[python] Mocking a class: Mock() or patch()?

I am using mock with Python and was wondering which of those two approaches is better (read: more pythonic).

Method one: Just create a mock object and use that. The code looks like:

def test_one (self):
    mock = Mock()
    mock.method.return_value = True 
    self.sut.something(mock) # This should called mock.method and checks the result. 
    self.assertTrue(mock.method.called)

Method two: Use patch to create a mock. The code looks like:

@patch("MyClass")
def test_two (self, mock):
    instance = mock.return_value
    instance.method.return_value = True
    self.sut.something(instance) # This should called mock.method and checks the result. 
    self.assertTrue(instance.method.called)

Both methods do the same thing. I am unsure of the differences.

Could anyone enlighten me?

I've got a YouTube video on this.

Short answer: Use mock when you're passing in the thing that you want mocked, and patch if you're not. Of the two, mock is strongly preferred because it means you're writing code with proper dependency injection.

Silly example:

# Use a mock to test this.
my_custom_tweeter(twitter_api, sentence):
    sentence.replace('cks','x')   # We're cool and hip.
    twitter_api.send(sentence)

# Use a patch to mock out twitter_api. You have to patch the Twitter() module/class 
# and have it return a mock. Much uglier, but sometimes necessary.
my_badly_written_tweeter(sentence):
    twitter_api = Twitter(user="XXX", password="YYY")
    sentence.replace('cks','x') 
    twitter_api.send(sentence)

Key points which explain difference and provide guidance upon working with unittest.mock

1. Use Mock if you want to replace some interface elements(passing args) of the object under test
2. Use patch if you want to replace internal call to some objects and imported modules of the object under test
3. Always provide spec from the object you are mocking
   • With patch you can always provide autospec
   • With Mock you can provide spec
   • Instead of Mock, you can use create_autospec, which intended to create Mock objects with specification.

In the question above the right answer would be to use Mock, or to be more precise create_autospec (because it will add spec to the mock methods of the class you are mocking), the defined spec on the mock will be helpful in case of an attempt to call method of the class which doesn't exists ( regardless signature), please see some

from unittest import TestCase
from unittest.mock import Mock, create_autospec, patch


class MyClass:
    
    @staticmethod
    def method(foo, bar):
        print(foo)


def something(some_class: MyClass):
    arg = 1
    # Would fail becuase of wrong parameters passed to methd.
    return some_class.method(arg)


def second(some_class: MyClass):
    arg = 1
    return some_class.unexisted_method(arg)


class TestSomethingTestCase(TestCase):
    def test_something_with_autospec(self):
        mock = create_autospec(MyClass)
        mock.method.return_value = True
        # Fails because of signature misuse.
        result = something(mock)
        self.assertTrue(result)
        self.assertTrue(mock.method.called)
    
    def test_something(self):
        mock = Mock()  # Note that Mock(spec=MyClass) will also pass, because signatures of mock don't have spec.
        mock.method.return_value = True
        
        result = something(mock)
        
        self.assertTrue(result)
        self.assertTrue(mock.method.called)
        
    def test_second_with_patch_autospec(self):
        with patch(f'{__name__}.MyClass', autospec=True) as mock:
            # Fails because of signature misuse.
            result = second(mock)
        self.assertTrue(result)
        self.assertTrue(mock.unexisted_method.called)


class TestSecondTestCase(TestCase):
    def test_second_with_autospec(self):
        mock = Mock(spec=MyClass)
        # Fails because of signature misuse.
        result = second(mock)
        self.assertTrue(result)
        self.assertTrue(mock.unexisted_method.called)
    
    def test_second_with_patch_autospec(self):
        with patch(f'{__name__}.MyClass', autospec=True) as mock:
            # Fails because of signature misuse.
            result = second(mock)
        self.assertTrue(result)
        self.assertTrue(mock.unexisted_method.called)
    
    def test_second(self):
        mock = Mock()
        mock.unexisted_method.return_value = True
        
        result = second(mock)
        
        self.assertTrue(result)
        self.assertTrue(mock.unexisted_method.called)

The test cases with defined spec used fail because methods called from something and second functions aren't complaint with MyClass, which means - they catch bugs, whereas default Mock will display.

As a side note there is one more option: use patch.object to mock just the class method which is called with.

The good use cases for patch would be the case when the class is used as inner part of function:

def something():
    arg = 1
    return MyClass.method(arg)

Then you will want to use patch as a decorator to mock the MyClass.