Parameters and arguments parser syntax error improvments

[Andykl]

BPO	46838
Nosy	@terryjreedy, @aroberge, @lysnikolaou, @pablogsal, @Andykl
PRs	bpo-46838: Syntax error improvements for function definitions #31590

^{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 = None
created_at = <Date 2022-02-23.21:23:21.121>
labels = ['interpreter-core', 'type-feature', '3.11']
title = 'Parameters and arguments parser syntax error improvments'
updated_at = <Date 2022-03-22.11:39:09.698>
user = 'https://github.com/Andykl'

bugs.python.org fields:

activity = <Date 2022-03-22.11:39:09.698>
actor = 'pablogsal'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Parser']
creation = <Date 2022-02-23.21:23:21.121>
creator = 'Andy_kl'
dependencies = []
files = []
hgrepos = []
issue_num = 46838
keywords = ['patch']
message_count = 6.0
messages = ['413856', '413858', '413860', '414038', '414052', '415760']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'aroberge', 'lys.nikolaou', 'pablogsal', 'Andy_kl']
pr_nums = ['31590']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46838'
versions = ['Python 3.11']

Linked PRs

• gh-90994: Improve error messages upon call arguments syntax errors #96893

[Andykl]

I saw that pablogsal welcomed improvments to the parser's suggestions, so here are the messages for parameters and arguments lists I think should be written instead of the current generic "invalid syntax".

>>> def foo(*arg, *arg): pass
SyntaxError: * argument may appear only once

>>> def foo(**arg, **arg): pass
SyntaxError: ** argument may appear only once

>>> def foo(arg1, /, arg2, /, arg3): pass
SyntaxError: / may appear only once

>>> def foo(*args, /, arg): pass
SyntaxError: / must be ahead of *

>>> def foo(/, arg): pass
SyntaxError: at least one argument must precede /

>>> def foo(arg=): pass
SyntaxError: expected default value expression

>>> def foo(*args=None): pass
SyntaxError: * argument cannot have default value

>>> def foo(**kwargs=None): pass
SyntaxError: ** argument cannot have default value

>>> foo(*args=[0])
SyntaxError: cannot assign to iterable argument unpacking

>>> foo(**args={"a": None})
SyntaxError: cannot assign to keyword argument unpacking

>>> foo(arg=)
SyntaxError: expected argument value expression

[Andykl]

I also sometimes write def foo(pos_only, /*, kwarg): pass. Perhaps this can be special cased with
SyntaxError: expected comma between / and *

[pablogsal]

Thanks a lot for the suggestions! I will try to give these a go to see if we can get them implemented. Parameter parsing is a bit hairy so not sure how lucky we will be but all of them make sense!

[terryjreedy]

With two exceptions, nice suggestions if feasible.

>>> def foo(*args=None): pass
SyntaxError: * argument cannot have default value

>>> def foo(**kwargs=None): pass
SyntaxError: ** argument cannot have default value

Good.

>>> foo(*args=[0])
SyntaxError: cannot assign to iterable argument unpacking

>>> foo(**args={"a": None})
SyntaxError: cannot assign to keyword argument unpacking

Incomprehensible. It seems to me that these should have same message as first two; message should not depend on proposed default.

[Andykl]

@terry.reedy, I based that error message on current

>>> foo(**{}, *())
SyntaxError: iterable argument unpacking follows keyword argument unpacking

and

>>> foo(__debug__=True)
SyntaxError: cannot assign to __debug__

but the final error message could be anything if it explicitly says "what's wrong".

[pablogsal]

New changeset 7d810b6 by Pablo Galindo Salgado in branch 'main':
bpo-46838: Syntax error improvements for function definitions (GH-31590)
7d810b6