Making a Python test think an installed package is not available

Question

I have a test that makes sure a specific (helpful) error message is raised, when a required package is not available.

def foo(caller):
    try:
        import pkg
    except ImportError:
        raise ImportError(f'Install "pkg" to use {caller}')
    pkg.bar()

with pytest.raises(ImportError, match='Install "pkg" to use test_function'):
   foo('test_function')

However, pkg is generally available, as other tests rely on it. Currently, I set up an additional virtual env without pkg just for this test. This seems like overkill.

Is it possible to "hide" an installed package within a module or function?

Does this answer your question? [How to mock an import](https://stackoverflow.com/questions/8658043/how-to-mock-an-import) — Capie, Feb 14 '20 at 13:48
@Capie I think mocking does the contrary? As in: provide something that is not there, versus: hide something that is there. Or am I overlooking something? In any case, I will clarify the question. — rvf, Feb 14 '20 at 14:12
well, i see you tagged pytest, so I assume you are trying to tests something. Mocking the import, you can set it to the value you want, be `module not found` in this case. That's how you do on test, so you can run them wherever without having to worry the state of the host machine. — Capie, Feb 14 '20 at 14:22
Similar question: https://stackoverflow.com/q/51044068/2650249 — hoefling, Feb 14 '20 at 14:44

score 6 · Accepted Answer · answered Feb 14 '20 at 15:19

I ended up with the following pytest-only solution, which appears to be more robust in the setting of a larger project.

import builtins
import pytest


@pytest.fixture
def hide_available_pkg(monkeypatch):
    import_orig = builtins.__import__

    def mocked_import(name, *args, **kwargs):
        if name == 'pkg':
            raise ImportError()
        return import_orig(name, *args, **kwargs)

    monkeypatch.setattr(builtins, '__import__', mocked_import)


@pytest.mark.usefixtures('hide_available_pkg')
def test_message():
    with pytest.raises(ImportError, match='Install "pkg" to use test_function'):
        foo('test_function')

score 3 · Answer 2 · answered Feb 14 '20 at 14:31

3

You can mock builtins.__import__.

from unittest import mock

import pytest


def foo(caller):
    try:
        import pkg
    except ImportError:
        raise ImportError(f'Install "pkg" to use {caller}')
    pkg.bar()

with mock.patch("builtins.__import__", side_effect=ImportError):
    with pytest.raises(ImportError, match='Install "pkg" to use test_function'):
        foo('test_function')

answered Feb 14 '20 at 14:31

Łukasz Rogalski

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Thanks, this works for the snippet I provided, so +1. There appear to be side effects, when additional imports happen, so I take a slightly different approach. – rvf Feb 14 '20 at 15:25

Making a Python test think an installed package is not available

2 Answers2