我如何使用 Python MonkeyPatch + Mock 断言调用了 function

Question

I have a test library using MonkeyPatch for testing some functionality. I would like to add a test to this library to test that a request to my endpoint calls functionB().

I have used Mock() library to create the mock function, that I then pass using MonkeyPatch.setattr(). However, the mock function is never getting called and the assertion always fails. What am I doing wrong?

mock_function_B = Mock()
monkeypatch.setattr(file, 'function_B', mock_function_B)
params = json.dumps({'foo': "bar"})
res = client.post('/some_endpoint', headers=content_type_json, data=params)
assert res.status_code == 201
assert mock_function_B.call_count == 1

I expect call_count to be 1, as I know that some_endpoint calls function_B (and from the logs, I see that it has been called) However, the assertion fails because call_count is 0.

Answer 1

It can be done in different ways, as an example you can mock function_B and count the calls in a local variable.

calls = []
monkeypatch.setattr(file, 'function_B', lambda a*, **k: calls.append('Called!'))
params = json.dumps({'foo': "bar"})
res = client.post('/some_endpoint', headers=content_type_json, data=params)
assert res.status_code == 201
assert len(calls) == 1

Answer 2

I you use the package pytest-mock , you can check for the number of calls of a mocked function and for the arguments the function was called with .

import sys

def my_fct(*args, **kwargs):
    return 7

def test_call_fct_count2(monkeypatch, mocker):

    # prepare: mock my_fct to return 99
    monkeypatch.setattr(__name__ + '.my_fct', lambda *args, **kwargs: 99)
    spy = mocker.spy(sys.modules[__name__], 'my_fct')  # how to use: spy(module/object, function or method as str)

    # use mocked function (and assert it is mocked)
    assert my_fct() == 99
    assert my_fct('arg0', 1, 'arg2') == 99
    assert my_fct(my_var=3) == 99

    # now we check how and how often my_fct-mock was called
    assert spy.call_count == 3
    my_fct.assert_any_call()
    my_fct.assert_any_call('arg0', 1, 'arg2')
    my_fct.assert_called_with(my_var=3)  # last call

__name__ is needed here as a placeholder for the current modules name. This is just the filename of the python script. When you refer to a function from another module , you need to specify that module as the first arg of setattr and spy .