[Python-Dev] PEP 0424: A method for exposing a length hint
tjreedy at udel.edu
Sun Jul 15 03:03:31 CEST 2012
On 7/14/2012 6:11 PM, Alex Gaynor wrote:
"This method is then used by various other functions (such +as ``map``)
to presize lists"
-- map no longer produces lists. This only makes sense in 3.x if you
mean that map can pass along the value of its inputs.
"Types can then define ``__length_hint__`` which are not
+sized, and thus should not define ``__len__``,"
is awkwardly phrased. I think you mean
"Types that are not sized and should not define __len__ can then define
What do 'sized' and 'should' mean? Some iterators know exactly how many
items they have yet to yield. The main implication of having a __len__
versus __length_hint__ methods seems to be it bool() value when empty.
If lists were to get a new keyword arg, so should the other classes
based on one internal array. I see this has been removed.
Generator functions are the nicest way to define iterators in Python.
Generator instances returned from generator functions cannot be given a
length hint. They are not directly helped. However ...
Not addressed in the PEP: do consumers of __length_hint look for it (and
__length__ before or after calling iter(input), or both? If before, then
the following should work.
class gwlh: # generator with length hint
def __init__(self, gen, len):
self.gen = gen
self.len = len
Do transformation iterators pass through hints from inputs? Does map(f,
iterable) look for len or hint on iterable? Ditto for some itertools,
like chain (add lengths). Any guidelines in the PEP
Terry Jan Reedy
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