[stdlib-sig] argparse PathType class -- validate file/directory paths without opening them

[Dan Lenski]

Dan Lenski <dlenski at gmail.com> writes:
> To support this case, I wrote the following PathType class, which
> will validate the suitability of a path for the specified usage
> (e.g. directory that exists, file that doesn't exist, etc.) before
> returning it unchanged.
> 
> Would this be useful for inclusion in the standard library? Any 
comments
> on style, parameter-naming, etc?

I received a couple suggestions in terms of normalizing the path with 
os.path.normpath (I've made it automatic) and changing it to an absolute 
path (optional).

Thanks,
Dan Lenski

# patharg.py

class PathType(object):
    def __init__(self, exists=True, type='file', dash_ok=True, 
abs=False):
        '''exists:
                True: a path that does exist
                False: a path that does not exist, in a valid parent 
directory
                None: don't care, in a valid parent directory
           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
           abs: if True, path will be coerced to an absolute path'''

        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 ArgumentTypeError('standard input/output (-) not 
allowed as directory path')
            elif self._type == 'symlink':
                raise ArgumentTypeError('standard input/output (-) not 
allowed as symlink path')
            elif not self._dash_ok:
                raise ArgumentTypeError('standard input/output (-) not 
allowed')
        else:
            np = os.path.abspath(string) if abs else 
os.path.normpath(string)
            e = os.path.exists(np)
            if self._exists==True:
                if not e:
                    raise ArgumentTypeError("path does not exist: '%s'" 
% np)

                if self._type is None:
                    pass
                elif self._type=='file':
                    if not os.path.isfile(np):
                        raise ArgumentTypeError("path is not a file: 
'%s'" % np)
                elif self._type=='symlink':
                    if not os.path.symlink(np):
                        raise ArgumentTypeError("path is not a symlink: 
'%s'" % np)
                elif self._type=='dir':
                    if not os.path.isdir(np):
                        raise ArgumentTypeError("path is not a 
directory: '%s'" % np)
                elif not self._type(np):
                    raise ArgumentTypeError("path not valid: '%s'" % np)
            else:
                if self._exists==False and e:
                    raise ArgumentTypeError("path exists: '%s'" % np)

                p = os.path.dirname(os.path.normpath(np)) or '.'
                if not os.path.isdir(p):
                    raise ArgumentTypeError("parent path is not a 
directory: '%s'" % p)
                elif not os.path.exists(p):
                    raise ArgumentTypeError("parent directory does not 
exist: '%s'" % p)

        return np