Pytest, unittest : Mocking out a non-existent imported module

[Swamp Thing]

TL;DR

A 'provider' module imports 'a_missing_module'. A 'client' module imports just one function from 'provider'. (It does not import the entire 'provider' module). How can we mock out 'a_missing_module' while testing?

The following provider module imports a_missing_module and implements a helper_function. (I haven't put in any code that actually uses the missing module, but there could be such code in general).

# provider.py
import a_missing_module

def helper_function():
    return 999

Now this client module imports the helper_function from the provider:

# client.py
from provider import helper_function

def client_function():
    return helper_function()

How can I write a test module that mocks out the import statement in the provider? In the past, I have used things like sys.modules['client.provider.missing_module'] = Mock() in cases where the client imports an entire provider module, but now it only imports a function from the provider, so I don't know how to proceed.

 

[Swamp Thing]

Ok, got it after some trial and error. You just do sys.modules['a_missing_module'] = Mock(). You don't need to worry about a full dotted path to the module, unlike the more usual kind of mocking/patching.