failed to mock function in the python

Question

I have a simple python code in the VS Code.

under a package my, there are two files aa.py and test_aa.py

aa.py

# Function to be tested
def add(a, b):
    return a + b

test_aa.py

from unittest.mock import MagicMock, patch
from my.aa import add

# Test case using MagicMock to mock the add function
@patch('my.aa.add')
def test_add_with_mock(mock_add):
    # Set the return value of the mocked add function
    mock_add.return_value = 10

    # Now any call to the add function will return the value set in the mock
    result = add(3, 5)

    # Assert that the add function was called with the correct arguments
    mock_add.assert_called_once_with(3, 5)

    # Assert the result of the add function
    assert result == 10

# Run the test
test_add_with_mock()

When I debug to run this unit test, result is 8 rather than 10, seem the mock doesn't work. I am confuse why it fails to mock add().

Any ideas how to fix it?

Answer 1

The reason is that you used:

from my.aa import add

So, you have a reference to the original add before it got patched. And that's what you call when you do result = add(3, 5).

Instead try:

from my import aa

def test(...):
    ...
    result = aa.add(3, 5)

Now your test should lookup the name "add" from the "aa" module namespace, which is where it got patched.