How to mock a function in python which is called in the global scope of a module using Pytest?

Question

I have written the following function in "sample.py" and corresponding test script in "test_sample.py". The main function roll_dice is in module named "dice.py". All these files are in a folder called "myapp".

Scenario 1

dice.py

import random
def roll_dice():
    print("rolling...")
    return random.randint(1, 6)

sample.py

from myapp.dice import roll_dice
def guess_number(num):
    result = roll_dice()
    if result == num:
        return "You won!"
    else:
        return "You lost!"

test_sample.py

@mock.patch("myapp.sample.roll_dice")
def test_guess_number(mock_roll_dice):
    mock_roll_dice.return_value = 3
    assert guess_number(3) == "You won!"

When I run the test using Pytest, it runs successfully. But when I make a small change in sample.py shown below, the test fails:

Scenario 2

sample.py

from myapp.dice import roll_dice
result = roll_dice() # Here is the change
def guess_number(num):
    if result == num:
        return "You won!"
    else:
        return "You lost!"

Rest everything remains same!

Instead of calling function inside another function, when I call it in the global scope of sample.py module, test fails. Can someone tell me how to mock the roll_dice function in Scenario 2?

My guess is that we can not mock function calls in global scope of a module. Is that correct?

Answer 1

In the scenario 2, roll_dice is call at import time. Complete test_sample.py should be:

from myapp.sample import guess_number      # Call to `roll_dice` is done here
                                           # before mock

@mock.patch("myapp.sample.roll_dice")
def test_guess_number(mock_roll_dice):
    mock_roll_dice.return_value = 3
    assert guess_number(3) == "You won!"

If you move import inside test function, it don't work too, because patch("myapp.sample.roll_dice") will import myapp.sample before mocking.

It will work with:

@mock.patch("myapp.dice.roll_dice")
def test_guess_number(mock_roll_dice):
    from myapp.sample import guess_number  # Call to `roll_dice` is done here
                                           # You shouldn't have imported `myapp.sample`
                                           # before
    mock_roll_dice.return_value = 3
    assert guess_number(3) == "You won!"

But be careful with code executed at import time (global variable initialization). At final the only thing you test is your knowledge of the mock library.