I noticed a difference between computation of column offset for SyntaxError and IndentationError (a subclass of SyntaxError).
It is slightly confusing.


def report(exc):
    print('lineno %s, offset %s' % (exc.lineno, exc.offset))
    raise

try:
    exec('except IOError:')
except Exception as e:
    report(e)

try:
    exec('    open(filepath)')
except Exception as e:
    report(e)


** OUTPUT **
lineno 1, offset 6
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "<stdin>", line 2, in <module>
  File "<string>", line 1
    except IOError:
         ^
SyntaxError: invalid syntax

lineno 1, offset 4
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "<stdin>", line 2, in <module>
  File "<string>", line 1
    open(filepath)
    ^
IndentationError: unexpected indent


** Analysis **

Case (1): offset is 6, and caret is below column 5 for SyntaxError
Case (2): offset is 4, and caret is below column 4 for IndentationError

Well, that would be because the invalid indent is detected when the 'o' is encountered, whereas the invalid syntax is detected when the 't' completes the keyword 'except'.

I don't think there is a bug here.

Perhaps Florent is pointing to the fact that for 'except X:', the syntax error is detected at the ' ' at offset 6 but the caret is backed up under the 't' at offset 5*, whereas it is not backed up to the space before '    open'. Which behavior do you propose to change?

Since these details of presentation are not documented, I do not consider the current difference a bug. I would not change them in current versions since change could confuse people or software. I suspect there is also danger of making some cases worse. For the same reasons, I would not change in future versions either without good offsetting reason. So I also think this should be closed.

On python-list, we tell people that the caret is a starting point for detecting the error, not an absolute pointer.

For syntax errors, Idle omits the traceback and highlights in red the presumed error. For the first, it marks the entire word 'except'. For the second, it marks the last whitespace before open. This is equivalent to backing up the caret.

* I suspect this backup is special to keywords. For 'except+', both systems also mark the keyword, not the '+'. Either the keyword is misused or is a regular name misspelled. In either case, someone thought it better to point to the keyword than the space or other character after. I agree. So did the Idle author who decided to highlight the keyword rather than the character after it.

I tried a few different lines that result in a syntax error and the position of the '^' always seems ok to me, so I'm not sure there's any bug here:

    foo:bar
       ^
    from import *
              ^
    try:+
        ^
    try+
       ^
    1?
     ^
    1+*1
      ^
    1+-*1
       ^
    [}]
     ^
    foo)
       ^

Hello again,

actually, the attached script demonstrates some issues with the computation of the offset.

In rare cases, the computed offset does not match the position of the caret '^' in the traceback. It makes it difficult to reuse exc.offset to display the exact position.

And this additional script (test_indenterror.py) demonstrates a different issue in the traceback module.

Indeed, there is an inconsistence between the traceback module and builtin exception print function. Behavior of traceback is intentional as noted in format_exception_only() (if this comment is not misleading).

The proposed patch deals with the case of the IndentationError in the traceback module.

Your diff file looks broken.

I'm not sure it's the traceback module should be fixed and not Python internals. Georg, what you say?

The previous patch fixes some cases where the caret was wrongly positioned.

However, there's probably stuff to fix in Python internals too.
See attached test cases (test_traceback_caret.py) which demonstrates other issues with non-ASCII chars or <tab> in the source code.

Patch updated with latest changes of trunk.

patch_indenterror_offset_v2.diff LGTM.

Florent, can you commit your patch?

New changeset 6d1372237607 by Florent Xicluna in branch '3.3':
Issue #17825: Cursor ^ is correctly positioned for SyntaxError and IndentationError.
http://hg.python.org/cpython/rev/6d1372237607

New changeset aeb204b8f6c4 by Florent Xicluna in branch 'default':
Issue #17825: Cursor ^ is correctly positioned for SyntaxError and IndentationError.
http://hg.python.org/cpython/rev/aeb204b8f6c4

New changeset 0041be34edbf by Florent Xicluna in branch '2.7':
Issue #17825: Cursor ^ is correctly positioned for SyntaxError and IndentationError.
http://hg.python.org/cpython/rev/0041be34edbf

Done.

The alignment error with non-ASCII chars should be fixed too, thanks to the work of Serhiy on issue #2382.