itertools.product not lazy

[stsouko]

BPO	40806
Nosy	@tim-one, @rhettinger, @miss-islington, @remilapeyre, @stsouko
PRs	bpo-40806: itertools.product docs clarify #20492 [3.9] bpo-40806: Clarify that itertools.product immediately consumes its inputs (GH-20492) #20498

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2020-05-28.16:50:13.281>
created_at = <Date 2020-05-28.12:55:46.015>
labels = ['3.8', 'type-bug', 'library']
title = 'itertools.product not lazy'
updated_at = <Date 2020-05-28.16:58:36.995>
user = 'https://github.com/stsouko'

bugs.python.org fields:

activity = <Date 2020-05-28.16:58:36.995>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2020-05-28.16:50:13.281>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2020-05-28.12:55:46.015>
creator = 'nougmanoff'
dependencies = []
files = []
hgrepos = []
issue_num = 40806
keywords = ['patch']
message_count = 5.0
messages = ['370201', '370203', '370246', '370249', '370250']
nosy_count = 5.0
nosy_names = ['tim.peters', 'rhettinger', 'miss-islington', 'remi.lapeyre', 'nougmanoff']
pr_nums = ['20492', '20498']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40806'
versions = ['Python 3.8']

[stsouko]

def x(y):
    while True:
        yield y

p = product(x(1), x(2))

next(p)  # this string will never be reached.

[remilapeyre]

Hi Ramil, itertools.product() expect its argument to be finite iterables, it needs to keep all their elements in memory anyway at it "cycles around" to produce all possible pairs.

Basically, product(x(1), x(2)) is equivalent to product(tuple(x(1)), tuple(x(2))).

I see that the documentation does not mention that the arguments must be finite, could you open a PR to improve it?

[rhettinger]

New changeset cfc6ce4 by Ramil Nugmanov in branch 'master':
bpo-40806: Clarify that itertools.product immediately consumes its inpt (GH-20492)
cfc6ce4

[rhettinger]

Updated the docs to note that inputs are fully consumed before the iterator is run.

FYI, notes on infinite iterable inputs likely belong the FAQs because they aren't specific to product(). Similar effects would be seen with list(x(1)), sorted(x(1)), set(x(1)), tuple(x(1)), etc.

[rhettinger]

New changeset e4cc3a7 by Miss Islington (bot) in branch '3.9':
bpo-40806: itertools.product immediately consumes its inputs (GH-20492) (GH-20498)
e4cc3a7