Monkey Patching in Python (Dynamic Behavior)

In Python, Monkey Patching refers to dynamically modifying or extending behavior of a class or module at runtime. This allows developers to change how functions, methods or classes behave without altering original source code.

Monkey patching is a powerful feature, but it should be used carefully because it can lead to unexpected behavior if not managed properly.

For Example: Suppose we have a module monk.py with a simple class:

Normally, calling func() would output:

func() is being called

For changing behavior at runtime we can replace func() method dynamically by assigning a new function to it:

Output

monkey_f() is being called

Explanation:

• A new function monkey_f is defined.
• monkey_f is assigned to monk.A.func.
• All instances of class A now call monkey_f() instead of original func().
• This demonstrates Python’s ability to modify behavior dynamically at runtime.

Why Use Monkey Patching?

Monkey patching is useful in several scenarios:

• Fixing Bugs in Third-Party Modules: Methods can be patched without modifying original library.
• Testing and Mocking: Methods can be replaced during unit testing to simulate specific behaviors.
• Dynamic Behavior in Frameworks: Some frameworks, such as Django, use monkey patching internally to extend classes dynamically.

Selective Monkey Patching: Instance vs. Class

Monkey patching allows modifying the behavior of classes or their instances at runtime.

Example 1: This example shows how to patch a method for a single instance without affecting other instances of same class.

Hello from monkey patch!
Hello from B!

Explanation:

• class B defines method greet and instances b1 and b2 are created.
• New function new_greet is defined and bound only to b1 using __get__.
• Calling b1.greet() executes patched method.
• Calling b2.greet() executes original method, demonstrating selective patching.

Example 2: This example shows how to patch square() for one instance so it acts as a cube calculator, while other instances keep original method.

27
9

Explanation:

• MathOps.square() normally returns square of a number.
• Two instances is created m1 and m2 and new method cube() is defined.
• Only m1 is patched, so m1.square(3) now returns cube (27).
• m2 keeps original behavior and returns square (9).

Note: Monkey patching can break other code and make maintenance difficult. Use it cautiously mainly for testing, quick fixes or prototyping.