The CFG configuration format is a text format for configuration files which is similar to, and a superset of, the JSON format. It dates from before its first announcement in 2008 and has the following aims:

• Allow a hierarchical configuration scheme with support for key-value mappings and lists.
• Support cross-references between one part of the configuration and another.
• Provide a string interpolation facility to easily build up configuration values from other configuration values.
• Provide the ability to compose configurations (using include and merge facilities).
• Provide the ability to access real application objects safely, where supported by the platform.
• Be completely declarative.

It overcomes a number of drawbacks of JSON when used as a configuration format:

• JSON is more verbose than necessary.
• JSON doesn’t allow comments.
• JSON doesn’t provide first-class support for dates and multi-line strings.
• JSON doesn’t allow trailing commas in lists and mappings.
• JSON doesn’t provide easy cross-referencing, interpolation, or composition.

This library is a reimplementation of an earlier version written in 2008. The latest version of that implementation is 0.4.2, released in May 2019. A new implementation was started in 2018 (with some changes to the format) and this differs from the earlier implementation in a number of ways:

• The format now uses true, false and null rather than True, False and None. This is for JSON compatibility.
• The format now uses ${A.B.C} for references rather than $A.B.C. This is to allow better expressivity in paths.
• Multiple strings following one another are concatenated into a single string.

• There is no support for writing configurations through the API, only for reading them.
• config is now a package rather than a module.
• The classes ConfigInputStream, ConfigOutputStream, ConfigList, ConfigMerger, ConfigReader, Container, Expression, Mapping, Namespace, Reference, SeqIter and Sequence are not in the new implementation.
• The ConfigResolutionError exception is not in the new implementation.
• The Config class in the new implementation is completely different.
• The functions defaultMergeResolve, defaultStreamOpener, isWord, makePath and overwriteMergeResolve are not in the new implementation.

If your code relies on specific features of the old implementation, be sure to specify config<0.5 in your dependencies.

This Python implementation is divided into three modules:

• config contains the high-level API which you will normally interact with.
• config.tokens contains code pertaining to tokens and lexical scanning of CFG.
• config.parser contains code pertaining to parsing CFG and returning Abstract Syntax Trees (ASTs).

You can use this package using pip install config>= 0.5.0 and then importing config in your code. You should install into a virtual environment.

A configuration is represented by an instance of the Config class. The constructor for this class can be passed a filename or a stream which contains the text for the configuration. The text is read in, parsed and converted to an object that you can then query. A simple example:

a: 'Hello, ' b: 'world!' c: { d: 'e' } 'f.g': 'h' christmas_morning: `2019-12-25 08:39:49` home: `$HOME` foo: `$FOO|bar` 

The configuration above can be loaded as shown below. In an interactive shell:

>>> import io, os, sys, config >>> cfg = config.Config('test0.cfg') 

Accessing elements of the configuration with a simple key is just like using a dictionary:

>>> cfg['a'] 'Hello, ' >>> cfg['b'] 'world!' 

As well as simple keys, elements can also be accessed using path strings:

>>> cfg['c.d'] 'e' 

Here, the desired value is obtained in a single step, by (under the hood) walking the path c.d – first getting the mapping at key c, and then the value at d in the resulting mapping.

Note that you can have simple keys which look like paths:

>>> cfg['f.g'] 'h' 

If a key is given that exists in the configuration, it is used as such, and if it is not present in the configuration, an attempt is made to interpret it as a path. Thus, f.g is present and accessed via key, whereas c.d is not an existing key, so is interpreted as a path.

You can also get native Python datetime objects from a configuration, by using an ISO date/time pattern in a backtick-string:

>>> cfg['christmas_morning'] # using `2019-12-25 08:39:49` datetime.datetime(2019, 12, 25, 8, 39, 49) 

Access to other Python objects is also possible using the backtick-string syntax, provided that they are either environment values or objects contained within importable modules:

>>> cfg['error'] # using `sys.stderr` <_io.TextIOWrapper name='<stderr>' mode='w' encoding='UTF-8'> >>> cfg['error'] is sys.stderr # Is it the exact same object? True >>> cfg['output'] # using `sys:stdout` <_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF-8'> >>> cfg['output'] is sys.stdout # Is it the exact same object? True 

To access an environment variable, use a backtick-string of the form $VARNAME:

>>> cfg['home'] == os.path.expanduser('~') # using `$HOME` True 

You can specify a default value to be used if an environment variable isn’t present using the $VARNAME|default-value form. Whatever string follows the pipe character (including the empty string) is returned if VARNAME is not a variable in the environment.

>>> cfg['foo'] 'bar' 

For more information, see the CFG documentation.