Improper use of the keyword-only syntax makes the parser crash

[avassalotti]

BPO	1573
Nosy	@amauryfa, @tiran, @avassalotti
Files	kwonly.patch kwonly2.patch

^{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 2007-12-09.21:50:46.338>
created_at = <Date 2007-12-08.18:13:57.020>
labels = ['interpreter-core', 'type-crash']
title = 'Improper use of the keyword-only syntax makes the parser crash'
updated_at = <Date 2008-01-06.22:29:44.524>
user = 'https://github.com/avassalotti'

bugs.python.org fields:

activity = <Date 2008-01-06.22:29:44.524>
actor = 'admin'
assignee = 'none'
closed = True
closed_date = <Date 2007-12-09.21:50:46.338>
closer = 'amaury.forgeotdarc'
components = ['Interpreter Core']
creation = <Date 2007-12-08.18:13:57.020>
creator = 'alexandre.vassalotti'
dependencies = []
files = ['8894', '8895']
hgrepos = []
issue_num = 1573
keywords = []
message_count = 12.0
messages = ['58299', '58300', '58301', '58302', '58303', '58304', '58306', '58308', '58309', '58310', '58319', '58330']
nosy_count = 3.0
nosy_names = ['amaury.forgeotdarc', 'christian.heimes', 'alexandre.vassalotti']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue1573'
versions = ['Python 3.0']

[avassalotti]

I found that the parser fails to handle correctly the (incorrect) case
where the single-star (*), used for delimiting keyword-only arguments,
is immediately followed by a **keywords parameter:

>>> def f(*, **kw):
...   pass
... 
python: Python/ast.c:652: handle_keywordonly_args: Assertion `kwonlyargs
!= ((void *)0)' failed.
[1]    7872 abort (core dumped)  ./python

[tiran]

Fixed in r59432

I've altered the assert(). It now checks if either kwonlyargs and
kwdefault or both not NULL or the next node is a DOUBLESTAR.

[amauryfa]

Err... I think it should raise a SyntaxError in this case.
See my attached patch.

[tiran]

Why do you want to forbid

def f(*, **kw)

? It's useful and it also works in release builds of Python 3.0a2. It
only breaks in debug builds because the assert() gets triggered.

[avassalotti]

Amaury is right. "def f(*, **kw): pass" should raise a SyntaxError. The
keyword-only delimiter is useless since the **kw parameter already only
accepts keywords.

[amauryfa]

Why do you want to forbid
def f(*, **kw)

Well, TOOWTDI and the like...
and the first time I saw it, it seemed that any number of parameters is
allowed!

[tiran]

Ah, you and Amaury are right! But I don't like Amaury's error message:

SyntaxError: no name for vararg

It doesn't explain what's wrong. How about

SyntaxError: keyword only arguments require at least one keyword

[tiran]

Kirk McDonald has an even better error message for us:

SyntaxError: Cannot specify keyword only arguments without named arguments

[tiran]

Here is another error message from Thomas Wouters 'named arguments must
follow bare *'

[amauryfa]

Right.
We should also replace the other occurence of "no name for vararg".
Here is another patch, against the current revision (59434).

[tiran]

I'm fine with your patch. Can you commit it please?

[amauryfa]

Committed revision 59443.