[issue34683] Caret positioned wrong for SyntaxError reported by ast.c

[Guido van Rossum]

New submission from Guido van Rossum <guido at python.org>:

Input file with a subtle error: a number where an assignment target is required:

for 1 in []: pass

Run it, it gives a SyntaxError.  Note how the caret pointing to the incorrect token is position one to the left of where you'd expect it:

  File "s.py", line 1
    for 1 in []: pass
       ^
SyntaxError: can't assign to literal

For every syntax error I've seen that's produced by ast.c this seems to be the case -- the caret is always positioned 1 too far left.

I tried to understand how this is happening but my AST-fu is lacking. It seems this has been happening since ast.c started added column numbers -- in Python 2.7 there's no caret at all, but in 3.4 and later there's a caret and it has the same problem. (Also in 3.3; I don't have 3.2 or older 3.x lying around to test.)

----------
messages: 325366
nosy: gvanrossum
priority: low
severity: normal
status: open
title: Caret positioned wrong for SyntaxError reported by ast.c
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34683>
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