Mock return values and side effects in PyTest - Test Execution Trace

Test Overview

This test checks how to use mocks in pytest to simulate different return values and side effects of a function. It verifies that the mocked function returns expected values and raises exceptions as planned.

Test Code - pytest

PyTest

import pytest
from unittest.mock import Mock

def test_mock_return_and_side_effect():
    mock_func = Mock()
    
    # Set return value
    mock_func.return_value = 10
    assert mock_func() == 10
    
    # Change return value
    mock_func.return_value = 20
    assert mock_func() == 20
    
    # Set side effect to raise exception
    mock_func.side_effect = ValueError("Mock error")
    with pytest.raises(ValueError) as exc_info:
        mock_func()
    assert str(exc_info.value) == "Mock error"
    
    # Set side effect to a list of return values
    mock_func.side_effect = [1, 2, 3]
    assert mock_func() == 1
    assert mock_func() == 2
    assert mock_func() == 3

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Create a mock function object	mock_func is a Mock object with no return value or side effect set	-	PASS
2	Set mock_func.return_value to 10	mock_func() will return 10 when called	Assert mock_func() == 10	PASS
3	Change mock_func.return_value to 20	mock_func() will now return 20	Assert mock_func() == 20	PASS
4	Set mock_func.side_effect to raise ValueError("Mock error")	Calling mock_func() raises ValueError with message 'Mock error'	Assert that calling mock_func() raises ValueError with correct message	PASS
5	Set mock_func.side_effect to list [1, 2, 3]	mock_func() will return 1, then 2, then 3 on consecutive calls	Assert mock_func() returns 1, then 2, then 3 on three calls	PASS

Failure Scenario

Failing Condition: If mock_func does not return the expected values or does not raise the expected exception

Execution Trace Quiz - 3 Questions

What does setting mock_func.return_value do?

A Makes mock_func() raise an exception

B Makes mock_func() print the value

C Makes mock_func() return the specified value when called

D Deletes the mock_func object

Key Result

Using mock's return_value and side_effect allows you to simulate different behaviors of dependencies, making your tests flexible and focused on the code under test.