pathlib: Path and PurePath cannot be subclassed #75140

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1. sbstp commented on Jul 18, 2017

   

   sbstpmannequin

   on Jul 18, 2017

   MannequinAuthor

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   Because of the special way Path and PurePath are instantiated, they can't be inherited like a normal class. Here's an example of the issue:

   >>> import pathlib
   >>> class MyPath(pathlib.Path):
   ...   pass
   ... 
   >>> p = MyPath('/home')
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "/usr/lib/python3.5/pathlib.py", line 969, in __new__
       self = cls._from_parts(args, init=False)
     File "/usr/lib/python3.5/pathlib.py", line 651, in _from_parts
       drv, root, parts = self._parse_args(args)
     File "/usr/lib/python3.5/pathlib.py", line 644, in _parse_args
       return cls._flavour.parse_parts(parts)
   AttributeError: type object 'MyPath' has no attribute '_flavour'

   A solution is to get the concrete type of Path via type(Path()) and inherit the class it yields, but it isn't pretty or intuitive. Perhaps a declaration that directs to the proper class could be added to the module.

   PlatformPath = WindowsPath if os.name == 'nt' else PosixPath
   PurePlatformPath = ...
2. sbstp

   added

   3.7 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-featureA feature request or enhancementA feature request or enhancement

   on Jul 18, 2017
3. stevendaprano commented on Jul 18, 2017

   

   stevendaprano

   on Jul 18, 2017

   Member

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   Python 3.4, 3.5 and 3.6 are in feature-freeze, so this enhancement can only apply to 3.7.

   You say that pathlib.Path "can't be subclassed", but then immediately show an example of subclassing it:

   >>> class MyPath(pathlib.Path):
   ...   pass
   ... 

   Which works fine. If you run:

   issubclass(MyClass, pathlib.Path)

   it returns True. Unfortunately, it looks like your subclass broke one of the class invariants, but you don't find out until you try to instantiate it:

   >>> p = MyPath('/home')
   Traceback (most recent call last):
     ...
   AttributeError: type object 'MyPath' has no attribute '_flavour'

   _flavour is a private attribute, and is not documented, so I don't think subclassing is supported. If that is the case:

   • the documentation should say that subclassing is not supported;
   • or the Path class should actively prohibit subclassing (will
     probably require a metaclass);
   • or both.

   If subclassing is supported, then I think there ought to be a better way than this:

   py> class MyPath(pathlib.Path):
   ... _flavour = pathlib.Path('.')._flavour
   ...
   py> MyPath('.')
   MyPath('.')
4. serhiy-storchaka commented on Jul 18, 2017

   

   serhiy-storchaka

   on Jul 18, 2017

   Member

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   This is a duplicate of bpo-24132.
5. serhiy-storchaka

   closed this as completedon Jul 18, 2017
6. ezio-melotti

   transferred this issue fromon Apr 10, 2022