How to mock.patch a class imported in another module

Where to patch:

patch() works by (temporarily) changing the object that a name points to with another one. There can be many names pointing to any individual object, so for patching to work you must ensure that you patch the name used by the system under test.

The basic principle is that you patch where an object is looked up, which is not necessarily the same place as where it is defined.

In your case, the lookup location is a.b.ClassC since you want to patch ClassC used in ClassA.

import mock

with mock.patch('a.b.ClassC') as class_c:
    instance = class_c.return_value  # instance returned by ClassC()
    b = ClassB()
    b.method1()
    assert instance.method3.called == True
    

You need to patch where ClassC is located so that's ClassC in b:

mock.patch('b.ClassC')

Or, in other words, ClassC is imported into module b and so that's the scope in which ClassC needs to be patched.