Explaining the 'self' variable to a beginner [duplicate]

Question

I'm pretty much ignorant of OOP jargon and concepts. I know conceptually what an object is, and that objects have methods. I even understand that in python, classes are objects! That's cool, I just don't know what it means. It isn't clicking with me.

I'm currently trying to understand a few detailed answers that I think will illuminate my understanding of python:

1. What does the "yield" keyword do in Python?
2. What is a metaclass in Python?

In the first answer, the author uses the following code as an example:

>>> class Bank(): # let's create a bank, building ATMs ...    crisis = False ...    def create_atm(self) : ...        while not self.crisis : ...            yield "$100" 

I don't immediately grok what self is pointing to. This is definitely a symptom of not understanding classes, which I will work on at some point. To clarify, in

>>> def func(): ...   for i in range(3): ...     print i 

I understand that i points to an item in the list range(3) which, since it is in a function, isn't global. But what does self "point to"?

Answer 1

I'll try to clear up some confusion about classes and objects for you first. Lets look at this block of code:

>>> class Bank(): # let's create a bank, building ATMs ...    crisis = False ...    def create_atm(self) : ...        while not self.crisis : ...            yield "$100" 

The comment there is a bit deceptive. The above code does not "create" a bank. It defines what a bank is. A bank is something which has a property called crisis, and a function create_atm. That's what the above code says.

Now let's actually create a bank:

>>> x = Bank() 

There, x is now a bank. x has a property crisis and a function create_atm. Calling x.create_atm(); in python is the same as calling Bank.create_atm(x);, so now self refers to x. If you add another bank called y, calling y.create_atm() will know to look at y's value of crisis, not x's since in that function self refers to y.

self is just a naming convention, but it is very good to stick with it. It's still worth pointing out that the code above is equivalent to:

>>> class Bank(): # let's create a bank, building ATMs ...    crisis = False ...    def create_atm(thisbank) : ...        while not thisbank.crisis : ...            yield "$100" 

Answer 2

It may help you to think of the obj.method(arg1, arg2) invocation syntax as purely syntactic sugar for calling method(obj, arg1, arg2) (except that method is looked up via obj's type, and isn't global).

If you view it that way, obj is the first argument to the function, which traditionally is named self in the parameter list. (You can, in fact, name it something else, and your code will work correctly, but other Python coders will frown at you.)