[Python-ideas] Proposal: A Reduce-Map Comprehension and a "last" builtin

[Tim Peters]

[Jacco van Dorp <j.van.dorp at deonet.nl>]
> I've sometimes thought that exhaust(iterator) or iterator.exhaust() would be
> a good thing to have - I've often wrote code doing basically "call this function
> for every element in this container, and idc about return values", but find
> myself using a list comprehension instead of generator. I guess it's such an
> edge case that exhaust(iterator) as builtin would be overkill (but perhaps
> itertools could have it ?), and most people don't pass around iterators, so
> (f(x) for x in y).exhaust() might not look natural to most people.

"The standard" clever way to do this is to create a 0-sized deque:

>>> from collections import deque
>>> deque((i for i in range(1000)), 0)
deque([], maxlen=0)

The deque constructor consumes the entire iterable "at C speed", but
throws all the results away because the deque's maximum size is too
small to hold any of them ;-)

> It could return the value for the last() semantics, but I think exhaustion
> would often be more important than the last value.

For last(),

>>> deque((i for i in range(1000)), 1)[0]
999

In that case the deque only has enough room to remember one element,
and so remembers the last one it sees.  Of course this generalizes to
larger values too:

>>> for x in deque((i for i in range(1000)), 5):
...     print(x)
995
996
997
998
999

I think I'd like to see itertools add a `drop(iterable, n=None)`
function.  If `n` is not given, it would consume the entire iterable.
Else for an integer n >= 0, it would return an iterator that skips
over the first `n` values of the input iterable.

`drop n xs` has been in Haskell forever, and is also in the Python
itertoolz package:

http://toolz.readthedocs.io/en/latest/api.html#toolz.itertoolz.drop

I'm not happy about switching the argument order from those, but would
really like to omit `n` as a a way to spell "pretend n is infinity",
so there would be no more need for the "empty deque" trick.