SyntaxError: trailing comma not allowed ... misleading

[aroberge]

BPO	44947
Nosy	@aroberge, @ambv, @lysnikolaou, @pablogsal, @miss-islington
PRs	bpo-44947: Refine the syntax error for trailing commas in import statements #27814 [3.10] bpo-44947: Refine the syntax error for trailing commas in import statements (GH-27814) #27816 [3.9] bpo-44947: Refine the syntax error for trailing commas in import statements (GH-27814) #27817

^{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 2021-08-18.21:04:56.153>
created_at = <Date 2021-08-18.11:48:58.213>
labels = ['interpreter-core', '3.9', '3.10']
title = 'SyntaxError: trailing comma not allowed ... misleading'
updated_at = <Date 2021-08-18.21:04:56.153>
user = 'https://github.com/aroberge'

bugs.python.org fields:

activity = <Date 2021-08-18.21:04:56.153>
actor = 'lukasz.langa'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-18.21:04:56.153>
closer = 'lukasz.langa'
components = ['Parser']
creation = <Date 2021-08-18.11:48:58.213>
creator = 'aroberge'
dependencies = []
files = []
hgrepos = []
issue_num = 44947
keywords = ['patch']
message_count = 8.0
messages = ['399837', '399839', '399843', '399859', '399860', '399867', '399868', '399871']
nosy_count = 5.0
nosy_names = ['aroberge', 'lukasz.langa', 'lys.nikolaou', 'pablogsal', 'miss-islington']
pr_nums = ['27814', '27816', '27817']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue44947'
versions = ['Python 3.9', 'Python 3.10']

[aroberge]

Consider the following four slightly different examples:

Python 3.10.0rc1 ...

>>> from math import sin and cos
  File "<stdin>", line 1
    from math import sin and cos
                         ^^^
SyntaxError: invalid syntax


>>> from math import sin, cos, and tan
  File "<stdin>", line 1
    from math import sin, cos, and tan
                               ^^^
SyntaxError: trailing comma not allowed without surrounding parentheses


>>> from math import (sin, cos,) and tan
  File "<stdin>", line 1
    from math import (sin, cos,) and tan
                                 ^^^
SyntaxError: invalid syntax


>>> from math import sin, cos and tan
  File "<stdin>", line 1
    from math import sin, cos and tan
                              ^^^
SyntaxError: invalid syntax

====
In all four cases, the keyword 'and' is correctly identified as causing the error. In the second case, the message given may suggest that adding parentheses is all that is needed to correct the problem; however, that is "obviously" not the case as shown in the third case.

**Perhaps** when a _keyword_ like 'and' is identified as a problem, a generally better message would be something like

SyntaxError: the keyword 'and' is not allowed here

leaving out all guesses like 'surrounding by parentheses', "meaning == instead of =", 'perhaps forgot a comma', etc., which are sometimes added by Python 3.10+ ?

I am fully and painfully aware that attempting to provide helpful and accurate error message is challenging...

[aroberge]

This message is not new to Python 3.10 as it is also shown with Python 3.9.5.

>>> from math import sin, cos, and tan
  File "<stdin>", line 1
    from math import sin, cos, and tan
                               ^
SyntaxError: trailing comma not allowed without surrounding parentheses

[pablogsal]

Hummm, interesting.

I wonder if this only happens with keywords or if this can be reproduced with other constructs

[aroberge]

Based on what I just read on https://github.com/davidhalter/parso/blob/master/parso/python/issue_list.txt
I gather that this exception is only raised in the context of an import statement with a trailing comma (usually followed by nothing).

[pablogsal]

The problem is not the keyword, is that we are parsing correctly until the "and" and we find the comma there. It happens with anything that invalidates more items:

>>> from math import sin, cos, $ tan
  File "<stdin>", line 1
    from math import sin, cos, $ tan
                               ^
SyntaxError: trailing comma not allowed without surrounding parentheses

[ambv]

New changeset b2f68b1 by Pablo Galindo Salgado in branch 'main':
bpo-44947: Refine the syntax error for trailing commas in import statements (GH-27814)
b2f68b1

[miss-islington]

New changeset 846a10f by Miss Islington (bot) in branch '3.10':
bpo-44947: Refine the syntax error for trailing commas in import statements (GH-27814)
846a10f

[ambv]

New changeset 4e4d35d by Łukasz Langa in branch '3.9':
[3.9] bpo-44947: Refine the syntax error for trailing commas in import statements (GH-27814) (GH-27817)
4e4d35d