[Python-ideas] for/else syntax

[Stephen J. Turnbull]

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!

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.