Python: changing methods and attributes at runtime

Question

I wish to create a class in Python that I can add and remove attributes and methods. How can I acomplish that?

Oh, and please don't ask why.

Answer 1

This example shows the differences between adding a method to a class and to an instance.

>>> class Dog():
...     def __init__(self, name):
...             self.name = name
...
>>> puppy = Dog('Skip')
>>> spot = Dog('Spot')
>>> def talk(self):
...     print 'Hi, my name is ' + self.name
...
>>> Dog.talk = talk # add method to class
>>> puppy.talk()
Hi, my name is Skip
>>> spot.talk()
Hi, my name is Spot
>>> del Dog.talk # remove method from class
>>> puppy.talk() # won't work anymore
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: Dog instance has no attribute 'talk'
>>> import types
>>> f = types.MethodType(talk, puppy, Dog)
>>> puppy.talk = f # add method to specific instance
>>> puppy.talk()
Hi, my name is Skip
>>> spot.talk() # won't work, since we only modified puppy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: Dog instance has no attribute 'talk'

Answer 2

A possibly interesting alternative to using types.MethodType in:

>>> f = types.MethodType(talk, puppy, Dog)
>>> puppy.talk = f # add method to specific instance

would be to exploit the fact that functions are descriptors:

>>> puppy.talk = talk.__get__(Dog, puppy)

Answer 3

I wish to create a class in Python that I can add and remove attributes and methods.

import types

class SpecialClass(object):
    @classmethod
    def removeVariable(cls, name):
        return delattr(cls, name)

    @classmethod
    def addMethod(cls, func):
        return setattr(cls, func.__name__, types.MethodType(func, cls))

def hello(self, n):
    print n

instance = SpecialClass()
SpecialClass.addMethod(hello)

>>> SpecialClass.hello(5)
5

>>> instance.hello(6)
6

>>> SpecialClass.removeVariable("hello")

>>> instance.hello(7)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'SpecialClass' object has no attribute 'hello'

>>> SpecialClass.hello(8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'SpecialClass' has no attribute 'hello'

Answer 4

I wish to create a class in Python that I can add and remove attributes and methods. How can I acomplish that?

You can add and remove attributes and methods to any class, and they'll be available to all instances of the class:

>>> def method1(self):
       pass

>>> def method1(self):
       print "method1"

>>> def method2(self):
       print "method2"

>>> class C():
       pass

>>> c = C()
>>> c.method()

Traceback (most recent call last):
  File "<pyshell#62>", line 1, in <module>
    c.method()
AttributeError: C instance has no attribute 'method'

>>> C.method = method1
>>> c.method()
    method1
>>> C.method = method2
>>> c.method()
    method2
>>> del C.method
>>> c.method()

Traceback (most recent call last):
  File "<pyshell#68>", line 1, in <module>
    c.method()
AttributeError: C instance has no attribute 'method'
>>> C.attribute = "foo"
>>> c.attribute
    'foo'
>>> c.attribute = "bar"
>>> c.attribute
    'bar'