[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[miss-islington]

miss-islington  added the comment:


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


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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

Change by Lysandros Nikolaou :


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pull_requests: +22901
pull_request: https://github.com/python/cpython/pull/24068

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:


New changeset 2ea320553298038bb7d6789e50e199332f66 by Lysandros Nikolaou in 
branch 'master':
bpo-40631: Disallow single parenthesized star target (GH-24027)
https://github.com/python/cpython/commit/2ea320553298038bb7d6789e50e199332f66


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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

Change by Lysandros Nikolaou :


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pull_requests: +22867
pull_request: https://github.com/python/cpython/pull/24027

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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pull_requests: +22859
pull_request: https://github.com/python/cpython/pull/24019

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

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

Apparently is 

commit bca701403253379409dece03053dbd739c0bd059 (HEAD)
Author: Lysandros Nikolaou 
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  atexitpwd
time


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

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

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.

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Guido van Rossum]

Guido van Rossum  added the comment:

> 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.

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Saiyang Gou]

Saiyang Gou  added the comment:

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

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

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

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

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.

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Guido van Rossum]

Guido van Rossum  added the comment:

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.

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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pull_requests: +22855
pull_request: https://github.com/python/cpython/pull/24015

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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keywords: +patch
pull_requests: +22854
stage: resolved -> patch review
pull_request: https://github.com/python/cpython/pull/24014

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


--
resolution: not a bug -> 
status: closed -> open

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Saiyang Gou]

Saiyang Gou  added the comment:

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 "", 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 "", line 1
del *x
^
SyntaxError: cannot delete starred
>>> del (*x)
  File "", line 1
del (*x)
 ^
SyntaxError: can't use starred expression here

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

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nosy: +gousaiyang

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

Change by Lysandros Nikolaou :


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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

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 "", line 1
SyntaxError: can't use starred expression here

Sorry for the noise!

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[issue40631] PEG Parser: Cannot used starred expression in parenthesised expr

[Lysandros Nikolaou]

New submission from Lysandros Nikolaou :

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 "", line 1, in 
  File "/Users/lysnikolaou/Repositories/cpython/Lib/ast.py", line 50, in parse
return compile(source, filename, mode, flags,
  File "", 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=[])"

--
assignee: lys.nikolaou
components: Interpreter Core
messages: 368936
nosy: gvanrossum, lys.nikolaou, pablogsal
priority: normal
severity: normal
status: open
title: PEG Parser: Cannot used starred expression in parenthesised expr
versions: Python 3.9

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