Difference Between Methods in Python

Question

I am new to python programming, and I've now reached the section on OOP in the Python book. I am confused about method definitions in python classes.

What is the difference between:

def __add__(self):
  pass

and

def add(self):
  pass

I'll be thankful if you could clarify this for me; thank you guys.

Usually an underscore at the beginning means that its meant for use internally for that object and not for public use. EDIT: it was single underscores originally, with double underscores it means you are implementing the behavior of `object + object` — , Mar 05 '14 at 18:05
well they are different names (also the one with underscores is a special magic function) — Joran Beasley, Mar 05 '14 at 18:07
__add__ is used to defined the addition (with a "+") operation, see here: http://effbot.org/pyref/__add__.htm — Svend, Mar 05 '14 at 18:08

score 1 · Accepted Answer · answered Mar 05 '14 at 18:07

Methods that begin and end with underscores like __add__(...) can be used to override existing functionality while a method like add(...) without the underscores is a new user-defined method. The difference would be:

With __add__(self) , you could call thisObj + otherObj

With add(self), you would call thisObj.add(otherObj)

So __add__, __sub__, __call__, etc. override existing operators or functionality.

Claudiu · Answer 2 · 2014-03-05T18:16:55.953

Fundamentally, there is no difference. One is a method called __add__ and the other is a method called add.

However, this is a convention which the Python interpreter, and various libraries, use to designate methods that are somehow "special". They are called magic methods. For example, a + b is essentially syntactic sugar for a.__add__(b). Really:

>>> (1).__add__(2)
3

This means if you create a class for which you want addition to be meaningful, instead of calling your method, say, addTo and then doing foo.addTo(bar), you can call the method __add__ and do foo + bar instead.

Other magic methods: a - b is equivalent to a.__sub__(b), len(a) is equivalent to a.__len__(), cmp(a, b) is equivalent to a.__cmp__(b). a = MyClass() causes the new object's __init__ method to be called. And many more.

score 0 · Answer 3 · answered Mar 05 '14 at 18:10

Methods in Python with double underscores before and after their names are called commonly called magic method, and are used to define certain behaviours for objects.

For example the my_object.__add__() method would be implicitly called when your object was used as part of an addition

>>> my_object + another_object

Here is a good resource of Python’s magic methods, aka “dunder” methods.

Difference Between Methods in Python

3 Answers3