Misleading error location reported by ellipsis preparser

[user202729]

Steps To Reproduce

for a in [0..2]:
	for b in 0..2:
		pass

Expected Behavior

The error is reported on the second line.

Actual Behavior

  Cell In[118], line 1
    for a in [Integer(0)..Integer(2)]:
                         ^
SyntaxError: invalid syntax

Additional Information

No response

Environment

• OS: Linux
• Sage Version: 10.4

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[DaveWitteMorris]

Thanks for the bug report. This error message is not actually coming from the preparser. The ellipsis preparser does raise an exception:

SyntaxError: unbalanced or missing delimiters

However, this exception is ignored (lines 1784 to 1787 of src/sage/repl/preparse.py):

try:
    L = parse_ellipsis(L, preparse_step=False)
except SyntaxError:
    pass

Because it raises an error instead of returning a value, the parse_ellipsis method doesn't end up doing anything, so the original (un-preparsed) code is passed to python. The first .. in the code hasn't been preparsed (it is still[Integer(0)..Integer(2)] instead of (ellipsis_range(Integer(1),Ellipsis,Integer(2))), so this is not valid python. This results in the SyntaxError that we see when the code is executed.

There doesn't seem to be much point in having parse_ellipsis raise an exception that is going to be ignored, so perhaps a way to fix the problem would be to add a boolean keyword argument to parse_ellipsis telling it not to raise a SyntaxError, and instead return a string after doing as much preparsing as possible. In the given example, this should mean that the syntax error comes up in the right place.

However, I don't plan to work on this (not soon, at least).