Error when copying instances of Generic subclasses

[izquierdo]

Trying to do the following

import copy
from typing import Generic, TypeVar

T = TypeVar('T')

class Test(Generic[T]):
    def __init__(self, t: T) -> None:
        self.t = t

test: Test[int] = Test(5)
copy.copy(test)

produces

Traceback (most recent call last):
  File "test.py", line 11, in <module>
    copy.copy(test)
  File "/usr/lib/python3.6/copy.py", line 88, in copy
    return copier(x)
TypeError: __copy__() takes 1 positional argument but 2 were given

whereas there is no error if not inheriting from Generic.

The most similar issue to this one is #458 but that snippet is trying to call __copy__ directly instead of copy.copy. There is also #306 but that one is trying to copy the type instead of an instance, and also switching the above snippet to copy.deepcopy does work. Finally, there is #468, it's possible that a fix for this issue would fall within its scope.

[ilevkivskyi]

Indeed, this is fixed by #468 (a.k.a. PEP 560), but the fix will be available only in Python 3.7. Therefore, I think it makes sense to also fix it in the backport (the crash also happens on Python 3.4).

[hynekcer]

A solution that works:

class MyClass(Generic[T]):
    __copy__ = None
    ...

This is safe:

1. The value __copy__ = None has the same effect for the function copy.copy() like an undefined __copy__ attribute. A possible problem is if it would overshadow a right __copy__ attribute.
2. The "Generic" type is the last ancestor of MyClass. (The only possible problem would be if it is used as generic mixin for a class with a __copy__ method, but it can be easily solved by a super(...).__copy__())

A general hack can be also

from typing import Generic

if sys.version_info[:2] == (3, 5):
    setattr(Generic, '__copy__', None)

(edited)

It works in some of the first modules of an application before the first copy.

Maybe this is also possible to do in typing_extensions.