Oatmeal Agitante

Why does len() on a filter object in py3 not just materialize the result instead of raising an error that's mostly unhelpful?

could cause the program to unexpectedly hang forever or use a huge amount of memory. Iterators dont have to be finite or small

well yes, but then you wouldn't call len on it in the first place. Also making me cast it into a list doesn't fix that issue.

so really it's an unhelpful "precaution"

I'd look at it this way: If it did what you want, then ppl reading code have to lookout for calls to len() as possibly allocating

As it is, they only have to look at list()/tuple(), etc., object constructors, as possbly allocating. easier to reason about

I see your point but it should be obvious that len just has to traverse the entirety of your data (absent storing the length)