Tuple unpacking in return and yield statements

@gvanrossum

BPO	32117
Nosy	@gvanrossum, @serhiy-storchaka, @cryvate, @dacut, @miss-islington, @jChapman
PRs	bpo-32117: Allow tuple unpacking in return and yield statements #4509 bpo-32117: Iterable unpacking in return and yield documentation #9487 Use assertEqual() instead of assertEquals(). #9721 bpo-38641: Add support of starred expressions in return/yield to lib2to3 #16994 bpo-32117: Updated Simpsons names in docs #19737

^{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 2018-10-05.18:18:03.512>
created_at = <Date 2017-11-22.23:31:49.357>
labels = ['interpreter-core', 'type-bug', '3.8']
title = 'Tuple unpacking in return and yield statements'
updated_at = <Date 2020-05-05.14:50:03.820>
user = 'https://github.com/dacut'

bugs.python.org fields:

activity = <Date 2020-05-05.14:50:03.820>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-05.18:18:03.512>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2017-11-22.23:31:49.357>
creator = 'dacut'
dependencies = []
files = []
hgrepos = []
issue_num = 32117
keywords = ['patch']
message_count = 17.0
messages = ['306761', '306783', '307257', '307264', '307266', '307267', '325327', '325421', '325459', '325460', '325461', '326063', '326064', '326122', '326823', '327160', '368161']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'python-dev', 'serhiy.storchaka', 'cryvate', 'dacut', 'miss-islington', 'Jordan Chapman']
pr_nums = ['4509', '9487', '9721', '16994', '19737']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32117'
versions = ['Python 3.8']

This stems from a query on StackOverflow: https://stackoverflow.com/questions/47272460/python-tuple-unpacking-in-return-statement/

Specifically, the following syntax is allowed:
def f():
rest = (4, 5, 6)
t = 1, 2, 3, *rest

While the following result in SyntaxError:
def g():
rest = (4, 5, 6)
return 1, 2, 3, *rest

def h():
    rest = (4, 5, 6)
    yield 1, 2, 3, *rest

Looking at the original commit that enabled tuple unpacking in assignment statements:
4905e80

I don't believe this difference is intentional.

My GitHub repo incorporates a fix for this; I'll file a pull request momentarily.

Since this changes the grammar, it should be first discussed on Python-Dev and approved by BDFL.

CLA processed, and BDFL has assented on python-dev. Serhiy, thoughts on next steps?

I think the language spec needs updating as well? In particular in

https://docs.python.org/3/reference/simple_stmts.html#the-return-statement

it seems expression_list should be replaced by starred_list.

Hm... that leaves the only production for expression_list as:
subscription ::= primary "[" expression_list "]"

And I'm not sure that this shouldn't also be replaced by starred_list. It's not accepted today, though:

In [6]: a[1,(4, 5, 6)]
File "<ipython-input-6-ba56159162e9>", line 1
a[1,(4, 5, 6)]
^
SyntaxError: invalid syntax

I will ask about this again on python-dev@

Oops, I wasn't looking broadly enough. This is also used in the augmented assignment statements syntax, e.g. a += 1, 2, 3

Move to 3.8.

BFDL approval: https://mail.python.org/pipermail/python-dev/2017-November/150842.html

Jordan, what's your GitHub account name? I hope you can check this out and make the changes I'm requesting on GitHub.

Here's my GitHub account: I'll make the changes and rebase as soon as I get home.

Sorry, I could have sworn that I pasted my link... https://github.com/jChapman

New changeset fd97d1f by Guido van Rossum (David Cuthbert) in branch 'master':
bpo-32117: Allow tuple unpacking in return and yield statements (gh-4509)
fd97d1f

Fixed by #4509.

New changeset 8fabae3 by Miss Islington (bot) (jChapman) in branch 'master':
bpo-32117: Iterable unpacking in return and yield documentation (GH-9487)
8fabae3

assertEquals() is deprecated, use assertEqual() instead. This causes tests failure when run with -Werror.

New changeset 4642d5f by Serhiy Storchaka in branch 'master':
Use assertEqual() instead of assertEquals(). (GH-9721)
4642d5f

New changeset 627f701 by Javier Buzzi in branch 'master':
bpo-32117: Updated Simpsons names in docs (GH-19737)
627f701

[syntax-errors] Tuple unpacking in `return` and `yield` before 3.8

[syntax-errors] Tuple unpacking in `return` and `yield` before Python…

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Tuple unpacking in return and yield statements #76298

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Tuple unpacking in return and yield statements #76298

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dacut commented on Nov 22, 2017

serhiy-storchaka commented on Nov 23, 2017

dacut commented on Nov 29, 2017

Cryvate commented on Nov 29, 2017

dacut commented on Nov 29, 2017

dacut commented on Nov 29, 2017

gvanrossum commented on Sep 14, 2018

jChapman commented on Sep 15, 2018

gvanrossum commented on Sep 15, 2018

jChapman commented on Sep 15, 2018

jChapman commented on Sep 15, 2018

gvanrossum commented on Sep 22, 2018

gvanrossum commented on Sep 22, 2018

miss-islington commented on Sep 23, 2018

serhiy-storchaka commented on Oct 1, 2018

serhiy-storchaka commented on Oct 5, 2018

miss-islington commented on May 5, 2020

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