gh-141510: Fix copy.deepcopy() for recursive frozendict

[vstinner]

• Issue: PEP 814: Add built-in frozendict type #141510

[eendebakpt]

Good catch!

[serhiy-storchaka]

After adding the pickle support we may be able to remove the special deepcopy code.

[vstinner]

I added the tests requested by @serhiy-storchaka.

[vstinner]

After adding the pickle support we may be able to remove the special deepcopy code.

I propose merging #144967 PR first (once it will be approved), and then rework this PR. See also @serhiy-storchaka's comment on the pickle PR:

If things are pickleable, they should also be deepcopyable. It is worth to have explicit deepcopy tests, because they can preserve additional invariants. For example, it should be possible to deepcopy a frozendict containing lambdas or modules.

We can add more tests in this deepcopy PR.

[vstinner]

After adding the pickle support we may be able to remove the special deepcopy code.

I merged main into this branch to retrieve the pickle change. If I remove _deepcopy_frozendict(), test_copy.test_deepcopy_frozendict() fails:

FAIL: test_deepcopy_frozendict (test.test_copy.TestCopy.test_deepcopy_frozendict)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/test/test_copy.py", line 442, in test_deepcopy_frozendict
    self.assertIs(y['foo'][0], y)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
AssertionError: frozendict({'foo': [frozendict({...})]}) is not frozendict({'foo': [frozendict({'foo': [...]})]})

[serhiy-storchaka]

If I remove _deepcopy_frozendict(), test_copy.test_deepcopy_frozendict() fails:

Hmm, I thought it is just an optimization. I wonder if deepcopy() can be generalized to handle this. But this is a different issue.

[serhiy-storchaka]

LGTM. 👍 (But maybe add few more assertions?)

[serhiy-storchaka]

Add also other assertions similar to the above ones:

        self.assertEqual(y, x)
        self.assertIsNot(x, y)
        self.assertIsNot(x[0], y[0])

[vstinner]

self.assertEqual(y, x) fails with RecursionError. I added the two other tests.