Any elegant way to add a method to an existing object in python?

Question

After a lot of searching, I have found that there are a few ways to add an bound method or unbound class methods to an existing instance objects

Such ways include approaches the code below is taking.

import types
class A(object):
 pass
def instance_func(self):
 print 'hi'
def class_func(self):
 print 'hi'
a = A()
# add bound methods to an instance using type.MethodType
a.instance_func = types.MethodType(instance_func, a) # using attribute
a.__dict__['instance_func'] = types.MethodType(instance_func, a) # using __dict__
# add bound methods to an class
A.instance_func = instance_func
A.__dict__['instance_func'] = instance_func
# add class methods to an class
A.class_func = classmethod(class_func)
A.__dict__['class_func'] = classmethod(class_func)

What makes me annoying is, typing the function's name, instance_func or class_func twice.

Is there any simple way to add an existing function to an class or instance without typing the function's name again?

For example, A.add_function_as_bound_method(f) will be far much elegant way to add an existing function to an instance or class since the function already has __name__ attribute.

Answer 1

Normally, functions stored in object dictionaries don't automatically turn into boundmethods when you look them up with dotted access.

That said, you can use functools.partial to pre-bind the function and store it in the object dictionary so it can be accessed like a method:

>>> from functools import partial
>>> class Dog:
 def __init__(self, name):
 self.name = name
>>> d = Dog('Fido')
>>> e = Dog('Buddy')
>>> def bark(self): # normal function
 print('Woof! %s is barking' % self.name)
>>> e.bark = partial(bark, e) # pre-bound and stored in the instance
>>> e.bark() # access like a normal method
Woof! Buddy is barking

This is a somewhat elegant way to add a method to an existing object (without needing to change its class and without affecting other existing objects).

Follow-up to Comment:

You can use a helper function to add the pre-bound function is a single step:

>>> def add_method(obj, func):
 'Bind a function and store it in an object'
 setattr(obj, func.__name__, partial(func, obj))

Use it like this:

>>> add_method(e, bark)
>>> e.bark()
Woof! Fido is barking

Hope this is exactly what you need :-)

• alec_djinn: Assuming the bark function to be encoded in a string, is it possible to do the same during class initialization like asked in stackoverflow.com/questions/52579310/…?