In Python you sometimes want to dynamically add methods to classes or class instances (objects). In Python code, the most obvious way to accomplish this might be something like the following but it has one caveat,

class MyObj(object):
    def __init__(self, val):
        self.val = val

def new_method(self, value):
    return self.val + value

obj = MyObj(3)
obj.method = new_method

… you can’t use self. Let’s try to actually call the method.

>>> obj.method(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: new_method() takes exactly 2 arguments (1 given)

Arg!

The new_method() function is added to the class as a property which is a function that takes two arguments. So how do we add a method that can use self? The answer is that we use the types module’s MethodType.

>>> from types import MethodType
>>> obj.method = MethodType(new_method, obj, MyObj)
>>> obj.method(5)
8

Ok, so now we can use self. The MethodType type actually binds the method to the instance and creates a “bound method”.

>>> obj.method
<bound method MyObj.new_method of <__main__.MyObj object at 0xb75c928c>>

Here, the method method is bound to the obj instance but is only available to that single instance. If we create another MyObj instance it won’t have the new method. Let’s create a new instance:

>>> obj2 = MyObj(2)
>>> obj2.method(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'MyObj' object has no attribute 'method

So if we create a new instance it doesn’t have the new method. In the case where we want to add a new method to a class and have all new instances of that Class have the new method we will need to add an “unbound method” to the class.

>>> MyObj.method = MethodType(new_method, None, MyObj)
>>> MyObj.method
<unbound method MyObj.new_method>
>>> obj2 = MyObj(2)
>>> obj2.method(5)
7

So now we can create any number of MyObj instances and they will all contain the new method. Pretty handy eh?