Tuple unpacking in return and yield statements

[dacut]

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']

[dacut]

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.

[serhiy-storchaka]

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

[dacut]

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

[Cryvate]

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.

[dacut]

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@

[dacut]

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

[gvanrossum]

Move to 3.8.

[jChapman]

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

[gvanrossum]

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

[jChapman]

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