How to detect the last element in a for loop

[Tom Verbeure]

>> 
>> Hello All,
>> 
>> I often have the case where I need to loop through a bunch of elements,
>> but do something special on for the last line of code.
> 
> That's not what you're doing below:
> 
>> Currently, I can solve it this way:
>> 
>>     first = 1
>>     for port in self.portDefsList:
>>         if first == 0:
>>             string += ", "
>>         else:
>>             first = 0
>>         string += str(port)
>>         pass
> 
> Here, you're doing something special (avoiding the += ", ") the FIRST
> time.  The pass is unconditional and adds absolutely nothing -- just
> take it away.

Duh. I copied the wrong piece of code. :-]

> Anyway, this specific need is met to perfection by:
>     string += ", ".join([ str(port) for port in self.portDefsList ])

Interesting. 
My focus wasn't really on joining strings, but I didn't know this either.

> This does assume that you have a sequence, not just an iterator, and can
> thus slice off the first or last items easily.  It's trickier but
> not difficult to do special-on-first on an iterator:
> 
> processfirst(iterator.next())
> for item in iterator:
>     processnormal(item)
> 
> the .next call returns and "consumes" the first item, so the following
> for starts from the second one, and all is sweetness and light.

I am not familiar with iterators as objects in Python. I understand that

    for a in myList:
        ...

will result in an implicit iterator on myList. How would you create an 
explicit iterator object of myList?

> Detecting the LAST item for special processing is trickier still if
> what you DO have is an iterator.  You basically have to "stagger"
> the output, e.g.:
> 
> saved = iterator.next()
> for item in iterator:
>     processnormal(saved)
>     saved = item
> processlast(saved)

Given that you have an explicit iterator, wouldn't it be trivial to add an 
'end()' method to this iterator to indicate the end of the sequence (just 
like C++ iterators) ?

This would result in:

for item in iterator:
    if iterator.next().end():
        do_something_special

    do_something_for_all
        

Thanks,
Tom