[FIXED] Pytest monkeypatch not working on flask view function

Issue

Suppose we have the following Flask view function to test. Specifically, we want to mock create_foo() as it writes to the filesystem.

# proj_root/some_project/views.py
from some_project import APP
from some_project.libraries.foo import create_foo

@APP.route('/run', methods=['POST']
def run():
    bar = create_foo()
    return 'Running'

Now we want to write a unit test for run() with the call to create_foo() mocked to avoid creating unnecessary files.

# proj_root/tests/test_views.py
from some_project import some_project

@pytest.fixture
def client():
    some_project.APP.config['TESTING'] = True
    with some_project.APP.test_client() as client:
        yield client

def test_run(client, monkeypatch):
    monkeypatch.setattr('some_project.libraries.foo.create_foo', lambda: None)
    response = client.post('/run')
    assert b'Running' in response.data

It seems that this approach should work even with the named create_foo import. The tests all pass, however the original code of create_foo is clearly being executed as a new file in the filesystem is created each time the test suite is run. What am I missing? I suspect it has something to do with the named imports based on some related questions but I'm not sure.

Solution

The correct monkeypatch is:

monkeypatch.setattr('some_project.views.create_foo', lambda: None)

The reason for this is pretty well explained here.

Answered By - Alex Waibel