reading help() - newbie question

[Lie Ryan]

On 05/31/10 20:19, Payal wrote:
> Hi,
> I am trying to learn Python (again) and have some basic doubts which I
> hope someone in the list can address. (English is not my first language and I
> have no CS background except I can write decent shell scripts)
> 
> When I type help(something) e.g. help(list), I see many methods like,
> __methodname__(). Are these something special? How do I use them and why
> put "__" around them?

Yes, the double-underscore are hooks to the various python protocols.
They defines, among all, operator overloading, class construction and
initialization, iterator protocol, descriptor protocol, type-casting, etc.

A typical usage of these double-underscore is to create a class that
overrides these functions, e.g.:

class Comparable(object):
    def __init__(self, value):
        self.value = value
    def __lt__(self, other):
        return self.value > other.value
    def __gt__(self, other):
        return self.value < other.value
    def __str__(self):
        return "Value: " + self.value

You should never create your own double-underscore method, just
override/use the ones that Python provide.

> One more simple query. Many times I see something like this,
> |      D.iteritems() -> an iterator over the (key, value) items of D
> What is this iterator they are talking about and how do I use these
> methods because simly saying D.iteritems() does not work?
> 

read about iterator protocol. Basically, the iterator protocol allows
for-looping over a user-defined class (e.g. for emulating a collection).
D.iteritems() returns an iterator object, which for-loop knows how to
iterate over to generate the stream of (key, value) pairs.