Python How to I check if last element has been reached in iterator tool chain?

Question

for elt in itertools.chain.from_iterable(node):

if elt is the last element:
  do statement

How do I achieve this

Asked By: Bruce

||

Source

Answer 1

You can’t per se. You’d need to store the current item, advance the iterator, and catch the StopIteration exception. And then you’d need to somehow signal that you have the last item.

Answered By: Ignacio Vazquez-Abrams

Answer 2

When the loop ends, the elt variable doesn’t go out of scope, and still holds the last value given to it by the loop. So you could just put the code at the end of the loop and operate on the elt variable. It’s not terribly pretty, but Python’s scoping rules aren’t pretty either.

The only problem with this (thanks, cvondrick) is that the loop might never execute, which would mean that elt doesn’t exist – we’d get a NameError. So the full way to do it would be roughly:

del elt # not necessary if we haven't use elt before, but just in case
for elt in itertools.chain.from_iterable(node):
    do_stuff_to_each(elt)
try:
    do_stuff_to_last(elt)
except NameError: # no last elt to do stuff to
    pass

Answered By: Chris Lutz

Answer 3

You can do this by manually advancing the iterator in a while loop using iter.next(), then catching the StopIteration exception:

>>> from itertools import chain
>>> it = chain([1,2,3],[4,5,6],[7,8,9])
>>> while True:
...     try:
...         elem = it.next()
...     except StopIteration:
...         print "Last element was:", elem, "... do something special now"
...         break
...     print "Got element:", elem
...     
... 
Got element: 1
Got element: 2
Got element: 3
Got element: 4
Got element: 5
Got element: 6
Got element: 7
Got element: 8
Got element: 9
Last element was: 9 ... do something special now
>>>

Answered By: Steven Kryskalla

Answer 4

I do something like this:

rng = len(mlist)
for i in range(rng):
    foo = mlist[i]
    foo.do_something_for_every_item_regardless()
    if i == rng - 1: #since we go from 0 to rng-1
        foo.last_item_only_operation()

Answered By: ether_joe

Answer 5


while True:
    try:
      print(next(myIterator))
    except StopIteration:
        break

Answered By: 吕九峦

Answer 6

Here’s a utility function that works like enumerate() except returns a bool if this was the last element.

Now you don’t have to add any additional lines, and this works on iterables besides lists.

def endify(itr):
    '''
    Like enumerate() except returns a bool if this is the last item instead of the number
    '''
    itr = iter(itr)
    has_item = False
    ended = False
    next_item = None
    while not ended:
        try:
            next_item = next(itr)
        except StopIteration:
            ended = True
        if has_item:
            yield ended, item
        has_item = True
        item = next_item

Look how clean this can be used now:

for end, i in endify(range(3)):
    print(f'Item: {i}, End: {end}')

Item: 0, End: False
Item: 1, End: False
Item: 2, End: True

Answered By: Dane Bouchie

Python How to I check if last element has been reached in iterator tool chain?

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