[Python-ideas] Make len() usable on a generator

[Guido van Rossum]

Support for len() on iterators isn't going to happen.

The "gut feeling" reason is that len() shouldn't "consume" anything. It
would lure people into thinking they can first call len() on the iterator
and then iterate over it -- while, if it is an actual iterator (like an I/O
stream), after calling len(), everything is consumed.

Yes, it's possible that you only want to count the number of lines, but
that's unusual, and the idiom for that is readily available (e.g. sum(1 for
...)). If the idiom occurs frequently in your code you can define a helper
function count(). The speed benefit of doing the loop in C would be minimal
in most cases.

-- 
--Guido van Rossum (python.org/~guido)
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