Special syntax error for `elif` after `else`

[swfarnsworth]

Feature or enhancement

Proposal:

I've implemented a special syntax error in the case that an elif follows an else. See here.

if i % 3 == 0:
    print("divisible by 3")
else:
    print("not divisible by 3")
elif i % 2 == 0:
    print("divisible by 2")
# SyntaxError: elif not allowed after else

This is currently the extent of the new behavior. If possible, I would be interested to implement behavior like this:

if isinstance(i, int):
    if i % 3 == 0:
        print("divisible by 3")
    else:
        print("not divisible by 3")
    elif isinstance(i, str):
        print("i is a string")
# SyntaxError: elif not allowed after else. Perhaps the elif is at the wrong indentation level?

This would be even more informative for beginners, but I'm not sure if the parser supports that level of state. In particular, we wouldn't want the latter syntax error (the one that suggests that the indentation level is wrong) to be raised if the invalid elif were within a for block.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-129858: Special syntax error for elif block after else #129902

[sobolevn]

I like the new error message! But, looks like Perhaps the elif is at the wrong indentation level? part would be really hard to generate, if even possible.

Please, send a PR with the first message for now :)

[StanFromIreland]

if even possible.

The first part was merged, is it time to close this?

[swfarnsworth]

I just spoke to @pablogsal about the stateful behaviour, and he confirmed that it's not possible; the behavior that is possible is already merged, so we can close this.