Replace a client library with a mock from a test?

Question

Is there a way to replace a client library (which communicates with a remote server) with a mock object from within a unittest?

Here's a diagram to explain what I'm attempting to do

    +---------------+
    |     tests     |----{ mock }
    +---------------+       |
            |               |
            v               |
    +---------------+       |
    |     model     |       |
    +---------------+       |
            |               |
            v               |
    +---------------+       |
    | client-module |<--{replaces}
    +---------------+
            ^
            :
            :
            v
    +---------------+
    |    service    |
    +---------------+

Since the tests import the model, which imports the client-module, there doesn't seem to be a way to apply the mock to the model's internals.

Answer 1

If model.py does an

import client_module

and doesn't use any features of it at import time, you can do

import model

...

model.client_module = MyMockModule()

where MyMockModule returns suitable mocks for stuff the real client_module provides. I haven't shown setUp/tearDown stuff to take care of this, but hopefully you get the idea,

If model does use stuff from client_module at import time, you'll need to replace sys.modules['client_module'] with the mocked module before importing model.