[Python-ideas] for/else syntax

[Michael Foord]

2009/10/2 Stephen J. Turnbull <stephen at xemacs.org>

> Yuvgoog Greenle writes:
>
>  > 1. Not allowing a for loop that has no "break" to have an "else". Just
> like
>  > "else" isn't allowed when there isn't an "except" in the "try". There
> really
>  > is no excuse, justification or sense in a "for" loop that has no "break"
> yet
>  > has an "else".
>
> +1.  File an RFE against PyLint for sure, and against Python too.
> That's a *great* idea!
>

Nonsense - there are several other ways to break out of a loop. Raising an
exception or returning for example.

Michael


>
> Bikeshedding follows.
>
>  > Antti's experiment got me asking, you can try this as well. Ask a person
> who
>  > knows python the following questions:
>  > Question #1: Do you know what happens when an "else" appears after a
> "for"
>  > loop?
>  >     if they answer yes then say nevermind.
>
> Er, no.  It's more important if what people "know" is wrong than if
> naive guesses are wrong.
>
>  >     otherwise: continue to question #2.
>  > Question #2: When would the code in the "else" block execute? If you
> don't
>  > know, it's ok to guess.
>
> I would fire a programmer who gave an answer.<0.5 wink>  He might do
> the same in his own code or in a review of someone else's.
>
>  > 3. Renaming this construct for py4k wouldn't be too crazy if you use the
>  > existing reserved words "not" and "break". Look at this code and please
> note
>  > that it doesn't need any comments to make it readable (as opposed to
> Nick's
>  > example, no offence):
>
> Nick's example is far more readable IMO.  No offense.  I can tell you
> why, too.  In his example, the "else" is immediately preceded by the
> "if ... break" suite.  Because "break" can only happen once in a loop,
> I read the else as the alternative to breaking out.  Sloppy, yes, but
> since this is the common idiom using for-else, it works for me. :-)
>
> OTOH, "break" is an imperative; "not break" looks like a prohibition
> to me, not the target of a branch.  You could use "for: ... if not
> break:" but that looks ugly to me, and worse requires several tokens
> of lookahead to disambiguate from an ordinary if block.
>
> I'll leave it up to GvR to decide whether for-else is "less Pythonic"
> than try-except-else.
>
>  > And I can even think of a few use cases for having just "break:". This
> block
>  > would execute only upon "break" occurring. That way you could have loops
>  > like this:
>  >
>  > for item in list_of_stuff:
>  >     if item.is_awesome():
>  >         break
>  > break:
>  >     celebrate(list_of_stuff)
>
> You'd need multiple breaks for this to make sense.  Else:
>
>    for item in list_of_stuff:
>        if item.is_awesome():
>             celebrate(list_of_stuff)
>            break
>
> Some people think that having multiple breaks is bad style.
>
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