Mock called_with does not ensure self/cls argument is used

[JonathanHuot]

BPO	31541
Nosy	@pablogsal, @JonathanHuot, @tirkarthi

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-11-13.03:21:01.320>
created_at = <Date 2017-09-21.09:35:48.433>
labels = ['3.7', 'invalid', 'type-bug', 'library']
title = 'Mock called_with does not ensure self/cls argument is used'
updated_at = <Date 2018-11-13.03:21:10.267>
user = 'https://github.com/JonathanHuot'

bugs.python.org fields:

activity = <Date 2018-11-13.03:21:10.267>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2018-11-13.03:21:01.320>
closer = 'pablogsal'
components = ['Library (Lib)']
creation = <Date 2017-09-21.09:35:48.433>
creator = 'jonathan.huot'
dependencies = []
files = []
hgrepos = []
issue_num = 31541
keywords = []
message_count = 3.0
messages = ['302677', '329721', '329800']
nosy_count = 3.0
nosy_names = ['pablogsal', 'jonathan.huot', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31541'
versions = ['Python 3.3', 'Python 3.4', 'Python 3.5', 'Python 3.6', 'Python 3.7']

[JonathanHuot]

Mock "assert_called_with" does not contain a possibility to verify if "self" or "cls" is used when mock is called.

So, in unittests, all tests are passing but code is broken. Example :

Steps to reproduce:
==================

class Something(object):
    def foobar(self):
        pass

    def foo(self):
        self.foobar()

    def bar(self):
        Something.foobar()  # this is broken


from unittest import mock
x = mock.Mock(spec=Something)
x.foo()
x.foo.assert_called_with()
x.bar()
x.bar.assert_called_with()  # this assertion pass!

# real code 
z = Something()
z.foo()
z.bar()  # raise exception

[tirkarthi]

mock can only verify if the function is called with the correct number of arguments as you have passed spec. It doesn't verify whether the function implementation is correct like calling Something.foobar() because it's not designed to verify the function implementation. It wouldn't raise an exception even when self.foo(1, 2) is called inside x.bar because runtime code is not executed and only the signature is checked with mock. I think this is an expected behavior and not a bug.

[pablogsal]

I agree with Karthikeyan: This is is the expected behaviour. The moment you use a mock, any implementation details are lost unless you use wraps or similar to also transfer the call to the/a real object.