[Python-ideas] Loop labels

[David Townshend]

The biggest problem is not the new syntax.  It's the new type of object
that would be needed, Label, which lies in some magical place half way
between a name and a keyword. What would be the result of the following
code?

loops = []
for i in range(4) as label:
    print(type(label), dir(label))
    loops.append(label)
for label in loops as newlabel:
    break label

David
On Mar 9, 2012 1:08 AM, "Westley Martínez" <anikom15 at gmail.com> wrote:

> On Fri, Mar 09, 2012 at 02:52:08AM +0800, Matt Joiner wrote:
> > Several languages allow break and continue statements to take a count
> > or loop label for greater flexibility.
> >
> > Dealing with nested loops and control flow around them is definitely
> > something I and probably most programmers deal with everyday.
> > Generally for very complex control flow one might employ functions,
> > and use return statements to work around any shortcomings. This is not
> > always ideal in CPython because of the performance cost of function
> > calls, and lack of anonymous functions. The other work around is
> > usually goto statements, which clearly aren't available or appropriate
> > in Python.
> >
> > So what about the following extensions?
> >
> > Allow for and while statements to be labelled using "as".
> >
> > Allow break and continue to take the name of a containing loop, or an
> > integer. The corresponding named loop, or the nth containing loop are
> > treated as though they are the nearest enclosing loop.
> >
> > loop_label ::=  identifier
> >
> > break_stmt ::=  "break" [decimalinteger | loop_label]
> >
> > continue_stmt ::=  "continue" [decimalinteger | loop_label]
> >
> > while_stmt ::=  "while" expression ["as" loop_label] ":" suite
> >                 ["else" ":" suite]
> >
> > for_stmt ::=  "for" target_list "in" expression_list ["as" loop_label]
> ":" suite
> >               ["else" ":" suite]
> >
> > Here's a really naive example:
> > for a in b as c:
> >     for d in e:
> >         break c # or break 2
> >     for f in g:
> >         continue c # or continue 2
>
> -1; if we do this we might as well add goto labels.
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