Iteratoration question

Rhodri James rhodri at wildebst.demon.co.uk
Fri Apr 3 03:33:23 CEST 2009


On Fri, 03 Apr 2009 02:07:38 +0100, grocery_stocker <cdalten at gmail.com>  
wrote:

> Okay, I was thinking more about this. I think this is also what is
> irking me. Say I have the following..
>
>>>> a = [1,2,3,4]
>>>> for x in a:
> ...     print x
> ...
> 1
> 2
> 3
> 4
>>>>
>
> Would 'a' somehow call __iter__ and next()? If so, does python just
> perform this magically?

No.  It's "for" that invokes the iteration protocol; that's pretty
much the definition of it.  You have read the iteration protocol
after it's been mentioned so many times now, haven't you?

"for" calls iter(a), which in turn calls a.__iter__(), to get an
iterator.  Once it's got, "for" calls next() on the iterator each
time round the loop.  Very approximately, that little for-loop
translates to:

a = [1,2,3,4]
i = iter(a)
try:
     while True:
         x = i.next()
	print x
except StopIteration:
     pass

-- 
Rhodri James *-* Wildebeeste Herder to the Masses



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