Monkey patch


A monkey patch is a way for a program to extend or modify supporting system software locally.

Etymology

The term monkey patch seems to have come from an earlier term, guerrilla patch, which referred to changing code sneakily – and possibly incompatibly with other such patches – at runtime. The word guerrilla, homophonous with gorilla, became monkey, possibly to make the patch sound less intimidating. An alternative etymology is that it refers to “monkeying about” with the code.
Despite the name's suggestion, the "monkey patch" is sometimes the official method of extending a program. For example, web browsers such as Firefox and Internet Explorer used to encourage this, although modern browsers now have an official Extensions system.

Definitions

The definition of the term varies depending upon the community using it. In Ruby, Python, and many other dynamic programming languages, the term monkey patch only refers to dynamic modifications of a class or module at runtime, motivated by the intent to patch existing third-party code as a workaround to a bug or feature which does not act as desired. Other forms of modifying classes at runtime have different names, based on their different intents. For example, in Zope and Plone, security patches are often delivered using dynamic class modification, but they are called hot fixes.

Applications

Monkey patching is used to:
Malicious, incompetently written, and/or poorly documented monkey patches can lead to problems:
The following Python example monkey-patches the value of Pi from the standard Python math library to make it compliant with the Indiana Pi Bill.

>>> import math
>>> math.pi
3.141592653589793
>>> math.pi = 3.2 # monkey-patch the value of Pi in the math module
>>> math.pi
3.2
>>> import math
>>> math.pi
3.141592653589793
>>>