Python: Mock a module without importing it or needing it to exist

Question

I am starting to use a python mock library for my testing. I want to mock a module that is imported within the namespace of the module under test without actually importing it o

Answer 1

I had a similar problem where the helpers library couldn't be loaded as it needed special hardware. Rather than make radical changes to the code that you want to test, an alternative is to insert a "fake" directory into sys.path as suggested by how to add a package to sys path for testing

import os, sys
fake_dir = os.path.join(os.path.dirname(__file__), 'fake')
assert(os.path.exists(fake_dir))
sys.path.insert(0, fake_dir)
import foo
from unittest.mock import sentinel
def foo_test():
    foo.helpers.helper_func.return_value = sentinel.foobar
    assert foo.foo_func() == sentinel.foobar

where fake is structured as:

.
├── fake/
│   └── helpers/
│       └── __init__.py
├── foo.py
└── helpers/

and __init__.py has

from unittest.mock import Mock
helper_func = Mock()