Python get the last element from generator items

Question

I'm super amazed using the generator instead of list. But I can't find any solution for this question.

What is the efficient way to get the first and last element from generator items? Because with list we can just do lst[0] and lst[-1]

Thanks for the help. I can't provide any codes since it's clearly that's just what I want to know :)

The only way to know what's at the end of a generator sequence is go through the whole thing. — Mark Ransom, Jul 17 '21 at 17:02
You might get some ideas from https://stackoverflow.com/q/661603/5987 but it won't be as efficient as `[-1]`. — Mark Ransom, Jul 17 '21 at 17:13
Generators don't necessarily have a last item, either. Consider `(i for i in itertools.count())`. (Or just `itertools.count()` if you don't want to nitpick over the difference between a generator and an iterator.) — chepner, Jul 17 '21 at 17:13

score 8 · Accepted Answer · answered Jul 17 '21 at 17:06

You have to iterate through the whole thing. Say you have this generator:

def foo():
    yield 0
    yield 1
    yield 2
    yield 3

The easiest way to get the first and last value would be to convert the generator into a list. Then access the values using list lookups.

data = list(foo())
print(data[0], data[-1])

If you want to avoid creating a container, you could use a for-loop to exhaust the generator.

gen = foo()
first = last = next(gen)
for last in gen: pass

print(first, last)

Note: You'll want to special case this when there are no values produced by the generator.

This is great. since I don't want to convert them to list, The first=last=next(gen) is what I'm looking for. I tried and it worked. Thank you very much — mark12345, Jul 17 '21 at 17:13

1 Answers1