">
<div class="code-block-caption"><span class="caption-text"><em>Retrieve the field construct’s coordinate reference