directory path types with argparse

Question

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() 

Answer 1

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