Demonstration of using Mock.side_effect to return a different value on each call.

side_effect.py

"""
Using Mock.side_effect with a list to provide multiple return values
"""

from unittest import mock


class MyClass:
    @staticmethod
    def hello(lang):
        # Imagine that this function makes an unreliable, expensive, or slow network call
        # to a translation service or something.
        # You will want to mock it in unit tests to ensure that the function
        # executes in reasonable time and without having to worry about network or credentials etc.
        # In this case, it's a function that gives a greeting in the requested
        # language. So you can imagine that it might check the current time and return
        # a morning, afternoon, or evening greeting.
        # Anything that returns a different result based on time of day needs to be mocked,
        # otherwise tests that pass in the morning will fail in the evening.
        if lang == 'deu':
            return 'guten Tag'
        if lang == 'nob':
            return 'god dag'

    @staticmethod
    def greet():
        return f'{MyClass.hello("deu")}, world! {MyClass.hello("nob")}, world!'


def bare_greet():
    return f'{bare_hello("cym")}, world! {bare_hello("fra")}, world!'


def bare_hello(lang):
    if lang == 'cym':
        return 'bore da'
    if lang == 'fra':
        return 'bonjour'


def test_class_greet():
    """
    You can patch out the method inside the test method,
    giving an iterable to side_effect to cope with the fact that it is called
    multiple times.
    """
    with mock.patch.object(MyClass, 'hello') as mock_hello:
        print('PATCHED: ')
        mock_hello.side_effect = [
            'guten abend', 'god kveld'
        ]
        # You can assert, as normal, that the value returned by the function is what you expect,
        # given the two values returned from the mock.
        assert MyClass.greet() == 'guten abend, world! god kveld, world!'
        # To be sure that the right arguments were provided in the right order,
        # you can use assert_has_calls.
        # This, in conjuction with asserting the value above, should give confidence that all is in order inside
        # `greet` and that
        assert mock_hello.call_count == 2
        mock_hello.assert_has_calls([mock.call('deu'), mock.call('nob')])


def test_bare_greet():
    with mock.patch('__main__.bare_hello') as mock_hello:
        print('PATCHED: ')
        mock_hello.side_effect = [
            'noswaith dda', 'bonsoir'
        ]
        assert bare_greet() == 'noswaith dda, world! bonsoir, world!'
        assert mock_hello.call_count == 2
        mock_hello.assert_has_calls([mock.call('cym'), mock.call('fra')])


if __name__ == '__main__':
    print('NORMAL (Class): ')
    MyClass.greet()
    test_class_greet()
    print('NORMAL (bare):')
    bare_greet()
    test_bare_greet()
    print('DONE')