Bug report

Bug description:

The following code works on py313 and below (this is a very cut-down version of some code in Matplotilb (matplotlib.path.Path)) but hit the maximum recursion depth on main.

# Add a code block here, if required
import copy

class Test:
    def __init__(self, a, b):
        self.a = a
        self.b = a
        self._state = True

    def __deepcopy__(self, memo=None):
        p = copy.deepcopy(super(), memo)
        p._state = False
        return p

t = Test(1, 2)
t2 = copy.deepcopy(t)

I bisected this to #125781 and suspect the problem is that because it is returning a new object it is escaping the memoization and looping.

I'm happy to be told we should not be doing this fix it on the Matplotlib side, but I would like to be sure that this was an expected consequence of the change.

attn @serhiy-storchaka

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

Linux