What is monkey patching?

Question

I am trying to understand, what is monkey patching or a monkey patch?

Is that something like methods/operators overloading or delegating?

Does it have anything common with these things?

Answer 1

No, it's not like any of those things. It's simply the dynamic replacement of attributes at runtime.

For instance, consider a class that has a method get_data. This method does an external lookup (on a database or web API, for example), and various other methods in the class call it. However, in a unit test, you don't want to depend on the external data source - so you dynamically replace the get_data method with a stub that returns some fixed data.

Because Python classes are mutable, and methods are just attributes of the class, you can do this as much as you like - and, in fact, you can even replace classes and functions in a module in exactly the same way.

But, as a commenter pointed out, use caution when monkeypatching:

1. If anything else besides your test logic calls get_data as well, it will also call your monkey-patched replacement rather than the original -- which can be good or bad. Just beware.

2. If some variable or attribute exists that also points to the get_data function by the time you replace it, this alias will not change its meaning and will continue to point to the original get_data. (Why? Python just rebinds the name get_data in your class to some other function object; other name bindings are not impacted at all.)

Answer 2

A MonkeyPatch is a piece of Python code which extends or modifies other code at runtime (typically at startup).

A simple example looks like this:

from SomeOtherProduct.SomeModule import SomeClass  def speak(self):     return "ook ook eee eee eee!"  SomeClass.speak = speak 

Source: MonkeyPatch page on Zope wiki.