itertools.product reference implementation creates temporaries

[mwallerb]

BPO	46379
Nosy	@rhettinger, @AlexWaygood, @mwallerb
PRs	bpo-46379: remove temporaries from itertools.product reference implementation. #30605

^{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 2022-01-14.20:30:01.483>
created_at = <Date 2022-01-14.17:41:54.751>
labels = ['3.11', 'type-feature', '3.9', '3.10', 'docs']
title = 'itertools.product reference implementation creates temporaries'
updated_at = <Date 2022-01-14.22:58:52.924>
user = 'https://github.com/mwallerb'

bugs.python.org fields:

activity = <Date 2022-01-14.22:58:52.924>
actor = 'mwallerb'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2022-01-14.20:30:01.483>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2022-01-14.17:41:54.751>
creator = 'mwallerb'
dependencies = []
files = []
hgrepos = []
issue_num = 46379
keywords = ['patch']
message_count = 5.0
messages = ['410573', '410575', '410598', '410599', '410607']
nosy_count = 4.0
nosy_names = ['rhettinger', 'docs@python', 'AlexWaygood', 'mwallerb']
pr_nums = ['30605']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46379'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[mwallerb]

The reference implementation of itertools.product creates large temporaries, which we need to remind people of at the top of the code block.

However, using generator magic, we don't need to do this and can even simplify the code in the process! Basically,we iterate over a generator of product(*seq[:-1]), and extend each of the values by every value in seq[-1].