I have a unit test where I want to check if a function was called. How do I do this withpytest and pytest-mock libraries?

For example, here is a unit test test_hello.py. In this test I call the function my_function and want to verify that it called hello with a given argument.

def hello(name):
    return f'Hello {name}'

def my_function():
    hello('Sam')

def test_hello(mocker):
    mocker.patch('hello')
    my_function()
    hello.assert_called_once_with('Sam')

The code above returns the following error:

target = 'hello'

    def _get_target(target):
        try:
>           target, attribute = target.rsplit('.', 1)
E           ValueError: not enough values to unpack (expected 2, got 1)

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/mock.py:1393: ValueError

During handling of the above exception, another exception occurred:

mocker = <pytest_mock.MockFixture object at 0x109c5e978>

    def test_hello(mocker):
>       mocker.patch('hello')

test_hello.py:8: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pytest_mock.py:156: in __call__
    return self._start_patch(self.mock_module.patch, *args, **kwargs)
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pytest_mock.py:134: in _start_patch
    p = mock_func(*args, **kwargs)
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/mock.py:1544: in patch
    getter, attribute = _get_target(target)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

target = 'hello'

    def _get_target(target):
        try:
            target, attribute = target.rsplit('.', 1)
        except (TypeError, ValueError):
            raise TypeError("Need a valid target to patch. You supplied: %r" %
>                           (target,))
E           TypeError: Need a valid target to patch. You supplied: 'hello'

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/mock.py:1396: TypeError

Solving the error by

assign a mocked_hello to mocked.patch

assign a side_effect to mocked func

def bonjour(name):
    return 'bonjour {}'.format(name)


def hello(name):
    return 'Hello {}'.format(name)


def my_function():
   return hello('Sam')


def test_hellow_differnt_from_module(mocker):
    # mocked func with `test_hello.py` as module name
    mocked_hello = mocker.patch('test_hello.hello')
    # assign side_effect to mocked func
    mocked_hello.side_effect = bonjour
    # the mocked func return_value changed by side_effect
    assert mocked_hello('Sam') == 'bonjour Sam'
    # the mocked func called with Sam, but with different return value
    mocked_hello.assert_called_with('Sam')

call a real function my_function() and the verify that it called hello

def test_my_function(mocker):
    mocker.patch('test_hello.hello', side_effect=bonjour)
    mf = my_function()
    hello.assert_called_with('Sam')
    assert mf == 'bonjour Sam'