What resides inside the python self?

Question

Recently I come across the following code in python

for var in self: 
    self.some_list.append(var)

I know little about self but the above code really made me to think so can some one explain me what the piece of code mean ?

Answer 1

The identifier self has no special meaning in python, it can hold whatever you put into it (either via an assignment or an argument of a function).

The only thing is that instance method when called will put the object on which the method was called as the first argument which is normally (by convention) called self.

Answer 2

When you iterate over some object with the for ... in ... syntax, you are really calling the __iter__ method of it. For example,

class Iterable(object):

    def __init__(self, *args):
        self.items = args 

    def dump(self):
        for val in self:
            print 'I have %s!' % str(val)

    def __iter__(self):
        for val in self.items:
            yield val

i = Iterable(1, 2, 3, 'foo', 'bar')
i.dump()

In the dump method the for val in self iterates over the self.values list as implemented by __iter__. If you drop the __iter__ method, Python won't know what to do and you will get a TypeError ('Iterable' object is not iterable).