Why is there no __iter__() for lists strings and tuples?

[Oren Tirosh]

On Fri, Dec 20, 2002 at 08:07:20PM +0000, Parzival Herzog wrote:
> >>> [1,2,3].__iter()__
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> AttributeError: 'list' object has no attribute '__iter__'
> >>>
> 
> So is this a bug? Are strings, lists and tuples sequences? If they are, why
> is there no __iter__() attribute for them?

Prior to Python 2.2 iteration used __getitem__ and IndexError. Python 2.2
adds support for iterators using the __iter__ function but still supports the
older form of iteration. Since strings, lists and tuples work just fine with
the older form there was no need to support __iter__ for these types.

for i in a:
   do_somewith_with(i)

is equivalent in Python < 2.2 to:

_tmpidx = 0
try:
    while 1:
        i = a[_tmpidx]
        _tmpidx = _tmpidx +1
        do_somewith_with(i)
except IndexError:
    pass


and in Python >= 2.2:

if hasattr(a, '__iter__'):
    _tmpiter = a.__iter__()
    try:
        while 1:
            i = _tmpiter.next()
            do_something_with(i)
    except StopIteration:
        pass
else:
    # fall back to < 2.2 behavior


	Oren