Auto-speccing can be done through the autospec argument to patch, or the create_autospec function. Auto-speccing creates mock objects that have the same attributes and methods as the objects they are replacing, and any functions and methods (including constructors) have the same call signature as the real object.

Given that, if your mock is used in illegal ways, for e.g a mock method is called with incorrect number of arguments, an exception will be thrown. When refactoring is happening and your tests still serve as living documentation, tests that are using mocks without autospeccing flag will continue to pass.

from mock import create_autospec
import unittest

def foo(a, b):

class FooTestCase(unittest.TestCase):

    def test_foo_spec(self):
        mock = create_autospec(foo)
        mock(1, 2)
        mock.assert_called_with(1, 2)
        mock.return_value = 10
        self.assertEqual(mock(1, 2), 10)
        self.assertRaises(TypeError, mock, 'only_one_arg')

if __name__ == '__main__':