Using Functions to Patch a Class

Overview

One of the things we noted in the previous example was that mypy assumed that the Callable attribute, callback, was a method of the Task class. This leads to a potentially confusing mypy error message, Invalid self argument "Task" to attribute function "callback" with type "Callable[[int], None]". In the previous example, the callable attribute was emphatically not a method.

The presence of the confusion means that a callable attribute can be treated as a method of a class. Since we can generally supply extra methods to a class, it means we can patch in additional methods at runtime.

Does it mean we should do this? It’s perhaps a bad idea, except in a very special situation.

Example

It is possible to add or change a function to an instantiated object, demonstrated as follows. First we’ll define a class, A, with a method, show_something():