directory path types with argparse

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