Test that module is NOT imported

Question

I need to test that specific imports of my library don't import along some other heavy modules (e.g. tensorflow - we'll use it in this question). Here's a detailed description:

The test should run some imports and check that TensorFlow is NOT imported. Preferably, it will check for each import on its own, so we can know which import brought tensorflow with it (in case the test fails).

IMPORTANT: the test must also work when running with pytest-xdist. This means that we need to bypass python's module caching.

Here's a general template of what this test might look like:

def test_not_importing_tensorflow():
    # possible setup 
    import mylib.some_module 
    # maybe some more imports - again, it's better to isolate each imported module if possible 
    # check that tensorflow is not imported 
    # possible cleanup thanks

Have you tried the solutions detailed here: stackoverflow.com/questions/4858100/…? Maybe you can for instance check that 'tensorflow' not in sys.modules.keys(). — globglogabgalab
– globglogabgalab, Commented Jan 28 at 14:04
@globglogabgalab Checking that tensorflow is not imported is the easy part. The challenging part is to make this test reliable when testing in a distributed environment using pytest-xdist. — noamgot
– noamgot, Commented Jan 28 at 16:26

noamgot · Accepted Answer · 2025-01-30 15:11:45Z

2

So eventually I came up with this solution:

import shlex
import subprocess
import sys

import pytest


def _test_package_not_imported(package: str, tested_modules: list[str]) -> None:
    import_str = " ;".join([f"import {module}" for module in tested_modules])
    command = shlex.split(f"{sys.executable} -c 'import sys; {import_str}; assert \"{package}\" not in sys.modules'")
    try:
        subprocess.run(command, check=True, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
    except subprocess.CalledProcessError:
        pytest.fail(f"{package} is imported")

# example of usage:
def test_no_tf_import():

    tested_modules = ["mylib.mymodule_1", "mylib.mymodule_2"]
    _test_package_not_imported("tensorflow", tested_modules)

The idea is to run a subprocess, which will provide a clean python environment, regardless of other modules imported during pytest-xdist's run.

answered Jan 30 at 15:11

noamgot

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2 Comments

Daniel Walker Jan 30 at 16:05

Don't you want subprocess.run to fail? That is, you don't want it to be imported.

noamgot Feb 2 at 7:41

@DanielWalker no - notice that the command does assert "package" not in sys.modules - it means package is not imported, we're good. Otherwise, a subprocess.CalledProcessError is thrown, we catch it and do pytest.fail (so it's marked as a failure rather than an error)

Acourages · Accepted Answer · 2025-01-29 12:40:42Z

-1

Use sys.modules to check imports

it checks that Tensowflow is imported in your memory or not

answered Jan 29 at 12:40

Acourages

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1 Comment

noamgot Jan 30 at 15:08

the check itself is easy - I asked about making it work with multi-processing (pytest-xdist)

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