Monkeypatch an executable in Python pytest

Question

In Python 3.10, I have a function like:

from shutil import which
def my_func():
    if which('myexecutable.sh'):
       # do stuff
    else:
       # do other stuff

I would like to write a unit test with Pytest that runs the first part code even though the executable is not present. What is the best way to do this?

I know that I can use monkeypatch.setenv() to set an environment variable, but that's not going to make the which() check pass. There's also the added challenge of making sure this is compatible on Windows and Linux.

Laurent · Accepted Answer · 2022-02-12 08:47:55Z

1

You could try like this:

# in script file
from shutil import which

def myfunc():
    if which("myexecutable.sh"):
        return "OK"
    else:
        ...

# in test file
import pytest
from script import myfunc


@pytest.fixture
def which(mocker):
    return mocker.patch("script.which", autospec=True)


def test_myfunc(which):
    assert myfunc() == "OK"

Running pytest outputs: 1 passed

answered Feb 12, 2022 at 8:47

Laurent

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1 Comment

Andrew Rosen Over a year ago

Now this is a neat trick!

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