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My python script needs to read files from a directory passed on the command line. I have defined a readable_dir type as below to be used with argparse for validating that the directory passed on the command line is existent and readable. Additionally, a default value (/tmp/non_existent_dir in the example below) has also been specified for the directory argument. The problem here is that argparse invokes readable_dir() on the default value even in a situation where a directory argument is explicitly passed in on the command line. This causes the script to crap out as the default path /tmp/non_existent_dir does not exist in a context where a directory is explicitly passed in on the command line. I could get around this by not specifying a default value and making this argument mandatory, or by deferring the validation until later in the script but is a more elegant solution that anyone is aware of?

#!/usr/bin/python import argparse import os def readable_dir(prospective_dir): if not os.path.isdir(prospective_dir): raise Exception("readable_dir:{0} is not a valid path".format(prospective_dir)) if os.access(prospective_dir, os.R_OK): return prospective_dir else: raise Exception("readable_dir:{0} is not a readable dir".format(prospective_dir)) parser = argparse.ArgumentParser(description='test', fromfile_prefix_chars="@") parser.add_argument('-l', '--launch_directory', type=readable_dir, default='/tmp/non_existent_dir') args = parser.parse_args()
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    Useful code sample. Raise should raise argparse.ArgumentTypeError, but otherwise, I'm digging the readable_dir type. Commented Jun 30, 2013 at 23:43

3 Answers 3

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I submitted a patch for "path arguments" to the Python standard library mailing list a few months ago.

With this PathType class, you can simply specify the following argument type to match only an existing directory--anything else will give an error message:

type = PathType(exists=True, type='dir')

Here's the code, which could be easily modified to require specific file/directory permissions as well:

from argparse import ArgumentTypeError as err import os class PathType(object): def __init__(self, exists=True, type='file', dash_ok=True): '''exists: True: a path that does exist False: a path that does not exist, in a valid parent directory None: don't care type: file, dir, symlink, None, or a function returning True for valid paths None: don't care dash_ok: whether to allow "-" as stdin/stdout''' assert exists in (True, False, None) assert type in ('file','dir','symlink',None) or hasattr(type,'__call__') self._exists = exists self._type = type self._dash_ok = dash_ok def __call__(self, string): if string=='-': # the special argument "-" means sys.std{in,out} if self._type == 'dir': raise err('standard input/output (-) not allowed as directory path') elif self._type == 'symlink': raise err('standard input/output (-) not allowed as symlink path') elif not self._dash_ok: raise err('standard input/output (-) not allowed') else: e = os.path.exists(string) if self._exists==True: if not e: raise err("path does not exist: '%s'" % string) if self._type is None: pass elif self._type=='file': if not os.path.isfile(string): raise err("path is not a file: '%s'" % string) elif self._type=='symlink': if not os.path.symlink(string): raise err("path is not a symlink: '%s'" % string) elif self._type=='dir': if not os.path.isdir(string): raise err("path is not a directory: '%s'" % string) elif not self._type(string): raise err("path not valid: '%s'" % string) else: if self._exists==False and e: raise err("path exists: '%s'" % string) p = os.path.dirname(os.path.normpath(string)) or '.' if not os.path.isdir(p): raise err("parent path is not a directory: '%s'" % p) elif not os.path.exists(p): raise err("parent directory does not exist: '%s'" % p) return string
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Comments

41

You can create a custom action instead of a type:

import argparse import os import tempfile import shutil import atexit class readable_dir(argparse.Action): def __call__(self, parser, namespace, values, option_string=None): prospective_dir=values if not os.path.isdir(prospective_dir): raise argparse.ArgumentTypeError("readable_dir:{0} is not a valid path".format(prospective_dir)) if os.access(prospective_dir, os.R_OK): setattr(namespace,self.dest,prospective_dir) else: raise argparse.ArgumentTypeError("readable_dir:{0} is not a readable dir".format(prospective_dir)) ldir = tempfile.mkdtemp() atexit.register(lambda dir=ldir: shutil.rmtree(ldir)) parser = argparse.ArgumentParser(description='test', fromfile_prefix_chars="@") parser.add_argument('-l', '--launch_directory', action=readable_dir, default=ldir) args = parser.parse_args() print (args)

But this seems a little fishy to me -- if no directory is given, it passes a non-readable directory which seems to defeat the purpose of checking if the directory is accessible in the first place.

Note that as pointed out in the comments, it might be nicer to
raise argparse.ArgumentError(self, ...) rather than argparse.ArgumentTypeError.

EDIT

As far as I'm aware, there is no way to validate the default argument. I suppose the argparse developers just assumed that if you're providing a default, then it should be valid. The quickest and easiest thing to do here is to simply validate the arguments immediately after you parse them. It looks like, you're just trying to get a temporary directory to do some work. If that's the case, you can use the tempfile module to get a new directory to work in. I updated my answer above to reflect this. I create a temporary directory, use that as the default argument (tempfile already guarantees the directory it creates will be writeable) and then I register it to be deleted when your program exits.

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4 Comments

mgilson, i had to change prospective_dir=values[0] to prospective_dir=values. Without this only the first character from the argument was being picked up. Your solution works when an explicit argument is passed in (in that the default value is not validated under these circumstances). However when no argument is passed in, the default value is NOT being validated, which is a problem.
@cravoori -- Some reason I thought values would be a list. I suppose that only happens when nargs=... is specified. Anyway, I don't think there's any way to coax argparse to do the validation after the arguments have been parsed (which is what you're really asking for). You'll have to do that yourself. I have updated my code so that there is always a valid directory for you to work in which will get deleted when your program exits. (directories specified on the command line won't get deleted).
please note the temp directory was used only an example
Minor suggestion: raising the error using raise argparse.ArgumentTypeError("message") causes a stack trace. If you use raise argparse.ArgumentError(self, "message") it gets formatted nicely without a stack trace.
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If your script can't work without a valid launch_directory then it should be made a mandatory argument:

parser.add_argument('launch_directory', type=readable_dir)

btw, you should use argparse.ArgumentTypeError instead of Exception in readable_dir().

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5 Comments

argparse.ArgumentError(self, "error string") is best of all, if you want the user to see a nice error message instead of a stack trace. For more info, see: stackoverflow.com/questions/9881933/…
@Skotch: readable_dir defines a type so ArgumentTypeError is appropriate here. I've fixed the typo: action -> type
J.F. Sebastian: I'm pretty sure it's a custom action we are talking about (see the definition of readable_dir given by mgilson above, it's being derived from argparse.Action). Passing a custom argparse action as a type will not work (at least it didn't when I tried).
@Apteryx: read the quesiton. readable_dir is a function. My answer has nothing to do with mgilson's one.
J.F. Sebastian: Sorry, you are right. And now that I understand, it makes more sense to go with a function defined type than to use a custom action. Thanks!

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