when an iterable object is exhausted or not

Sat Aug 4 16:11:56 EDT 2012

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.