`copy.copy` ignores methods of the object which are not part of the class

[LukasGibeh]

Bug report

Bug description:

from types import MethodType
from copy import copy, deepcopy

class MyClass:
    def __copy__(self):
        print("Call __copy__ of class")
        return self

    def __deepcopy__(self, memo):
        print("Call __deepcopy__ of class")
        return self

obj1 = MyClass()
obj2 = MyClass()

def copy_func(self):
    print("New copy function")
    return self

def deepcopy_func(self, memo):
    print("New deepcopy function")
    return self

obj11 = copy(obj1)
# > Call __copy__ of class
obj12 = deepcopy(obj1)
# > Call __deepcopy__ of class

obj21 = copy(obj2)
# > Call __copy__ of class
obj22 = deepcopy(obj2)
# > Call __deepcopy__ of class

obj1.__copy__ = copy_func.__get__(obj1)
obj1.__deepcopy__ = deepcopy_func.__get__(obj1)

obj2.__copy__ = MethodType(copy_func, obj2)
obj2.__deepcopy__ = MethodType(deepcopy_func, obj2)

obj13 = copy(obj1)
# > Call __copy__ of class
obj14 = deepcopy(obj1)
# > New deepcopy function

obj23 = copy(obj2)
# > Call __copy__ of class
obj24 = deepcopy(obj2)
# > New deepcopy function

Knowing, that binding additional methods to an object after instantiation might not be recommended, the behavior shown above confuses me. My expectation is that copy.copy and copy.deepcopy both use the __copy__/__deepcopy__ method of the actual object passed to it (code). However, copy.copy does not, but rather uses the __copy__ method of the class which could be different from that of the object (code).

Historically, the behavior of copy.copy and copy.deepcopy was first identical (c06e3ac, using the method of the class in both cases), but lateron changed (e690883).

Apart from the differences between the behavior of copy.copy and copy.deepcopy w.r.t. the __copy__/__deepcopy__ methods, the copy.copy method itself takes two different approaches when looking for methods to create the copy: __copy__ is searched inside the class, and in case it is not found, the __reduce_ex__/__reduce__ are looked up in the object (code).

CPython versions tested on:

3.11

Operating systems tested on:

Linux

[picnixz]

cc @serhiy-storchaka (not sure which one should be the correct behaviour as it has changed). The docs say:

In order for a class to define its own copy implementation, it can define special methods __copy_\_() and __deepcopy__().

So one could say that the instance's lookup is more of a bug. in this case

[serhiy-storchaka]

Inconsistencies between the pickle and copy modules are well known. They have not yet been fixed because no one dares to do this. Fixing it in one way or in other may break existing code, and we do not know what way will break more code. On other hand, leaving this issue unresolved, keeps potential bugs in user code.

[srinivasreddy]

@serhiy-storchaka Shouldn't we discuss and take a decision? Or was it already decided to leave as is ?