Mocking a module import in pytest

问题

I am writing a pytest plugin that should test software that's designed to work inside a set of specific environments.

The software I'm writing is run inside a bigger framework, which makes certain Python modules available only when running my Python software inside the framework.

In order to test my software, I'm required to "mock" or fake an entire module (actually, quite a few). I'll need to implement its functionality in some kind of similar-looking way, but my question is how should I make this fake Python module available to my software's code, using a py.test plugin?

For example, let's assume I have the following code in one of my source files:

import fwlib

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

However, the fwlib module is only made available by the framework I run my software from, and I cannot test inside it.

How would I make sure, from within a pytest plugin, that a module named fwlib is already defined in sys.modules? Granted, I'll need to implement fwlib.sum myself. I'm looking for recommendations on how to do just that.

回答1:

~~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 (if anyone reading this does find the correct directory structure to get it working, please post an answer!).

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 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

来源：https://stackoverflow.com/questions/43162722/mocking-a-module-import-in-pytest

标签

python

pytest