[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[Guido van Rossum]

Change by Guido van Rossum :


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

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[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[miss-islington]

miss-islington  added the comment:


New changeset 15bc9ab301d73f20bff47a12ef05326feb40f797 by Guido van Rossum in 
branch 'master':
bpo-40612: Fix SyntaxError edge cases in traceback formatting (GH-20072)
https://github.com/python/cpython/commit/15bc9ab301d73f20bff47a12ef05326feb40f797


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nosy: +miss-islington

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[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

Understood.

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[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[Guido van Rossum]

Guido van Rossum  added the comment:

(And, to be clear, I don't *want* to strip trailing spaces.)

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[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[Guido van Rossum]

Guido van Rossum  added the comment:

My current PR does not strip trailing whitespace. It only strips a single 
trailing newline (since this is usually but not always present, and we don't 
want to its presence to cause an extra blank line, nor do we want its absence 
to cause the text line and the caret line to be run together).

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[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

Agreed on everything.

One thing I don't really understand is if you propose to also strip trailing 
whitespace. Does "limit the offset to just past the end of the source text" 
include whitespace or not?

For example, the linked PR does not change this behavior:

➜  cpython git:(pr/20072) ./python
Python 3.9.0a6+ (heads/pr/20072:6df7662ca5, May 14 2020, 20:37:50) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> compile('1 +  ', '', 'exec')
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 1
1 +  
 ^
SyntaxError: invalid syntax

Should we clip just past the end of `1 +` here?

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[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[Guido van Rossum]

Change by Guido van Rossum :


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keywords: +patch
pull_requests: +19378
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/20072

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[issue40612] Make traceback module's formatting of SyntaxError more similar to system formatting

[Guido van Rossum]

New submission from Guido van Rossum :

The traceback module formats several edge cases of SyntaxError different than 
CPython's default formatting.

- There's an off-by-one error if the column offset (printed as a caret) points 
past the last character (example: 'a +='). The clipping is wrong and the caret 
points to the last character.

- If the offset is <= 0, it appears the code silently adds the length of the 
source text.

- The system formatting suppresses the caret if the offset is -1; the only way 
to suppress the caret with the traceback module is setting the offset to None 
(or setting the source text to None).

- The system formatting can position the caret way past the end of the source 
text; the traceback module clips (also see the first bullet).

I propose to make the traceback module behave the same way as the system module 
in all cases. I also propose to make both suppress the caret if the offset is 
<= 0. Finally I propose to make the system formatting limit the offset to just 
past the end of the source text.

I propose not to bother changing anything in 3.8 or before.

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messages: 368759
nosy: gvanrossum, lys.nikolaou, pablogsal
priority: normal
severity: normal
status: open
title: Make traceback module's formatting of SyntaxError more similar to system 
formatting
versions: Python 3.9

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