class LoggingService(object):
    """
    Just a contrived logging class for demonstration purposes
    """
    def log_error(self, error):
        pass


class UserService(object):
    """
    Provide a method to authenticate the user by performing some expensive DB or network operation.
    """
    def validate_credentials(self, username, password):
        print('> UserService::validate_credentials')
        return username == 'iainjames88' and password == 'secret'


class User(LoggingService, UserService):
    """
    A User model class for demonstration purposes. In production, this code authenticates user credentials by calling
    super().validate_credentials and having the MRO resolve which class should handle this call.
    """
    def __init__(self, username, password):
        self.username = username
        self.password = password

    def authenticate(self):
        if super().validate_credentials(self.username, self.password):
            return True
        super().log_error('Incorrect username/password combination')
        return False

class MockUserService(UserService):
    """
    Provide an implementation for validate_credentials() method. Now, calls from super() stop here when part of MRO.
    """
    def validate_credentials(self, username, password):
        print('> MockUserService::validate_credentials')
        return True


class MockUser(User, MockUserService):
    """
    A wrapper class around User to change it's MRO so that MockUserService is injected before UserService.
    """
    pass

if __name__ == '__main__':
    # Normal useage of the User class which uses UserService to resolve super().validate_credentials() calls.
    user = User('iainjames88', 'secret')
    print(user.authenticate())

    # Use the wrapper class MockUser which positions the MockUserService before UserService in the MRO. Since the class
    # MockUserService provides an implementation for validate_credentials() calls to super().validate_credentials() from
    # MockUser class will be resolved by MockUserService and not passed to the next in line.
    mock_user = MockUser('iainjames88', 'secret')
    print(mock_user.authenticate())

>>> print(MockUser('foo', 'bar').authenticate())
> MockUserService::validate_credentials
True

def validate_credentials(self, username, password):
    print('> MockUserService::validate_credentials')
    assert username_ok(username), 'username expected to be ok'
    assert password_ok(password), 'password expected to be ok'
    return True

>>> MockUser.mro()
[<class '__main__.MockUser'>,
 <class '__main__.User'>,
 <class '__main__.LoggingService'>,
 <class '__main__.MockUserService'>,
 <class '__main__.UserService'>,
 <class 'object'>]