Repository navigation
Inheriting from collections.MutableMapping and typing.MutableMapping breaks MutableMapping.register #222
Description
import collections
from typing import MutableMapping, TypeVar, Generic
_KT = TypeVar('_KT')
_VT = TypeVar('_VT')
class MyMutableMapping(collections.MutableMapping, MutableMapping[_KT, _VT], Generic[_KT, _VT]):
pass
class OtherMutableMapping(object):
pass
collections.MutableMapping.register(OtherMutableMapping)
Running this code under python2 results in the following error:
raceback (most recent call last):
File "t2.py", line 13, in <module>
collections.MutableMapping.register(OtherMutableMapping)
File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/abc.py", line 109, in register
if issubclass(subclass, cls):
File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/abc.py", line 180, in __subclasscheck__
if issubclass(subclass, scls):
File "/usr/local/lib/python2.7/site-packages/typing.py", line 1056, in __subclasscheck__
return issubclass(cls, self.__extra__)
File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/abc.py", line 180, in __subclasscheck__
if issubclass(subclass, scls):
File "/usr/local/lib/python2.7/site-packages/typing.py", line 1056, in __subclasscheck__
return issubclass(cls, self.__extra__)
File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/abc.py", line 180, in __subclasscheck__
<snip>
if issubclass(subclass, scls):
File "/usr/local/lib/python2.7/site-packages/typing.py", line 1056, in __subclasscheck__
return issubclass(cls, self.__extra__)
File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/abc.py", line 180, in __subclasscheck__
if issubclass(subclass, scls):
File "/usr/local/lib/python2.7/site-packages/typing.py", line 1056, in __subclasscheck__
return issubclass(cls, self.__extra__)
File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/abc.py", line 151, in __subclasscheck__
if subclass in cls._abc_cache:
RuntimeError: maximum recursion depth exceeded
Activity
bintoro commentedon May 18, 2016
The issue is not specific to
register. Inheriting from thecollectionsandtypingvariants of the same collection at the same time is not really safe because it breaks subsequent subclass checks against either ABC. (Checks that immediately returnTrueor are in the cache will still work, but others won't.)The registration attempt results in
issubclass(OtherMutableMapping, collections.MutableMapping)as a sanity check. Since there is no direct match, the query is delegated to any subclasses ofcollections.MutableMapping, which now includeMyMutableMapping.The resulting
issubclass(OtherMutableMapping, MyMutableMapping), in turn, leads back to the original call via the__extra__linkage fromMyMutableMappingtocollections.MutableMapping; hence the infinite recursion.The problem is fixed in the first commit of #207.
gvanrossum commentedon May 18, 2016
@bintoro: Thanks for reminding me of that! I've been wanting to check whether your commit would fix this (or, actually, http://bugs.python.org/issue27014, but I think that's the same issue).
I still don't have the time or the stomach for the rest of your PR #207 -- I wonder if you could resubmit just the first commit, assuming it doesn't do much more than fix this particular issue? (If you don't have the time I can probably figure out how to cherry-pick it; the main issue I expect is the merge conflict with e599f84.
bintoro commentedon May 18, 2016
Yes, 27014 is the same issue. And yeah, I can split off the first commit, since it only adds a few tests. I'm not terribly motivated to deal with the rest of the conflict, either.