Python Cookbook

Adding Methods to a Class at Runtime

Credit: Brett Cannon

Problem

You want to add a method to a class at an arbitrary point in your code for highly dynamic customization.

Solution

The best way to perform this task works for both classic and new-style classes:

def funcToMethod(func, clas, method_name=None):
    setattr(clas, method_name or func._ _name_ _, func)

If a method of the specified name already exists in the class, funcToMethod replaces it with the new implementation.

Discussion

Ruby can add a method to a class at an arbitrary point in your code. I figured Python must have a way for allowing this to happen, and it turned out it did. There are several minor possible variations, but this recipe is very direct and compact, and works for both classic and new-style classes. The method just added is available instantly to all existing instances and to those not yet created. If you specify method_name, that name is used as the method name; otherwise, the method name is the same as the name of the function.

You can use this recipe for highly dynamic customization of a running program. On command, you can load a function from a module and install it as a method of a class (even in place of another previous implementation), thus instantly changing the behavior of all existing and new instances of the class.

One thing to make sure of is that the function has a first argument for the instance that will be passed to it (which is, conventionally, always named self). Also, this approach works only ...