pytest-mock for enhanced mocking - Test Execution Trace

Test Overview

This test uses pytest-mock to mock a function call and verify that it was called with expected arguments. It checks that the mocked function returns the mocked value and that the call count and arguments are correct.

Test Code - pytest

PyTest

import pytest

def fetch_data():
    # Imagine this fetches data from a remote API
    return "real data"

def process_data():
    data = fetch_data()
    return f"Processed {data}"

def test_process_data_calls_fetch_data(mocker):
    mock_fetch = mocker.patch(__name__ + ".fetch_data", return_value="mocked data")
    result = process_data()
    assert result == "Processed mocked data"
    mock_fetch.assert_called_once()
    mock_fetch.assert_called_with()

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	-	PASS
2	mocker.patch replaces fetch_data with a mock returning 'mocked data'	fetch_data function is mocked	-	PASS
3	process_data() is called, which internally calls mocked fetch_data	process_data calls mocked fetch_data, receives 'mocked data'	-	PASS
4	Assert that process_data() returns 'Processed mocked data'	Returned value is 'Processed mocked data'	assert result == 'Processed mocked data'	PASS
5	Assert that fetch_data was called exactly once	Mock call count is 1	mock_fetch.assert_called_once()	PASS
6	Assert that fetch_data was called with no arguments	Mock call arguments are empty	mock_fetch.assert_called_with()	PASS
7	Test ends successfully	All assertions passed	-	PASS

Failure Scenario

Failing Condition: fetch_data is not called or returns unexpected value

Execution Trace Quiz - 3 Questions

What does mocker.patch do in this test?

A Skips the fetch_data call

B Runs the real fetch_data function

C Replaces fetch_data with a mock that returns 'mocked data'

D Raises an exception

Key Result

Use pytest-mock's mocker.patch to replace functions during tests. This lets you control return values and verify calls without changing real code.