unittest mock with __new__ function lead to TypeError

[Fryguest]

Bug report

Bug description:

import unittest
from unittest.mock import patch

class Summary(object):

    def __init__(self, path):
        print("Summary init")

    def __del__(self):
        print("Summary del")

    def get_summary(self):
        print("get_summary")

class MockSummary:
    def __init__(self, path):
        print("Mock start")

    def __del__(self):
        print("Mock end")

    def get_summary(self):
        print("mock_get_summary")

class TestSummary(unittest.TestCase):

    def setUp(self):
        pass

    def tearDown(self):
        pass

    def test_summary(self):
        # 1 success case
        summary1 = Summary('path')
        summary1.get_summary()

        # 2 mock case
        mock = patch.object(Summary, '__new__', return_value=MockSummary('path'))
        mock.start()
        summary2 = Summary('path')
        summary2.get_summary()
        mock.stop()

        # 3 fail case
        summary3 = Summary('path')
        summary3.get_summary()

if __name__ == '__main__':
    unittest.main()

When run this python unittest file, it turns out with error:

Summary init
get_summary
Mock start
mock_get_summary
ESummary del

======================================================================
ERROR: test_summary (__main__.TestSummary.test_summary)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/admin/workspace/python/testSummaryAll.py", line 46, in test_summary
    summary3 = Summary('path')
               ^^^^^^^^^^^^^^^
TypeError: object.__new__() takes exactly one argument (the type to instantiate)

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (errors=1)
Mock end

All the instance init after the mock stop will be failed.

CPython versions tested on:

3.11

Operating systems tested on:

Linux

[graingert]

looking at the unittest.mock._patch code it's doing something like this:

import sys
 
class Summary:
    def __init__(self, path):
        pass
 
def main():
    Summary.__new__ = lambda: None
    del Summary.__new__
    Summary('path')
 
if __name__ == "__main__":
    sys.exit(main())

Which fails in the same way.

It's odd as Summary.__dict__ is the same before and after the assignment and del operation

It looks like the slot and the dict is out of sync

[Fryguest]

I find the discussion in pytest: pytest-dev/pytest#8009.
This may caused by the special behavior for function __new__

BTW, this issue will impact not only the current function but the whole process
Below test will fails in the same way.

class TestSummary(unittest.TestCase):

    def setUp(self):
        pass

    def tearDown(self):
        pass

    def test_1(self):
        # 1 success case
        summary1 = Summary('path')
        summary1.get_summary()

        # 2 mock case
        mock = patch.object(Summary, '__new__', return_value=MockSummary('path'))
        mock.start()
        summary2 = Summary('path')
        summary2.get_summary()
        mock.stop()

    def test_2(self):
        # 3 fail case
        summary3 = Summary('path')
        summary3.get_summary()

This will be very inconvenient since some unittest tool such as nose2 will run all test file in one process.
Which means if in one file we use the mock of function __new__ for the class, all other file can not init the instance of same class anymore.