What is monkey patching in Python and should you use it

Definition

Monkey patching is the practice of modifying or extending code at runtime without altering the original source. In Python, this usually means reassigning methods or attributes on classes or modules.

Example

class Dog:
    def bark(self):
        return "Woof"

# Monkey patch at runtime
Dog.bark = lambda self: "Meow"

d = Dog()
print(d.bark())  # Meow

Common Use Cases

Monkey patching appears most often in testing, where external dependencies are stubbed:

import requests

def mock_get(url, **kwargs):
    class MockResponse:
        status_code = 200
        def json(self):
            return {"mocked": True}
    return MockResponse()

requests.get = mock_get

For safer testing patterns, consider Python testing with pytest and its built-in monkeypatch fixture. If you need to swap behavior cleanly in production, Python decorators explained offer a more maintainable approach.

Should You Use It?

Avoid monkey patching in production code. It harms readability, breaks encapsulation, and can cause subtle bugs when libraries update. Prefer composition, dependency injection, or official extension points.