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Why uses the ‘self’ variable of define ‘class’ in Python ? Or who can explain what is ‘self’ ?

11/08/2013, Posted in  python

Question:

What is the purpose of the self word in Python? I understand it refers to the specific object created from that class, but I can’t see why it explicitly needs to be added to every function as a parameter. To illustrate, in Ruby I can do this:

class myClass def myFunc(name) @name = name end end

Which I understand, quite easily. However in Python I need to include self:

class myClass: def myFunc(self, name): self.name = name

Can anyone talk me through this? It is not something I’ve come across in my (admittedly limited) experience. Any help would be appreciated.

Answer:

1.

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 self..

2.

I have been confused by this as well for quite a while and I don’t believe that the reason for this has got much to do with the often-pronounced explicit is better than implicit but that it is just following a simple analogy there.

Let’s take a simple vector class:

class Vector(object): def __init__(self, x, y): self.x = x self.y = y

Now, 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)

And, 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. Now, 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_global v = 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 self.

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

v_instance.length()

is internally transformed to

Vector.length(v_instance)

it is easy to see where the self fits in. You don’t not 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.

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