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

[Thomas Chaumeny]

Yes, it has to exhaust the generator to find the length, but that also is
what list(generator) or sum(generator) have to do and yet they are allowed
constructions.

Actually I don't think that calling len(generator) would be very useful
with a generator variable, but with an "anonymous" generator using a
comprehension like:
valid_customers = len(customer for customer in customers if
customer.is_valid())

that would be useful.

On Fri, Oct 3, 2014 at 5:17 PM, Chris Angelico <rosuav at gmail.com> wrote:

> On Sat, Oct 4, 2014 at 1:09 AM, Thomas Chaumeny <t.chaumeny at gmail.com>
> wrote:
> > I have just come across some code counting a generator comprehension
> > expression by doing len([foo for foo in bar if some_condition]) and I
> > realized it might be better if we could just use len(foo for foo in bar
> if
> > some_condition) as it would avoid a list allocation in memory.
> >
> > Another possibility is to write sum(1 for foo in bar if some_condition),
> but
> > that is not optimal either as it generates a lot of intermediate
> additions
> > which should not be needed.
> >
> > Sure, len(generator) might lead to an infinite loop but since
> sum(generator)
> > is allowed in Python I see no reason why len(generator) isn't.
>
> I think len() would be confusing, as it has to exhaust the generator
> to find out the length. The easiest would be to roll your own:
>
> def genlen(gen):
>     len = 0
>     for len, _ in enumerate(gen, 1): pass
>     return len
>
> At least then it's clear that it'll iterate over it completely.
>
> ChrisA
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