Itertools Combinations/Permutations size [duplicate]

Question

Is there anyway to see the len() of an itertools.Combination or other object, really, without materializing it to a list?
I can get the cardinality of combs or permutations with the factorials,... but I want something that generalizes.

Thanks

Answer 1

For any iterable it, you can do:

length = sum(1 for ignore in it)

That doesn't create a list, so the memory footprint is small. But for many kinds of iterables, it also consumes it (for example, if it is a generator, it's consumed and can't be restarted; if it is a list, it's not consumed). There is no generally "non-destructive" way to determine the length of an arbitrary iterable.

Also note that the code above will run "forever" if it delivers an unbounded sequence of objects.

Answer 2

No need to create a list. You can count the number of items in an iterable without storing the entire set:

sum(1 for _ in myIterable)