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=[])"

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!

Well, there is actually a bug:

f1b4a742d8fc">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

f1b4a742d8fc">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,
f1b4a742d8fc">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".

Honestly this seems like a bug in 3.8 to me (if it indeed behaves like this):

>>> (*x), y
(1, 2, 3)

Every time I mistakenly tried (*x) I really meant (*x,), so it's surprising that (*x), y would be interpreted as (*x, y) rather than flagging (*x) as an error.

Please don't "fix" this even if it is a regression.

It makes sense to me to be able to do `(*a), b` if I can do `*a, b`, but I don't really have a strong opinion on it.

Parenthesis can be added around expression. But `*a` is not an expression (as well as `+`, `or`, `1:5`, you cannot surround them with parenthesis).

Also the current behavior allows `(*x), y = 1` assignment. If `(*x)` is to be totally disallowed, `(*x), y = 1` should also be rejected.

> It makes sense to me to be able to do `(*a), b` if I can do `*a, b`, but I don't really have a strong opinion on it.

I disagree. *a is not an expression, so the normal rules for parenthesizing those don't apply. I've always thought of *a as a feature of the "comma" syntax. Note too that (**a) is not valid and never was. Also note that 2.7 doesn't support f((*a)). In fact 3.4 doesn't either -- but 3.5 does.

I don't know how this slipped into earlier Python 3 versions -- apparently there aren't tests for this, and it's not used in popular 3rd code either, or we would have found out when we first implemented PEP 617.

Most likely it's due to the general problem where the parser would just accept parenthesized stuff in various places where it shouldn't (e.g. also f(a=1) could be spelled as f((a)=1) -- this was fixed in 3.8).

> Also the current behavior allows `(*x), y = 1` assignment. If `(*x)` is to be totally disallowed, `(*x), y = 1` should also be rejected.

I agree.

Yup, this all sounds much more reasonable. Thanks for the explanation, Guido.

> Also the current behavior allows `(*x), y = 1` assignment. If `(*x)` is to be totally disallowed, `(*x), y = 1` should also be rejected.

This is allowed in 3.9.1 and 3.10.0a2, but not allowed in 3.9.0 and 3.10.0a1. I'll work on finding out when this got messed up and fix it.

> I'll work on finding out when this got messed up and fix it.

Apparently is 

commit bca701403253379409dece03053dbd739c0bd059 (HEAD)
Author: Lysandros Nikolaou <lisandrosnik@gmail.com>
Date:   Tue Oct 27 00:42:04 2020 +0200

    bpo-42123: Run the parser two times and only enable invalid rules on the second run (GH-22111)


~/github/python/master v3.10.0a2~61 9s
❯ ./python -c "(*x), y = 1,2"

~/github/python/master v3.10.0a2~61
❯ git checkout HEAD^
Previous HEAD position was bca7014032 bpo-42123: Run the parser two times and only enable invalid rules on the second run (GH-22111)
HEAD is now at c8c4200b65 bpo-42157: Convert unicodedata.UCD to heap type (GH-22991)

~/github/python/master remotes/welikeparsers/master
❯ make -j -s
 CC='gcc -pthread' LDSHARED='gcc -pthread -shared    ' OPT='-g -Og -Wall'       _TCLTK_INCLUDES='' _TCLTK_LIBS=''       ./python -E ./setup.py -q build

The following modules found by detect_modules() in setup.py, have been
built by the Makefile instead, as configured by the Setup files:
_abc                  atexit                pwd
time


~/github/python/master remotes/welikeparsers/master
❯ ./python -c "(*x), y = 1,2"
  File "<string>", line 1
    (*x), y = 1,2
     ^
SyntaxError: can't use starred expression here

New changeset 2ea320dddd553298038bb7d6789e50e199332f66 by Lysandros Nikolaou in branch 'master':
bpo-40631: Disallow single parenthesized star target (GH-24027)
https://github.com/python/cpython/commit/2ea320dddd553298038bb7d6789e50e199332f66

New changeset 9a608ac17c284008d3c2986a4a8b194f84488e56 by Lysandros Nikolaou in branch '3.9':
[3.9] bpo-40631: Disallow single parenthesized star target (GH-24027) (GH-24068)
https://github.com/python/cpython/commit/9a608ac17c284008d3c2986a4a8b194f84488e56