How Can I Add Methods to Existing Python Objects?

Adding Methods to Existing Object Instances in Python

In Python, it is generally discouraged to add methods to existing objects (i.e., not in the class definition). However, there are some exceptions when it may be necessary.

Bound Methods vs. Functions

In Python, methods are distinguished from functions by being bound to an object instance. When a method is called, the instance is automatically passed as the first argument.

Functions, on the other hand, are not bound to any specific instance and must be explicitly invoked with the appropriate arguments.

Modifying Class Attributes

If you want to add a method to all instances of a class, you can simply modify the class definition:

class A:
    def bar(self):
        print("bar")

a = A()
a.bar()  # Output: "bar"

Modifying Individual Instances

However, if you need to add a method to a specific instance of a class, things become slightly more complicated. You can't directly assign a function to an instance attribute, as it will not be bound:

def barFighters():
    print("barFighters")

a.barFighters = barFighters
a.barFighters()  # TypeError: barFighters() takes exactly 1 argument

Binding Functions with MethodType

To properly bind a function to an object instance, you can use the MethodType function from the types module:

import types

a.barFighters = types.MethodType(barFighters, a)
a.barFighters()  # Output: "barFighters"

This method binding ensures that the function is invoked with the instance as the first argument when called.

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