GeezerVison is showing github.com — Invalid mapping patterns give confusing SyntaxErrors · Issue #88534 · python/cpython
Invalid mapping patterns give confusing SyntaxErrors #88534Labels 3.11only security fixesonly security fixesinterpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error Description
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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'] Activitybrandtbucher commented on 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 commented on Jun 9, 2021 Perhaps we need something like invalid_mapping_pattern or invalid_mapping_pattern_double_star rules? pablogsal commented on 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 commented on Jun 9, 2021 The backtracking with the soft keyword may make this very annoying, by the way. pablogsal commented on 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 commented on 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 commented on Jun 9, 2021 Oh, turns out I already added machinery to solved this but I was missing a piece! brandtbucher commented on Jun 9, 2021 Wow, that was quite a roller-coaster ride. Thanks Pablo. :) brandtbucher commented on Jun 9, 2021 I found a similar one, by the way (not related to mapping patterns):
>>> match ...:
... case 42 as _:
... pass
File "<stdin>", line 2
case 42 as _:
^
SyntaxError: expected ':'Is this covered by your fix? pablogsal commented on Jun 9, 2021
Is this covered by your fix?
No, that is not a backtracking error (is not going all the way to the "match" expression). pablogsal commented on Jun 9, 2021 I will fix that one in another PR pablogsal commented on 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 commented on 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 commented on 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 commented on Jun 9, 2021 Like "SyntaxError: can't capture into a wildcard (consider removing)". miss-islington commented on 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 commented on 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.
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 commented on Jun 9, 2021 Checkout PR 26632, and see if this works for you pablogsal commented on Jun 10, 2021 pablogsal commented on Jun 18, 2021 furkanonder commented on May 9, 2023
MetadataMetadata3.11only security fixesonly security fixesinterpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error DevelopmentNo branches or pull requestsIssue actions
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