iterating and detecting the last

[Skaperen]

i am iterating over a list (or tuple). for each item except the last item i need to do a specific thing. for the last item i need to do something different. the solution i am thinking about is to get the initial len() of the list and count as the iterating proceeds. if the counting is up, i detect the last when the count equals the length. another way is iterating over range(len(seq)-1) and indexing the sequence like:

for x in range(len(seq)-1):
     foo(seq[x])
bar(seq[-1])

there are many ways to do this. which would be the best? which would be most pythonic?

[Yoriz]

items = [1, 2, 3, 4]

for item in items[:-1]:
    print(item)
print(f"last item = {items[-1]}")

Output:
1
2
3
last item = 4

[bowlofred]

If you know it's a sequence (which list and tuples are), I'd just iterate over obj[:-1].

If it's something that isn't a sequence (like a generator), then you could just stick the current object in a cache and operate on it after you read the next item.

for element in seq[:-1]:
    foo(element)
bar(seq[-1])

or:

i = iter(obj)
cache = next(i)
for item in i:
    foo(cache)
    cache = item
bar(cache)

[Gribouillis]

The cache strategy is also implemented in more_itertools.peekable()

for item in (p := peekable(iterable)):
    if not p:
        print(f"Item {item} is the last one")
    else:
         print("Not the last item")