Why is my monkeypatch not working in pytest?

What will you learn?

In this tutorial, you will master the art of troubleshooting and fixing issues related to monkeypatching in pytest.

Introduction to the Problem and Solution

When conducting tests in Python using pytest, the monkeypatch feature comes in handy for dynamically altering the behavior of functions or objects during testing. However, there are instances where the monkeypatch fails to function as expected, resulting in test failures. This guide delves into common reasons why your monkeypatch may not be operating correctly and provides effective solutions to rectify these issues.

To tackle the challenge of a malfunctioning monkeypatch in pytest, it is crucial to grasp how monkeypatch operates and understand potential causes for its failure. By following specific troubleshooting steps and implementing best practices, you can ensure that your test cases run seamlessly without encountering any monkeypatch-related hurdles.


import pytest

# Example function to be tested
def get_data():
    return "Original data"

# Test function using monkeypatch
def test_get_data(monkeypatch):
    # Define a mock function for get_data()
    def mock_get_data():
        return "Mocked data"

    # Apply the patch for get_data() using our mock function
    monkey.patch('path.to.get_data', mock_get_data)

    # Now when calling get_data(), it should return our mocked data
    assert get_data() == "Mocked data"

# Run this test with: python -m pytest your_test_file.py 

# Copyright PHD


  • Understanding Monkey Patching: Monkey patching involves dynamically modifying functions or objects during runtime.
  • Common Issues: Problems like incorrect usage of monkey.patch(), providing an incorrect target path, or encountering scope-related errors.
  • Solution Steps:
    1. Ensure correct usage of the monkey.patch() method.
    2. Verify the accuracy of the target path being patched.
    3. Confirm that the patching scope aligns with the test requirements.
    Why is my monkeypatch not changing the behavior of my function?

    If your monkeypatch fails to alter function behavior, it may be due to specifying an incorrect path while applying the patch.

    How can I debug issues related to monkeypatching?

    Debugging problems with monkeypatching can be done by inserting print/debug statements within your test functions or running tests with increased verbosity (pytest -v) for more detailed insights.

    Is it necessary to always use monkey.patch() explicitly?

    No, at times you can leverage fixtures that handle monkeypatching implicitly based on their scope.

    Can I apply multiple patches within a single test case?

    Yes, you have the flexibility to apply multiple patches sequentially within a single test case as required.

    What happens if I forget to undo a patch after my test completes?

    Leaving patches applied post-test completion might result in unexpected behaviors elsewhere in your codebase; remember always to clean up after each test case.

    Should I always prefer using mocks over actual implementations when testing?

    The choice between mocks and real implementations depends on what aspect you wish to focus on during testing. Real implementations are suitable for certain scenarios while mocks excel in isolated unit tests.

    How do I determine which functions or objects should be patched in my tests?

    Identify external components impacting your current unit under test’s behavior but requiring isolation during testing (e.g., API calls).

    Can fixture scoping influence how my patches behave across different tests?

    Fixture scoping dictates when a fixture gets set up and torn down; comprehending fixture scopes is vital when dealing with patches shared among multiple tests.


    In conclusion, addressing issues related to monkeypatch functionality in pytest demands meticulous attention towards accurate usage and awareness of potential challenges that may surface during implementation. By adhering to best practices and effectively utilizing debugging techniques, developers can establish reliable and efficient testing procedures within their Python projects.

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