PEG Parser: Cannot used starred expression in parenthesised expr

[lysnikolaou]

BPO	40631
Nosy	@gvanrossum, @serhiy-storchaka, @lysnikolaou, @pablogsal, @miss-islington, @gousaiyang
PRs	bpo-40631: Single-element starred elements in parenthesised expressions cannot be parsed #24014 bpo-40631: Improve error message when deleting single starred element #24015 bpo-40631: Disallow star targets with parentheses around #24019 bpo-40631: Disallow single parenthesized star target #24027 [3.9] bpo-40631: Disallow single parenthesized star target (GH-24027) #24068

^{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/lysnikolaou'
closed_at = <Date 2021-01-03.02:01:34.480>
created_at = <Date 2020-05-15.12:04:27.756>
labels = ['interpreter-core', '3.9']
title = 'PEG Parser: Cannot used starred expression in parenthesised expr'
updated_at = <Date 2021-01-03.02:01:34.480>
user = 'https://github.com/lysnikolaou'

bugs.python.org fields:

activity = <Date 2021-01-03.02:01:34.480>
actor = 'pablogsal'
assignee = 'lys.nikolaou'
closed = True
closed_date = <Date 2021-01-03.02:01:34.480>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2020-05-15.12:04:27.756>
creator = 'lys.nikolaou'
dependencies = []
files = []
hgrepos = []
issue_num = 40631
keywords = ['patch']
message_count = 12.0
messages = ['368936', '368938', '384086', '384089', '384091', '384092', '384093', '384094', '384096', '384098', '384244', '384247']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'lys.nikolaou', 'pablogsal', 'miss-islington', 'gousaiyang']
pr_nums = ['24014', '24015', '24019', '24027', '24068']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue40631'
versions = ['Python 3.9']

[lysnikolaou]

The new PEG parser fails when a parenthesised expression with a single child (a group) contains a starred expression. Example:

╰─ ./python.exe
Python 3.9.0a6+ (heads/master-dirty:4a12d12186, May 15 2020, 14:53:45)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ast
>>> ast.parse('(*a)')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/lysnikolaou/Repositories/cpython/Lib/ast.py", line 50, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 1
    (*a)
        ^
SyntaxError: invalid syntax

This was valid syntax up until now:

╰─ ./python.exe -X oldparser
Python 3.9.0a6+ (heads/master-dirty:4a12d12186, May 15 2020, 14:53:45)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ast
>>> ast.dump(ast.parse('(*a)'))
"Module(body=[Expr(value=Starred(value=Name(id='a', ctx=Load()), ctx=Load()))], type_ignores=[])"

[lysnikolaou]

Whoops, false alarm.

It's just that we moved the check for invalid starred expressions to the parser, while it previously was in the compiler.

╰─ ./python.exe -X oldparser
Python 3.9.0a6+ (heads/master-dirty:003708bcf8, May 15 2020, 15:08:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> (*a)
  File "<stdin>", line 1
SyntaxError: can't use starred expression here

Sorry for the noise!

[gousaiyang]

Well, there is actually a bug:

root@f1b4a742d8fc:/# python3.9
Python 3.9.1 (default, Dec  8 2020, 03:24:52)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x = [1, 2]
>>> y = 3
>>> *x, y
(1, 2, 3)
>>> (*x), y
  File "<stdin>", line 1
    (*x), y
     ^
SyntaxError: can't use starred expression here

root@f1b4a742d8fc:/# python3.8
Python 3.8.6 (default, Oct  6 2020, 03:22:36)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x = [1, 2]
>>> y = 3
>>> *x, y
(1, 2, 3)
>>> (*x), y
(1, 2, 3)

This is different from the previous message where the starred expression is "alone" (and thus invalid). Since this bug happens in 3.9 but not in 3.8, it might be due to the PEG parser.

Also,
root@f1b4a742d8fc:/# python3.9
Python 3.9.1 (default, Dec  8 2020, 03:24:52)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> del *x
  File "<stdin>", line 1
    del *x
        ^
SyntaxError: cannot delete starred
>>> del (*x)
  File "<stdin>", line 1
    del (*x)
         ^
SyntaxError: can't use starred expression here

The latter case should also report "SyntaxError: cannot delete starred".