In computer programming, monkey patching is a technique used to dynamically update the behavior of a piece of code at run-time. It is used to extend or modify the runtime code of dynamic languages such as Smalltalk, JavaScript, Objective-C, Ruby, Perl, Python, Groovy, and Lisp without altering the original source code.

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, nearly 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 (messing with it). 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 (including Firefox) 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:

Pitfalls

Malicious, incompetently written, and/or poorly documented monkey patches can lead to problems:

Examples

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.