[New-bugs-announce] [issue34372] Compiler could output more accurate line numbers

[Arusekk]

New submission from Arusekk <arek_koz at o2.pl>:

If this is a duplicate, please excuse me.

In particular, the most noticeable inaccuracy happens when the postfix if-else expression is involved. Maybe there are more of them.
The problem is quite self-explaining. The module named 'dis' will be helpful to reproduce the issue.

>>> import dis
>>> code = """(
... [
...     call1(),
...     call2()
... ]
... + call3()
... * call4()
... )"""
>>> dis.dis(code)
  3           0 LOAD_NAME                0 (call1)
              3 CALL_FUNCTION            0 (0 positional, 0 keyword pair)

  4           6 LOAD_NAME                1 (call2)
              9 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             12 BUILD_LIST               2

  6          15 LOAD_NAME                2 (call3)
             18 CALL_FUNCTION            0 (0 positional, 0 keyword pair)

  7          21 LOAD_NAME                3 (call4)
             24 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             27 BINARY_MULTIPLY
             28 BINARY_ADD
             29 RETURN_VALUE
>>> dis.dis(code.replace("+", "if").replace("*", "else"))
  6           0 LOAD_NAME                0 (call3)
              3 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
              6 POP_JUMP_IF_FALSE       25
              9 LOAD_NAME                1 (call1)
             12 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             15 LOAD_NAME                2 (call2)
             18 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             21 BUILD_LIST               2
             24 RETURN_VALUE

  7     >>   25 LOAD_NAME                3 (call4)
             28 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             31 RETURN_VALUE

I used this code to show the difference between if-else and some arithmetics.

AFAICT the feature is possible to implement, as lnotab can contain negative line differences.

I don't know whether it is just a bug or a fully intended feature, but it would be quite an enhancement to have better line number tracking, useful for debugging.

If this is implemented, it may be worth further backporting.

Possible reasons in the upstream Python/compile.c (using < instead of !=):

https://github.com/python/cpython/blob/077059e0f086cf8c8b7fb9d1f053e38ddc743f59/Python/compile.c#L4092

https://github.com/python/cpython/blob/077059e0f086cf8c8b7fb9d1f053e38ddc743f59/Python/compile.c#L4438

----------
components: Interpreter Core
messages: 323371
nosy: Arusekk
priority: normal
severity: normal
status: open
title: Compiler could output more accurate line numbers
type: behavior
versions: Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34372>
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