Monkeypatch not working on "from module import X"?

[thomjah-computas]

I'm trying to mock uuid.uuid4() to give a constant value in the test case, but it is not triggering when the code uses from uuid import uuid4.

from uuid import UUID, uuid4
import uuid

import pytest


def make_plain_uuid():
    return uuid4()


def make_modular_uuid():
    return uuid.uuid4()


def test_patch_uuid_plain(monkeypatch: pytest.MonkeyPatch):
    fixed_uuid = UUID("c559e06e-f0d7-4047-abf3-20faab54dfb6")

    def mock_uuid():
        return fixed_uuid

    monkeypatch.setattr(uuid, "uuid4", mock_uuid, raising=True)

    actual = make_plain_uuid()

    assert actual == fixed_uuid


def test_patch_uuid_modular(monkeypatch: pytest.MonkeyPatch):
    fixed_uuid = UUID("c559e06e-f0d7-4047-abf3-20faab54dfb6")

    def mock_uuid():
        return fixed_uuid

    monkeypatch.setattr(uuid, "uuid4", mock_uuid, raising=True)

    actual = make_modular_uuid()

    assert actual == fixed_uuid


def test_is_uuid4_the_same():
    from_module_id = id(uuid4)
    modular_id = id(uuid.uuid4)

    assert from_module_id == modular_id

test_patch_uuid_plain fails as it doesn't patch the function. Is this a known problem with the monkeypatch, and is there some way to fix it?

It is running platform darwin -- Python 3.10.4, pytest-7.1.2, pluggy-1.0.0

[RonnyPfannschmidt]

That's expected behaviour, monkeypatch patches where you tell it, not everywhere it could possibly find something

[thomjah-computas]

Is there a way to patch uuid4() when it's imported as from uuid import uuid4 in the application code?

[nicoddemus]

You will need to patch the application module, not uuid, say if the code that calls uuid4 lives in app.utils, you need to call monkeypatch.setattr(app.utils, "uuid4", ...).

There is a complete explanation in the mock docs, which apply for monkeypatch.setattr as well.

[thomjah-computas]

Thanks, that worked. Maybe it should be demonstrated in the documentation https://docs.pytest.org/en/7.1.x/how-to/monkeypatch.html . It's not so obvious how to fix it.

[nicoddemus]

Indeed, opened #10217 for that. 👍