How to define a custom function of an instance using setattr

Question

I'm trying to add a method to a class dynamically, but I keep running into an error where self is not passed to a the new function. For instance:

class Dummy():
    def say_hi(self):
        print("hi")


def new_method(self):
    print("bye")


dummy = Dummy()
setattr(dummy, "say_bye", new_method)

dummy.say_bye()

results in the following error:

Traceback (most recent call last):
  File "main.py", line 13, in <module>
    dummy.say_bye()
TypeError: new_method() missing 1 required positional argument: 'self'

What am I doing wrong?

Answer 1

Use types.MethodType feature:

from types import MethodType

class Dummy():
    def say_hi(self):
        print("hi")


def new_method(self):
    print("bye")


dummy = Dummy()
dummy.say_bye = MethodType(new_method, dummy)

dummy.say_bye()   # bye

Answer 2

You are setting the function new_method as an attribute of the dummy object.

If you do print(dummy.__dict__) you'll see something like this:

{'say_bye': <function new_method at 0x7fcd949af668>}

This means that your dummy object has the function new_method as an attribute, so when you do dummy.say_bye() , you're calling the function you have as an attribute without any argument.

It is not a function of the Dummy class, it is just a function that your dummy object has as an attribute.

You can achieve the functionality you are looking for using RomanPerekhrest's answer.

Hope it helps.

Cheers!