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Invalid mapping patterns give confusing SyntaxErrors #88534
Description
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 2021-06-09.19:47:35.477>
labels = ['interpreter-core', 'type-bug', '3.11']
title = 'Invalid mapping patterns give confusing SyntaxErrors'
updated_at = <Date 2021-06-18.21:16:02.181>
user = 'https://github.com/brandtbucher'bugs.python.org fields:
activity = <Date 2021-06-18.21:16:02.181>
actor = 'pablogsal'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Parser']
creation = <Date 2021-06-09.19:47:35.477>
creator = 'brandtbucher'
dependencies = []
files = []
hgrepos = []
issue_num = 44368
keywords = ['patch']
message_count = 20.0
messages = ['395453', '395454', '395458', '395460', '395461', '395462', '395467', '395469', '395470', '395473', '395477', '395478', '395479', '395481', '395482', '395485', '395486', '395487', '395584', '396092']
nosy_count = 4.0
nosy_names = ['lys.nikolaou', 'pablogsal', 'miss-islington', 'brandtbucher']
pr_nums = ['26630', '26631', '26632', '26792']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44368'
versions = ['Python 3.11']
Activity
brandtbucher commentedon Jun 9, 2021
Here are a few that I found. Not sure when they were introduced:
match ...:
case {**rest, "key": value}:
pass
match ...:
case {"first": first, **rest, "last": last}:
pass
match ...:
case {**_}:
pass
These all give the following error while parsing the second line:
File "<stdin>", line 1
match ...:
^^^^^^^^^
SyntaxError: invalid syntax. Perhaps you forgot a comma?
brandtbucher commentedon Jun 9, 2021
Perhaps we need something like invalid_mapping_pattern or invalid_mapping_pattern_double_star rules?
pablogsal commentedon Jun 9, 2021
Probably, otherwise is going to hurt because the syntax errors that you describe trigger when two names are place together, which in general is a missing comma.
The error message also doesn't say: "you are missing a comma" it says that the most typical reason is a missing comma, which is important to distinguish :)
pablogsal commentedon Jun 9, 2021
The backtracking with the soft keyword may make this very annoying, by the way.
pablogsal commentedon Jun 9, 2021
Oh, wait, I think I misunderstood the problem. The problem is that the parser is backtracking and identifying match as a name.
Indeed, the soft keyword is a pain :(
pablogsal commentedon Jun 9, 2021
I think it actually will be very useful to explain that these cases are invalid in the Syntax error, which will also solve this problem.
This on the other hand shows a bigger problem: any generic syntax error that happens inside "match" will probably end identifying the keyword as a name, even if we don't have specific errors, the parser will point to it and complain about "match".
pablogsal commentedon Jun 9, 2021
Oh, turns out I already added machinery to solved this but I was missing a piece!
brandtbucher commentedon Jun 9, 2021
Wow, that was quite a roller-coaster ride. Thanks Pablo. :)
brandtbucher commentedon Jun 9, 2021
I found a similar one, by the way (not related to mapping patterns):
Is this covered by your fix?
pablogsal commentedon Jun 9, 2021
No, that is not a backtracking error (is not going all the way to the "match" expression).
pablogsal commentedon Jun 9, 2021
I will fix that one in another PR
pablogsal commentedon Jun 9, 2021
New changeset 457ce60 by Pablo Galindo in branch 'main':
bpo-44368: Ensure we don't raise incorrect custom syntax errors with soft keywords (GH-26630)
457ce60
pablogsal commentedon Jun 9, 2021
Oh, this one is actually correct:
match ...:
case 42 as _:
pass
File "<stdin>", line 2
case 42 as _:
^
SyntaxError: expected ':'
That is literally expecting a ":" and that's the error. It has identified "case 42" correctly and it now expects a ":". Is just that the error marker is wrong because it has reached the "as and _" as part of the parsing so the error is there.
That is going to be a bit tricky to "fix"
brandtbucher commentedon Jun 9, 2021
Could we just try parsing "as _" and raise if so? That wouldn't conflict with any existing rules, and that way we could actually have a helpful error message.
brandtbucher commentedon Jun 9, 2021
Like "SyntaxError: can't capture into a wildcard (consider removing)".
miss-islington commentedon Jun 9, 2021
New changeset f807a4f by Miss Islington (bot) in branch '3.10':
bpo-44368: Ensure we don't raise incorrect custom syntax errors with soft keywords (GH-26630)
f807a4f
pablogsal commentedon Jun 9, 2021
No, the same happens with other targets such as:
File "/home/pablogsal/github/python/master/lel.py", line 2
case 42 as 1+1:
^
SyntaxError: expected ':'
pablogsal commentedon Jun 9, 2021
Checkout PR 26632, and see if this works for you
pablogsal commentedon Jun 10, 2021
New changeset 0507303 by Pablo Galindo in branch 'main':
bpo-44368: Improve syntax errors with invalid as pattern targets (GH-26632)
0507303
pablogsal commentedon Jun 18, 2021
New changeset a8c418d by Pablo Galindo in branch '3.10':
[3.10] bpo-44368: Improve syntax errors with invalid as pattern targets (GH-26632) (GH-26792)
a8c418d
furkanonder commentedon May 9, 2023
@pablogsal @brandtbucher The issue seems to be solved. We can close the issue.