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

Is it possible to override += in Python?

Contents hide

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

Yes, override the __iadd__ method. Example:

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

Method 2

In addition to what’s correctly given in answers above, it is worth explicitly clarifying that when __iadd__ is overriden, the x += y operation does NOT end with the end of __iadd__ method.

Instead, it ends with x = x.__iadd__(y). In other words, Python assigns the return value of your __iadd__ implementation to the object you’re “adding to”, AFTER the implementation completes.

This means it is possible to mutate the left side of the x += y operation so that the final implicit step fails. Consider what can happen when you are adding to something that’s within a list:

>>> x[1] += y # x has two items

Now, if your __iadd__ implementation (a method of an object at x[1]) erroneously or on purpose removes the first item (x[0]) from the beginning of the list, Python will then run your __iadd__ method) & try to assign its return value to x[1]. Which will no longer exist (it will be at x[0]), resulting in an ÌndexError.

Or, if your __iadd__ inserts something to beginning of x of the above example, your object will be at x[2], not x[1], and whatever was earlier at x[0] will now be at x[1]and be assigned the return value of the __iadd__ invocation.

Unless one understands what’s happening, resulting bugs can be a nightmare to fix.

Method 3

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?

Method 4

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.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

0 0 votes

Article Rating