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

July 24, 2015


      Dan Lenski <dlenski@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