Python mockito - Mocking a class which is being instantiated from the testable function

Question

I am bit lost while writing the test case for UserCompanyRateLimitValidation class. I am finding difficulty in mocking the class which is being instantiated from inside this class.

class UserCompanyRateLimitValidation:
    def __init__(self, user_public_key):
        self.adapter = UserAdapter(user_public_key)
        container = self.adapter.get_user_company_rate_limit()
        super(UserCompanyRateLimitValidation, self).__init__(container,\
                                            UserCompanyRateLimitValidation.TYPE)

I have to test this class. I have written test case something like this. I have tried to mock the UserAdapter class but I am not able to do so completely.

def test_case_1():
   self.user_public_key = 'TEST_USER_PUBLIC_KEY_XXXXXX1234567890XXXXX'
   UserAdapter_mock = mock(UserAdapter)
   when(UserAdapter_mock).get_user_company_rate_limit().\
                                          thenReturn(get_fake_container_object())

   self.test_obj = UserCompanyRateLimitValidation(self.user_public_key)

Here if you see I have mocked get_user_company_rate_limit() call from the testable function, container = self.adapter.get_user_company_rate_limit() but I am still not able to figure out the way in which I can mock this call,

 self.adapter = UserAdapter(user_public_key)

Answer 1

It is quite simple if you know the trick.

Creating an object in Python is very much like a function call to the class object. UserCompanyRateLimitValidation is 'invoking' UserAdapter(user_public_key). You want to stub the return value of that 'call' to return UserAdapter_mock.

You can stub this like you would stub a function in a module. The line you're missing is:

when(module_declaring_UserAdapter)\
    .UserAdapter(self.user_public_key)\
    .thenReturn(UserAdapter_mock)

After that, calling module_declaring_UserAdapter.UserAdapter(self.user_public_key) will return UserAdapter_mock.

Here's the link to the section in the manual: https://code.google.com/p/mockito-python/wiki/Stubbing#Modules

You have to be careful to choose the right module_declaring_UserAdapter, due to the way the from ... import ... statement works. From your code, I'd say you have to pick the module in which UserCompanyRateLimitValidation is declared.

Answer 2

Here is another way of looking at it. Say I have this code in which I would like to mock MyClass:

from some.module import MyClass

class AnotherClass:

    def __init__(self):
        self.my_class = MyClass()

One would typically call the imports as shown above. With some slight modification of the import, we can get it into a state where MyClass it can be mocked using mockito:

from some import module

class AnotherClass:

    def __init__(self):
        self.my_class = module.MyClass()

Then the mocking would work like so:

from some import module

when(module).MyClass().thenReturn(mock())