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July 2015

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argparse PathType class -- validate file/directory paths without opening them
by Dan Lenski July 24, 2015

July 24, 2015

Hi, The standard argparse library supports a convenient FileType class (https://docs.python.org/3/library/argparse.html#filetype-objects) which automatically converts file paths to open filehandles. I've found that I frequently need to validate that arguments correspond to valid file or directory paths, but WITHOUT subsequently opening these paths as files. This is useful when the path has to be subsequently passed off to another program via subprocess, etc. 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? Thanks, Dan Lenski # standalone pathtype.py 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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