No way to create an abstract classmethod

[della]

BPO	5867
Nosy	@gvanrossum, @terryjreedy, @pitrou, @benjaminp, @merwok, @durban
Files	issue5867.diff: Patch (py3k branch) issue5867a.diff: New patch (+ tests) (py3k branch) abstractclassstaticmethod.diff: Patch adding the abc.abstractclassmethod and abc.abstractstatismethod decorators abstractclassstaticmethod+doc.diff: New patch (+doc)

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2010-08-17.00:53:12.003>
created_at = <Date 2009-04-28.14:24:11.112>
labels = ['type-feature', 'library']
title = 'No way to create an abstract classmethod'
updated_at = <Date 2010-08-17.00:53:12.001>
user = 'https://bugs.python.org/della'

bugs.python.org fields:

activity = <Date 2010-08-17.00:53:12.001>
actor = 'benjamin.peterson'
assignee = 'none'
closed = True
closed_date = <Date 2010-08-17.00:53:12.003>
closer = 'benjamin.peterson'
components = ['Library (Lib)']
creation = <Date 2009-04-28.14:24:11.112>
creator = 'della'
dependencies = []
files = ['18510', '18512', '18519', '18542']
hgrepos = []
issue_num = 5867
keywords = ['patch']
message_count = 13.0
messages = ['86744', '86922', '113790', '113820', '113822', '113826', '113833', '113838', '113877', '113973', '114038', '114088', '114091']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'terry.reedy', 'pitrou', 'benjamin.peterson', 'eric.araujo', 'della', 'daniel.urban']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = 'patch review'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue5867'
versions = ['Python 3.2']

[della]

Is there a way to define an abstract classmethod? The two obvious ways
don't seem to work properly.

Python 3.0.1+ (r301:69556, Apr 15 2009, 17:25:52) 
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import abc
>>> class C(metaclass=abc.ABCMeta):
...     @abc.abstractmethod
...     @classmethod
...     def f(cls): print(42)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in C
  File "/usr/lib/python3.0/abc.py", line 24, in abstractmethod
    funcobj.__isabstractmethod__ = True
AttributeError: 'classmethod' object has no attribute '__isabstractmethod__'
>>> class C(metaclass=abc.ABCMeta):
...     @classmethod
...     @abc.abstractmethod
...     def f(cls): print(42)
... 
>>> class D(C): pass
... 
>>> D.f()
42

[terryjreedy]

Please ask questions like this first on python-list or the c.l.p or
gmane mirrors.

[durban]

@abstractmethod
@classmethod
def ...
doesn't work because classmethod objects doesn't have a __dict__, so setting arbitrary attributes don't work, and abstractmethod tries to set the __isabstractmethod__ atribute to True.

The other order:
@classmethod
@AbstractMethod
def ...
doesn't work, because the abstractmethod decorator sets the function's __isabstractmethod__ attribute to True, but when ABCMeta.__new__ checks the object in the namespace of the class, it won't find it, because the classmethod object won't have an __isabstractmethod__ attribute.

The situation is the same with staticmethod.

One possible solution would be adding a descriptor to classmethod (and staticmethod), with the name "__isabstractmethod__", which on __get__ would check its underlying callable for this attribute, and on __set__ would set this attribute on that callable. I think this way both order should work.

[durban]

Here is a patch, which adds a descriptor to classmethod and staticmethod.
Pseudocode:

__get__(self, inst, owner):
if getattr(inst.callable, '__isabstractmethod__', False):
return True
return False

__set__(self, inst, value):
inst.callable.__isabstractmethod__ = bool(value)

[pitrou]

The patch doesn't check that instantiating these methods work at all.

[durban]

If I understand correctly, some tests are needed for the instantiation of classes with abstract static/classmethods. I added them in issue5867a.diff.

[pitrou]

Thank you. I'm not an ABC expert but it looks ok. Guido, what do you think?

[gvanrossum]

As you figured out it is not yet supported.

I object to making changes to the classmethod implementation.

I expect the best thing to do is to add a new

@abc.abstractclassmethod

decorator defined in pure Python (maybe the definition of abstractproperty provides a hint on how to do this).

You may want to define @abc.abstractstaticmethod as well.

[durban]

I'm attaching a new patch adding the abc.abstractclassmethod and abc.abstractstaticmethod decorators.

[pitrou]

The patch looks fine code-wise, but it also needs a doc addition in Doc/library/abc.rst.

[durban]

I'm attaching a new patch containing also some documentation for the two new decorators. The doc is rather terse, and english is not my first language, so please let me know if some corrections are needed.

[merwok]

Looks good.

[benjaminp]

Applied in r84124. Thanks for the patch.