unittest.mock.patch.object holds references for too long

Bug report

Bug description:

import sys
from unittest import mock
import json

def main():
    obj = 'asdfa' * 2000

    print("BEGIN", sys.getrefcount(obj))
    for i in range(10):
        with mock.patch.object(json, 'dumps', return_value=3):
            from json import dumps
            dumps(obj)
    del dumps
    print("END", sys.getrefcount(obj))
    import gc; gc.collect()
    print("AFTERGC", sys.getrefcount(obj))


if __name__ == '__main__':
    main()

The code prints:

BEGIN 2
END 12
AFTERGC 2

Because it is the user's data that is leaked, the leaked object could be huge and could cause non-trivial problems.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-139937

Start with unittest.mock.patch.object and the supplied reproduction, then read linked PR gh-139937 for the current investigation. Verify the reference counts before and after repeated patch contexts and garbage collection; done means the user's object no longer remains referenced after the contexts end.