python self-less

class, python, self

Solution

Python methods are just functions that are bound to the class or instance of a class. The only difference is that a method (aka bound function) expects the instance object as the first argument. Additionally when you invoke a method from an instance, it automatically passes the instance as the first argument. So by defining `self` in a method, you're telling it the namespace to work with.

This way when you specify `self.a` the method knows you're modifying the instance variable `a` that is part of the instance namespace.

Python scoping works from the inside out, so each function (or method) has its own namespace. If you create a variable `a` locally from within the method `p` (these names suck BTW), it is distinct from that of `self.a`. Example using your code:

class d:
    def __init__(self,arg):
        self.a = arg
    def p(self):
        a = self.a - 99
        print "my a= ", a
        print "instance a= ",self.a


x = d(1)
y = d(2)
x.p()
y.p()

Which yields:

my a=  -98
instance a=  1
my a=  -97
instance a=  2

Lastly, you don't have to call the first variable `self`. You could call it whatever you want, although you really shouldn't. It's convention to define and reference `self` from within methods, so if you care at all about other people reading your code without wanting to kill you, stick to the convention!

Further reading:

- Python Classes tutorial

Problem

Original source