[New-bugs-announce] [issue39031] Inconsistent lineno and col_offset info when parsing elif

[Lysandros Nikolaou]

New submission from Lysandros Nikolaou <lisandrosnik at gmail.com>:

While working on pegen, we came across an inconsistency on how line number and column offset info is stored for (el)if nodes. When parsing a very simple if-elif construct like

if a:
    pass
elif b:
    pass

the following parse tree gets generated:

Module(
    body=[
        If(
            test=Name(id="a", ctx=Load(), lineno=1, col_offset=3, end_lineno=1, end_col_offset=4),
            body=[Pass(lineno=2, col_offset=4, end_lineno=2, end_col_offset=8)],
            orelse=[
                If(
                    test=Name(
                        id="b", ctx=Load(), lineno=3, col_offset=5, end_lineno=3, end_col_offset=6
                    ),
                    body=[Pass(lineno=4, col_offset=4, end_lineno=4, end_col_offset=8)],
                    orelse=[],
                    lineno=3,
                    col_offset=5,
                    end_lineno=4,
                    end_col_offset=8,
                )
            ],
            lineno=1,
            col_offset=0,
            end_lineno=4,
            end_col_offset=8,
        )
    ],
    type_ignores=[],
)

There is the inconsistency that the column offset for the if statement is 0, thus the if statement starts with the keyword if, whereas the column offset for elif if 5, which means that the elif keyword is skipped.

As Guido suggests over at https://github.com/gvanrossum/pegen/issues/107#issuecomment-565135047 we could very easily change Python/ast.c so that the elif statement start with the elif keyword as well.

I have a PR ready!

----------
messages: 358304
nosy: lys.nikolaou
priority: normal
severity: normal
status: open
title: Inconsistent lineno and col_offset info when parsing elif
type: behavior
versions: Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39031>
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