A Standard

[Andrew Koenig]

Greg> Andrew Koenig wrote:

>> For example, in C++, every random-access iterator is also a forward
>> iterator.  Accordingly, every algorithm that operators on forward
>> iterators will also operate on random-access iterators.

Greg> And in Python, every sequence (i.e. something that
Greg> implements __getitem__) can be iterated over, so
Greg> every algorithm that operates on iterables will
Greg> also operate on sequences.

You mean it is impossible for a user-defined type to implement
__getitem__ without also implementing __iter__?

Greg> Seems to me the concepts are quite similar, it's
Greg> just that the terminology is slightly different.

Greg>     STL                           Python
Greg>     --------------------------    ------------
Greg>     random-access iterator        sequence
Greg>     forward iterator              iterable

There is a key difference: A C++ forward iterator is a first-class
value, which means that you can copy it and use it again later to
iterate over the same sequence a second time.  In general, you can't
do that with a Python iterator.  Moreover, you can modify the values
of the elements of the sequence to which a forward iterator refers
(unless the elements themselves are of a type the prevents
modification).

Therefore a C++ forward iterator allows more operations than Python
iterators.  Python iterators are much closer to C++ input iterators
than they are to forward iterators.

-- 
Andrew Koenig, ark at research.att.com, http://www.research.att.com/info/ark