Detect if generator function is empty, otherwise iterate over it

Let us suppose we have a generator function gen(), which we do not know if it is empty or not.

If it is empty, we would like to execute a special function foo(), and otherwise we would like to execute a function over each element of the iterator bar(elem).

I can do it like this:

is_empty = True
for elem in gen():
    is_empty = False
    bar(elem)
if is_empty: foo()

But that does not feel very pythonic. Any other approach?

You can't tell directly whether a generator is empty. This is by design. A key principle behind generators is they do not hold in memory all items of a generated sequence.

But you can do something like this:

from itertools import chain

def check_first_and_iterate(iterable):
    try:
        first = next(iterable)
        for item in chain([first], item):
            bar(item)
    except StopIteration:
        foo()

check_first_and_iterate(iterable)