What is Monkey Patching and How Does it Work in Python?

What is Monkey Patching?

Monkey patching is a technique in programming that involves dynamically altering the attributes of a class or module at runtime. It is not the same as method or operator overloading or delegation.

How it Works

In Python, classes are mutable, and methods are attributes of the class. Monkey patching involves dynamically replacing these attributes with modified versions, allowing you to alter the behavior of the class or module.

Example

Consider a class with a get_data method that retrieves data from an external source. In a unit test, you may want to replace the get_data method with a stub that returns fixed data without relying on the external source.

import unittest

class MyTest(unittest.TestCase):
    def test_data(self):
        # Monkey patch the original get_data method
        original_data = my_module.get_data_orig
        my_module.get_data = my_module.get_data_stub

        # Now, calling get_data will use the test stub
        my_data = my_module.get_data()

        # Restore the original get_data method
        my_module.get_data = original_data

Cautions

While monkey patching can be useful for testing and debugging, it is important to use it cautiously:

• It can affect the behavior of other code that relies on the original attributes.
• It may not affect alias variables that point to the original attribute before it was patched.

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