As reported at: http://code.google.com/p/mock/issues/detail?id=209

>>> from unittest import mock
[107971 refs]
>>> mock.mock_open
<function mock_open at 0x10cff9d20>
[107974 refs]
>>> a = mock.mock_open()
[109965 refs]
>>> a.reset_mock()
...

The best way to solve this seems to be to track a set of visited ids (mocks we've reset) and not recurse into mocks we've already done. This is similar to the patch proposed on the google code issue - but not identical as that uses a list and has it as the default argument to reset_mock.

Hi all,

I've fixed the infinite recursion in `reset_mock()`. It has been solved by tracking a set of visited ids as suggested.

>>> from unittest import mock
>>> a = mock.mock_open()
>>> a.reset_mock()
>>> a
<MagicMock name='open' spec='builtin_function_or_method' id='4449688192'>

The proposed patch does solve the infinite recursion bug, but a different problem appears when resetting the same mock multiple times: it only works the first time.

Using the patch as it stands:

>>> from unittest.mock import mock_open
>>> mo = mock_open()
>>> a = mo()
>>> mo.call_count
1
>>> mo.reset_mock()
>>> mo.call_count
0
>>> b = mo()
>>> mo.call_count
1
>>> mo.reset_mock()
>>> mo.call_count
1

And here from a version with an added print(visited) statement:

>>> from unittest.mock import mock_open
>>> mo = mock_open()
>>> a = mo()
>>> mo.call_count
1
>>> mo.reset_mock()
[]
[139803191795152]
[139803191795152, 139803181189008]
[139803191795152, 139803181189008, 139803213598416]
[139803191795152, 139803181189008, 139803213598416, 139803213652048]
[139803191795152, 139803181189008, 139803213598416, 139803213652048]
>>> mo.call_count
0
>>> b = mo()
>>> mo.call_count
1
>>> mo.reset_mock()
[139803191795152, 139803181189008, 139803213598416, 139803213652048, 139803213598288]
>>> mo.call_count
1
>>> mo.reset_mock(visited=[])
[]
[139803191795152]
[139803191795152, 139803181189008]
[139803191795152, 139803181189008, 139803213598416]
[139803191795152, 139803181189008, 139803213598416, 139803213652048]
[139803191795152, 139803181189008, 139803213598416, 139803213652048]
>>> mo.call_count
0

As you can see, for some reason I don't quite grasp, the 'visited' parameter persists across calls to reset_mock(), meaning that the very first call does indeed reset it but subsequent calls do not.

As the last two calls show, one can force a reset by explicitly providing an empty list, but this is starting to become a change in API and not just a bugfix...

Sorry Michael, I should have read your second comment more closely since you already pointed out that using a list as default argument is bad.

It is, however, easily fixed by changing to this:

def reset_mock(self, visited=None):
    "Restore the mock object to its initial state."
    if visited is None:
        visited = []
    if id(self) in visited:
        return

I should have read more carefully, too! Thanks to both.

And here's an actual patch with the corrected code

I've been bitten by this issue with a custom psycopg2 mock.

>>> cur = mock.Mock()
>>> 
>>> cur.connection.cursor.return_value = cur
>>> cur.reset_mock()
RuntimeError: maximum recursion depth exceeded

the patch looks ok, except the mix of tab and spaces :-)

Applying this to 3.4 and up with a test.

Laurent, it would be good to sign the CLA - since your change here is minimal and Nicola has, I'm just going ahead.

New changeset 4c8cb603ab42 by Robert Collins in branch '3.4':
- Issue #18622: unittest.mock.mock_open().reset_mock would recurse infinitely.
https://hg.python.org/cpython/rev/4c8cb603ab42

New changeset c0ec61cf5a7d by Robert Collins in branch '3.5':
- Issue #18622: unittest.mock.mock_open().reset_mock would recurse infinitely.
https://hg.python.org/cpython/rev/c0ec61cf5a7d

New changeset a4fe32477df6 by Robert Collins in branch 'default':
- Issue #18622: unittest.mock.mock_open().reset_mock would recurse infinitely.
https://hg.python.org/cpython/rev/a4fe32477df6