How to Use mocker Fixture in pytest for Easy Mocking

In pytest, use the mocker fixture from the pytest-mock plugin to replace parts of your code with mocks during tests. Simply add mocker as a test argument and call methods like mocker.patch() to mock functions or objects.

Syntax

The mocker fixture is provided by the pytest-mock plugin. You use it by adding mocker as a parameter to your test function. Then you can call methods like mocker.patch(target, new=mocked_value) to replace the target with a mock.

Key parts:

• mocker: the fixture that gives mocking tools
• patch(): replaces a function or object temporarily
• target: the full path to the function or object to mock
• new: the value or mock to use instead

python

import module

def test_example(mocker):
    mocked_func = mocker.patch('module.function')
    mocked_func.return_value = 42
    result = module.function()
    assert result == 42

Example

This example shows how to mock a function get_data from a module data_source. The mock replaces the real function to return a fixed value during the test.

python

import pytest

# Imagine this is in data_source.py
# def get_data():
#     return 'real data'

import data_source

def test_mock_get_data(mocker):
    mocker.patch('data_source.get_data', return_value='mocked data')
    result = data_source.get_data()
    assert result == 'mocked data'

Output

============================= test session starts =============================
collected 1 item

test_example.py .                                                          [100%]

============================== 1 passed in 0.01s ==============================

Common Pitfalls

Common mistakes when using mocker include:

• Not installing or importing pytest-mock plugin, so mocker is unavailable.
• Using incorrect target strings in patch() (must be full import path).
• Forgetting to set return_value or side effects on the mock.
• Trying to patch objects not imported or used in the tested code's namespace.

python

import module

def test_wrong_patch(mocker):
    # Wrong target string - will not patch correctly
    mocker.patch('wrong.module.func', return_value=10)

# Correct way

def test_right_patch(mocker):
    mock = mocker.patch('module.func')
    mock.return_value = 10
    assert module.func() == 10

Quick Reference

• mocker.patch(target, new=...): Replace target with new mock or value.
• mocker.spy(target): Wrap target to spy calls without changing behavior.
• mocker.stub(): Create a simple mock object.
• Always use full import path for target.
• Set return_value or side_effect on mocks to control behavior.

Key Takeaways

Use the pytest-mock plugin to get the mocker fixture in pytest tests.

Add mocker as a test function argument to access mocking methods like patch().

Always patch using the full import path of the function or object you want to mock.

Set return_value or side_effect on mocks to control their behavior during tests.

Check that pytest-mock is installed and imported to avoid fixture errors.