What is the purpose of the word 'self'?
The reason you need to use
self. is because Python does not use the
@ syntax to refer to instance attributes. Python decided to do methods in a way that makes the instance to which the method belongs be passed automatically, but not received automatically: the first parameter of methods is the instance the method is called on. That makes methods entirely the same as functions, and leaves the actual name to use up to you (although
self is the convention, and people will generally frown at you when you use something else.)
self is not special to the code, it's just another object.
Python could have done something else to distinguish normal names from attributes -- special syntax like Ruby has, or requiring declarations like C++ and Java do, or perhaps something yet more different -- but it didn't. Python's all for making things explicit, making it obvious what's what, and although it doesn't do it entirely everywhere, it does do it for instance attributes. That's why assigning to an instance attribute needs to know what instance to assign to, and that's why it needs
Let's say you have a class
ClassA which contains a method
methodA defined as:
def methodA(self, arg1, arg2): # do something
ObjectA is an instance of this class.
ObjectA.methodA(arg1, arg2) is called, python internally converts it for you as:
ClassA.methodA(ObjectA, arg1, arg2)
self variable refers to the object itself.
Let’s take a simple vector class:
class Vector: def __init__(self, x, y): self.x = x self.y = y
We want to have a method which calculates the length. What would it look like if we wanted to define it inside the class?
def length(self): return math.sqrt(self.x ** 2 + self.y ** 2)
What should it look like when we were to define it as a global method/function?
def length_global(vector): return math.sqrt(vector.x ** 2 + vector.y ** 2)
So the whole structure stays the same. How can me make use of this? If we assume for a moment that we hadn’t written a
length method for our
Vector class, we could do this:
Vector.length_new = length_globalv = Vector(3, 4)print(v.length_new()) # 5.0
This works because the first parameter of
length_global, can be re-used as the
self parameter in
length_new. This would not be possible without an explicit
Another way of understanding the need for the explicit
self is to see where Python adds some syntactical sugar. When you keep in mind, that basically, a call like
is internally transformed to
it is easy to see where the
self fits in. You don't actually write instance methods in Python; what you write is class methods which must take an instance as a first parameter. And therefore, you’ll have to place the instance parameter somewhere explicitly.