如何使用 pytest 模拟导入的模块

Question

Something similar has been asked before, but I'm struggling to get this to work.

How do I mock an import module from another file

I have one file:
b.py (named to be consistent with the linked docs)

import cv2   # module 'a' in the linked docs

def get_video_frame(path):
    vidcap = cv2.VideoCapture(path)  # `a.SomeClass` in the linked docs
    vidcap.isOpened()
    ...

test_b.py

import b
import pytest # with pytest-mock installed

def test_get_frame(mocker):

    mock_vidcap = mocker.Mock()
    mock_vidcap.isOpened.side_effect = AssertionError

    mock_cv2 = mocker.patch('cv2.VideoCapture')
    mock_cv2.return_value = mock_vidcap

    b.get_video_frame('foo')    # Doesn't fail

    mock_vidcap.isOpened.assert_called()   # fails

I set the tests up like this because in where to patch it specifies that if

In this case the class we want to patch is being looked up on the a module and so we have to patch a.SomeClass instead:

@patch(‘a.SomeClass’)

I've tried a few other combinations of patching, but it exhibits the same behavior, which suggests I'm not successfully patching the module. If the patch were to be applied b.get_video_frame('foo') would fail due to the side_effect ; having assert_called fail, supports this.

Edit in an effort to reduce the length of the question I left off the rest of get_video_frame . Unfortunitly, the parts left off we're the critical parts. The full function is:

def get_video_frame(path):
    vidcap = cv2.VideoCapture(path)  # `a.SomeClass` in the linked docs
    is_open = vidcap.isOpened()
    while True:
        is_open, frame = vidcap.read()
        if is_open:
            yield frame
        else:
            break

Answer 1

This line just creates a generator:

b.get_video_frame('foo')

The line is_open = vidcap.isOpened() is never reached, because in the test function the generator remains frozen at the start, therefore the side effect never raises.

You are otherwise using mocker and patch correctly.