Syntax errors in else blocks are reported at else

[encukou]

Since #29513, syntax errors in else and elif blocks, like:

if 1:
    pass
else:
    This is invalid syntax (sic)

are reported at the else:

  File "/tmp/repro.py", line 3
    else:
    ^^^^^^
SyntaxError: 'else' must match a valid statement here

This is quite unhelpful when a small typo is hiding in a large block.

@lysnikolaou, could you take a look?

Linked PRs

• gh-119724: Revert "bpo-45759: Better error messages for non-matching 'elif'/'else' statements (#29513)" #119974
• [3.13] gh-119724: Revert "bpo-45759: Better error messages for non-matching 'elif'/'else' statements (GH-29513)" (GH-119974) #120013

[encukou]

@lysnikolaou Do you think this can be solved soon?

Beta 2 is coming up; if this can't be fixed quickly it might be better to revert #29513 and try again.

Marking as a potential release blocker.

[lysnikolaou]

I gave this a go yesterday, but I couldn't come up with something that both fixes this and keeps the current behavior when there's an unmatched else/elif. I'll give another try later and report back.

[erlend-aasland]

Would you consider this an improvement?

Python 3.14.0a0 (heads/autoconf/curses-dirty:739af749138, Jun  3 2024, 13:59:59) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> if 1:
...     pass
... else:
...     This is invalid syntax (sic)
  File "<unknown>", line 4
    This is invalid syntax (sic)
                                ^
SyntaxError: 'else' must match a valid statement here
>>> 

[encukou]

Hard to say with that example. Here is a less minimal one; IMO the error should be “near” the bad line:

if 1:
    pass
else:
    while True:
        do_necessary_things()
        if should_end:
            retrun True  # sic
        do_correct_things()
        do_good_things()

[erlend-aasland]

In this case, yes it shows the offending line, but not the exact column.

[pablogsal]

Yeah, I think the correct thing to do here is reverting

[lysnikolaou]

I couldn't make it work well. Let's revert and reevaluate.

[pablogsal]

yeah, the ratio benefit/complexity it's quite important for these error messages and I prefer to have something dummer than showing incorrect things to the user or having to maintain some gigantic rule so it's fine to just not do it