[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:


New changeset 58fb156edda1a0e924a38bfed494bd06cb09c9a3 by Pablo Galindo in 
branch 'master':
bpo-42997: Improve error message for missing : before suites (GH-24292)
https://github.com/python/cpython/commit/58fb156edda1a0e924a38bfed494bd06cb09c9a3


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[issue42997] Improve error message for missing : before suites

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

> Yeah, but the cut is "absolute" (because of the syntax Error). IIRC the 
> normal cut only affects that rule, no?

Right.

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> It's basically a positive lookahead with a cut, but the extra feature that it 
> raises a SyntaxError, right?

Yeah, but the cut is "absolute" (because of the syntax Error). IIRC the normal 
cut only affects that rule, no?

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> See also issue1634034.

The problem with issue1634034 is that is not anymore possible without manual 
changes to the grammar with the PEG parser (or new infrastructure). The new 
parser does not have a defined concept of "i expected this token and now I am 
going to hard fail because is not there", but instead it will backtrack and try 
something else. We also did some work on something similar and we deemed it not 
useful as it was:

bugs.python.org/issue40599

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[issue42997] Improve error message for missing : before suites

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

I propose to go with the first approach. I really like that and I can see us 
using it in various place in the future. It's basically a positive lookahead 
with a cut, but the extra feature that it raises a SyntaxError, right?

I'll have a look at the PR now.

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[issue42997] Improve error message for missing : before suites

[Andre Roberge]

Change by Andre Roberge :


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nosy: +aroberge

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[issue42997] Improve error message for missing : before suites

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

See also issue1634034.

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nosy: +serhiy.storchaka

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

PR 24293 implements the second idea (only new rules and no new machinery)

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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pull_requests: +23115
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/24293

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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nosy: +gvanrossum

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

There are several ways to implement this:

In PR24292 I implemented this idea by adding a new element to the grammar: 
'&&'. This allows to hard-expect a token: if the token is not there the parsing 
hard-fails immediately without trying anything else and displays an appropriate 
error message

The other possibility if we think that PR24292 is overkill is manually adding 
an "invalid_wathever" for every possible compound statement option that parses 
the statement with a !':' and then raises appropriately. This option is more 
verbose in the grammar but requires no new machinery.

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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keywords: +patch
pull_requests: +23113
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/24292

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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nosy: +lys.nikolaou

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[issue42997] Improve error message for missing : before suites

[Pablo Galindo Salgado]

New submission from Pablo Galindo Salgado :

Instead of displaying a generic syntax error:

Python 3.8.6 (default, Oct 10 2020, 18:31:21)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> for x in range
  File "", line 1
for x in range
 ^
SyntaxError: invalid syntax
>>>

we could display:

>>> for x in range
  File "", line 1
for x in range
  ^
SyntaxError: expected ':'

The same idea applies for every suite that has a missing ':'

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messages: 385463
nosy: pablogsal
priority: normal
severity: normal
status: open
title: Improve error message for missing : before suites
versions: Python 3.10

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