PyMock

Mocking in Python made easy!

Install

PyMock can be installed with pip

$ pip install python-mock

Note: PyMock require Python version >= 3.7 to work properly.

How is it different?

The typical way of mocking in Python is with MagicMock or simply Mock (https://docs.python.org/3/library/unittest.mock.html).

def test_something():
    blog = MagicMock()
    blog.get_post.return_value = Post()
    assert isinstance(blog.get_post(1), Post)

While this technically work the syntax can be difficult to remember, provides limited support for when your return values depend on input arguments and even if you're using a decent IDE there is no type hinting available to help with code completion.

PyMock attempts to solve all of these issues by offering a simple syntax and full control over how your mocks should respond in different situations. The same example as above can be written using PyMock:

def test_something():
    blog = PyMock.create(Blog)
    PyMock.setup(blog.get_post(1)).returns(Post())
    assert isinstance(blog.get_post(1), Post)

The benefits here are many, for example

• Just by looking at the first line it's very clear what type of object you intend to mock
• PyMock support setting different return values for different input arguments. In this case it's blog.get_post(1) that should return an instance of Post, not blog.get_post(2) or any other call to the same method.
• Because PyMock know exactly what the mocked type is, code completion work just as if it were an object of that type.

Feedback

PyMock is an early state of development and is likely to contain bugs. Quite possibly lots of them. Don't let that hold you back from trying it out and let me know how it works for you! All kinds of feedback are valuable, good and bad

• Bug reports
• Performance
• Syntax
• Overall experience
• Success stories
• Anything else you want to share :)

Examples

Below are a few examples on how PyMock can be used.

from unittest import TestCase

from pymock import PyMock, Is


class Post:
    def get_title(self) -> str:
        pass


class Blog:
    def get_post(self, id: int) -> Post:
        pass


class CustomException(Exception):
    pass


class TestExamples(TestCase):

    def test_return_object(self):
        post = Post()
        mock = PyMock.create(Blog)
        PyMock.setup(mock.get_post(123)).returns(post)

        self.assertEqual(post, mock.get_post(123))

    def test_setup_multiple_values(self):
        post1 = Post()
        post2 = Post()
        mock = PyMock.create(Blog)
        PyMock.setup(mock.get_post(1)).returns(post1)
        PyMock.setup(mock.get_post(2)).returns(post2)

        self.assertEqual(post1, mock.get_post(1))
        self.assertEqual(post2, mock.get_post(2))

    def test_raise_exception(self):
        mock = PyMock.create(Blog)
        PyMock.setup(mock.get_post(1)).raises(CustomException())

        with self.assertRaises(CustomException):
            mock.get_post(1)

    def test_match_instance_type(self):
        post = Post()
        mock = PyMock.create(Blog)
        PyMock.setup(mock.get_post(Is.type(int))).returns(post)

        self.assertEqual(post, mock.get_post(12345))

    def test_recursive_mocking(self):
        mock = PyMock.create(Blog)
        PyMock.setup(mock.get_post(123).get_title()).returns("PyMock is awesome")

        self.assertEqual("PyMock is awesome", mock.get_post(123).get_title())

    def test_mock_function(self):
        def my_function():
            pass
        mock = PyMock.create(my_function)
        PyMock.setup(mock()).returns("my_function return value")

        self.assertEqual("my_function return value", mock())