Signify that a SyntaxError comes from an fstring in the error message

[lysnikolaou]

BPO	41084
Nosy	@gvanrossum, @vstinner, @ericvsmith, @lysnikolaou, @pablogsal, @miss-islington
PRs	bpo-41084: Adjust message when an f-string expression causes a SyntaxError #21084 [3.9] bpo-41084: Adjust message when an f-string expression causes a SyntaxError (GH-21084) #21188 [3.9] bpo-41084: Fix test_fstring failure when using the old parser #21212

^{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 2020-06-29.13:03:43.826>
created_at = <Date 2020-06-22.23:53:42.299>
labels = ['interpreter-core', 'type-feature', '3.9', '3.10']
title = 'Signify that a SyntaxError comes from an fstring in the error message'
updated_at = <Date 2020-06-29.13:03:43.825>
user = 'https://github.com/lysnikolaou'

bugs.python.org fields:

activity = <Date 2020-06-29.13:03:43.825>
actor = 'lys.nikolaou'
assignee = 'none'
closed = True
closed_date = <Date 2020-06-29.13:03:43.826>
closer = 'lys.nikolaou'
components = ['Interpreter Core']
creation = <Date 2020-06-22.23:53:42.299>
creator = 'lys.nikolaou'
dependencies = []
files = []
hgrepos = []
issue_num = 41084
keywords = ['patch']
message_count = 11.0
messages = ['372134', '372136', '372150', '372422', '372471', '372472', '372477', '372478', '372562', '372570', '372572']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'vstinner', 'eric.smith', 'lys.nikolaou', 'pablogsal', 'miss-islington']
pr_nums = ['21084', '21188', '21212']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41084'
versions = ['Python 3.9', 'Python 3.10']

[lysnikolaou]

It's relatively easy to identify if a SyntaxError occurs when parsing an fstring expression or not. The idea is to slightly change the error message to start with "f-string: " when it does (in the same way in which SyntaxError messages are produced by the hand-written fstring parser). I have a working PR that produces the following:

$ cat a.py
f'{a $ b}'
$ ./python.exe a.py
  File "/Users/lysnikolaou/Repositories/cpython/a.py", line 1
    (a $ b)
       ^
SyntaxError: f-string: invalid syntax

Thoughts?

[ericvsmith]

I think it's a good idea.

My only reservation would be: if the parsing of f-strings is moved into the parser, would it be possible to maintain the error new messages?

[lysnikolaou]

My only reservation would be: if the parsing of f-strings is moved into the parser, would it be possible to maintain the error new messages?

Probably without a single change.

[pablogsal]

New changeset 2e0a920 by Lysandros Nikolaou in branch 'master':
bpo-41084: Adjust message when an f-string expression causes a SyntaxError (GH-21084)
2e0a920

[lysnikolaou]

Should we maybe backport this to 3.9 as well?

[pablogsal]

I would say yes unless someone sees an obvious impediment

[ericvsmith]

AFAIK, we don't guarantee the stability of error messages, so I think backporting is fine. It's all the better that this is just prefixing something to an existing error message.

[miss-islington]

New changeset cb0dc52 by Miss Islington (bot) in branch '3.9':
bpo-41084: Adjust message when an f-string expression causes a SyntaxError (GH-21084)
cb0dc52

[vstinner]

The change broke the old parser in Python 3.9, I reopen the issue:

AMD64 Arch Linux VintageParser 3.9:
https://buildbot.python.org/all/#/builders/765/builds/129

FAIL: test_format_specifier_expressions (test.test_fstring.TestCase) (str="f'{4:{/5}}'")
FAIL: test_invalid_syntax_error_message (test.test_fstring.TestCase)
FAIL: test_lambda (test.test_fstring.TestCase) (str="f'{lambda x:x}'")
FAIL: test_parens_in_expressions (test.test_fstring.TestCase) (str="f'{,}'")
FAIL: test_parens_in_expressions (test.test_fstring.TestCase) (str="f'{,}'")

[pablogsal]

New changeset 990efe0 by Lysandros Nikolaou in branch '3.9':
[3.9] bpo-41084: Fix test_fstring failure when using the old parser (GH-21212)
990efe0

[lysnikolaou]

Thanks, Victor!

The fix is merged, so I'm closing this again.