Misleading information about slice assignment in docs

[stefanchrobot]

BPO	17939
Nosy	@terryjreedy, @ezio-melotti

^{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 = None
created_at = <Date 2013-05-08.21:31:39.955>
labels = ['type-feature', '3.8', '3.9', '3.10', 'docs']
title = 'Misleading information about slice assignment in docs'
updated_at = <Date 2020-11-17.15:23:56.125>
user = 'https://bugs.python.org/stefanchrobot'

bugs.python.org fields:

activity = <Date 2020-11-17.15:23:56.125>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2013-05-08.21:31:39.955>
creator = 'stefanchrobot'
dependencies = []
files = []
hgrepos = []
issue_num = 17939
keywords = []
message_count = 2.0
messages = ['188738', '188870']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'ezio.melotti', 'docs@python', 'stefanchrobot']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue17939'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[stefanchrobot]

http://docs.python.org/3/reference/simple_stmts.html#assignment-statements

The docs says:

"If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (such as a list). The assigned object should be a sequence object of the same type."

This seems wrong, because the assigned object can be any iterable:

a = [4, 5, 6]
a[0:0] = range(1, 4)
# a is now [1, 2, 3, 4, 5, 6]

[terryjreedy]

Since range objects have a known length, that example is not enough to show 'any iterable'. However, generators do not even have a __length_hint__ and they work too.

a = [1,2,3]
a[0:1] = (i for i in range(4))
print(a)
>>> 
[0, 1, 2, 3, 2, 3]