Overriding "+=" in Python? (__iadd__() method)

Yes, override the __iadd__ method. Example:

def __iadd__(self, other):
    self.number += other.number
    return self    

In addition to overloading __iadd__ (remember to return self!), you can also fallback on __add__, as x += y will work like x = x + y. (This is one of the pitfalls of the += operator.)

>>> class A(object):
...   def __init__(self, x):
...     self.x = x
...   def __add__(self, other):
...     return A(self.x + other.x)
>>> a = A(42)
>>> b = A(3)
>>> print a.x, b.x
42 3
>>> old_id = id(a)
>>> a += b
>>> print a.x
45
>>> print old_id == id(a)
False

It even trips up experts:

class Resource(object):
  class_counter = 0
  def __init__(self):
    self.id = self.class_counter
    self.class_counter += 1

x = Resource()
y = Resource()

What values do you expect x.id, y.id, and Resource.class_counter to have?

http://docs.python.org/reference/datamodel.html#emulating-numeric-types

For instance, to execute the statement x += y, where x is an instance of a class that has an __iadd__() method, x.__iadd__(y) is called.