tuple unpacking on assignment should be lazy for unpacked generators

[MarioWenzel]

BPO	25542
Nosy	@vadmium

^{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 = None
closed_at = <Date 2015-11-03.19:36:33.408>
created_at = <Date 2015-11-03.12:30:08.504>
labels = ['interpreter-core', 'type-feature']
title = 'tuple unpacking on assignment should be lazy for unpacked generators'
updated_at = <Date 2015-11-03.19:36:33.407>
user = 'https://bugs.python.org/MarioWenzel'

bugs.python.org fields:

activity = <Date 2015-11-03.19:36:33.407>
actor = 'Mario Wenzel'
assignee = 'none'
closed = True
closed_date = <Date 2015-11-03.19:36:33.408>
closer = 'Mario Wenzel'
components = ['Interpreter Core']
creation = <Date 2015-11-03.12:30:08.504>
creator = 'Mario Wenzel'
dependencies = []
files = []
hgrepos = []
issue_num = 25542
keywords = []
message_count = 3.0
messages = ['253985', '253988', '254013']
nosy_count = 2.0
nosy_names = ['martin.panter', 'Mario Wenzel']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue25542'
versions = ['Python 3.6']

[MarioWenzel]

if I have an assignment
a = <some_generator> then a is the generator.

But if I do any kind of unpacking
*a, = <some_generator> # a list of all items
a, *aa = <some_generator> # first item in a, list of rest in as

If the right-hand side is a generator expression, I would expect that
a, *aa unpacks the first element into a and puts the whole generator (having yielded the first element) back into aa.

So the star-operator of tuple unpacking with the right-hand-side having a generator should be lazy and not exhaust the whole generator.

[vadmium]

But when the starred element is not the final one, it couldn’t be lazy, so you would end up with an inconsistency.

a, *rest, z = <generator>

Also, it would be inconsistent with other kinds of iterables, which get turned into list objects.

>>> a, *aa = (1, 2)
>>> aa
[2]
>>> a, *aa = "abc"
>>> aa
['b', 'c']

[MarioWenzel]

You are right. I didn't even know
head, *middle, end = <generator>
worked.

Thanks