The QGIS expression engine is a powerful tool that can be used to create very complex expressions. The expression engine cannot however replace every action available in QGIS, and sometimes you may want to create a custom function to achieve your goal, this can be because you want to use external tools, run algorithms that are available in PyQGIS but not as part of the expression engine or to simply have a function that performs a series of actions similar to a model.
This is where custom functions come in.
Writing your own custom functions is a way to extend the capabilities of the expression engine, allowing you to simplify your expressions and add new functionalities, and share those new capabilities with others.

We will go over writing custom expression functions with an example function as well as talk about how you should document your functions to make their function help look like the built-in functions’ help.