when an iterable object is exhausted or not

[MRAB]

On 04/08/2012 20:20, Franck Ditter wrote:
> Two similar iterable objects but with a different behavior :
>
> $$$ i = range(2,5)
> $$$ for x in i : print(x,end=' ')
>
> 2 3 4
> $$$ for x in i : print(x,end=' ')        # i is not exhausted
>
> 2 3 4
>
> --------- Compare with :
>
> $$$ i = filter(lambda c : c.isdigit(), 'a1b2c3')
> $$$ for x in i : print(x,end=' ')
>
> 1 2 3
> $$$ for x in i : print(x,end=' ')        # i is exhausted
>
> $$$
>
> IMHO, this should not happen in Py3k.
> What is the rationale of this (bad ?) design, which forces the programmer
> to memorize which one is exhaustable and which one is not ?...
>
'range' returns a 'range' object:

 >>> i = range(2,5)
 >>> i
range(2, 5)

The 'for' loop passes it to 'iter' to get an iterator:

 >>> iter(i)
<range_iterator object at 0x0135DB30>

More importantly:

 >>> iter(i) is i
False

In other words, when asked for an iterator, the 'range' object always
creates a new one.


On the other hand, 'filter' returns a 'filter' object:

 >>> i = filter(lambda c : c.isdigit(), 'a1b2c3')
 >>> i
<filter object at 0x01360B30>

The 'for' loop passes it to 'iter' to get an iterator:

 >>> iter(i)
<filter object at 0x01360B30>

More importantly:

 >>> iter(i) is i
True

In other words, the 'filter' object returns _itself_ as the iterator.