Metaclass hash, eq and isinstance

Question

I have implemented a class and its metaclass, both defining binary dunder methods including __eq__ and __hash__. The metaclass' __eq__ method should simply instantiate the class and then pass the comparision to the class operator. But when I use isinstance on my classes, I end up in an infinite recursion because isinstance seems to be calling the metaclass' __eq__ method which in turn calls the class' __eq__ method which calls isinstance again.

from abc import ABCMeta

class Metaclass(ABCMeta):
    def __eq__(cls, other):
        return cls() == other

    def __hash__(cls):
        return 0 #would be implemented accordingly with a correct hastype

class Myclass(metaclass=Metaclass):
    def __eq__(self, other):
        if isinstance(other, Myclass):
            """dostuff"""
        else:
            return NotImplemented

class Childclass(Myclass): pass

isinstance(Childclass, Myclass)

This is the result for me in python3.6.7:

  [...]
    File "./test.py", line 14, in __eq__
    if isinstance(other, Myclass):
  File "/usr/lib/python3.6/abc.py", line 193, in __instancecheck__
    return cls.__subclasscheck__(subclass)
  File "/usr/lib/python3.6/abc.py", line 218, in __subclasscheck__
    if cls in getattr(subclass, '__mro__', ()):
  File "./test.py", line 7, in __eq__
    return cls() == other
  File "./test.py", line 14, in __eq__
    if isinstance(other, Myclass):
  File "/usr/lib/python3.6/abc.py", line 193, in __instancecheck__
    return cls.__subclasscheck__(subclass)
  File "/usr/lib/python3.6/abc.py", line 218, in __subclasscheck__
    if cls in getattr(subclass, '__mro__', ()):
  File "./test.py", line 7, in __eq__
    return cls() == other
  File "./test.py", line 14, in __eq__
    if isinstance(other, Myclass):
  File "/usr/lib/python3.6/abc.py", line 193, in __instancecheck__
    return cls.__subclasscheck__(subclass)
  File "/usr/lib/python3.6/abc.py", line 218, in __subclasscheck__
    if cls in getattr(subclass, '__mro__', ()):
  File "./test.py", line 7, in __eq__
    return cls() == other
RecursionError: maximum recursion depth exceeded while calling a Python object

blueteeth · Answer 1 · 2019-12-21T11:47:51.560

1

isinstance(Childclass, Myclass)

Is never going to do what you think because isinstance takes an instance of a class not the class itself.

And lo and behold, if you instantiate Childclass, this call succeeds.

isinstance(Childclass(), Myclass) # => True

I've just run this, and I can't see an error. Running Python 3.7.5.

In [35]: from abc import ABCMeta
    ...:
    ...: class Metaclass(ABCMeta):
    ...:     def __eq__(cls, other):
    ...:         return cls() == other
    ...:
    ...:     def __hash__(cls):
    ...:         return 0 #would be implemented accordingly with a correct hastype
    ...:
    ...: class Myclass(metaclass=Metaclass):
    ...:     def __eq__(self, other):
    ...:         if isinstance(other, Myclass):
    ...:             """dostuff"""
    ...:         else:
    ...:             return NotImplemented
    ...:
    ...: class Childclass(Myclass): pass
    ...:

In [36]: isinstance(Childclass, Myclass)
Out[36]: False

edited Dec 21 '19 at 11:47

answered Dec 21 '19 at 11:39

blueteeth

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but then it should just return False, shouldn't it? Just as `isinstance(str, str)` does? – Torilla Dec 21 '19 at 11:42
I've just run it and it does return False. – blueteeth Dec 21 '19 at 11:45
What Python version do you use. I keep getting the infinite recursion stated above... – Torilla Dec 21 '19 at 11:48
Python 3.7.5, and this is in IPython 7.9.0, but I get the same result in the standard Python interpreter. – blueteeth Dec 21 '19 at 11:50
You are right, it also runs fine in python3.7.5... Seems to be a version problem then. Still wondering why this would happen. The error happens for me in python3.6.7 – Torilla Dec 21 '19 at 11:56

Metaclass hash, __eq__ and isinstance

1 Answers1

Metaclass hash, eq and isinstance