Mocking a module import in pytest

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pytest provides a fixture for this use-case: monkeypatch.syspath_prepend.

You may prepend a path to sys.path list of import locations. Write a fake fwlib.py and include it in your tests, appending the directory as necessary. Like the other test modules, it needn’t be included with the distribution.

After playing with this myself, I couldn’t actually figure out how to get the fixture to mock module level imports correctly from the library code. By the time the tests run, the library code was already imported and then it is too late to patch.

However, I can offer a different solution that works: you may inject the name from within conftest.py, which gets imported first. The subsequent import statement within the code under test will just re-use the object already present in sys.modules.

Package structure:

$ tree .
.
├── conftest.py
├── lib
│   └── my_lib.py
└── tests
    └── test_my_lib.py

2 directories, 3 files

Contents of files:

# conftest.py
import sys

def fwlib_sum(a, b):
    return a + b

module = type(sys)('fwlib')
module.sum = fwlib_sum
sys.modules['fwlib'] = module

library file:

# lib/my_lib.py
import fwlib

def fw_sum(a, b):
    return fwlib.sum(a, b)

test file:

# lib/test_my_lib.py
import my_lib

def test_sum():
    assert my_lib.fw_sum(1, 2) == 3

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