[Python-ideas] Break multiple loop levels

[Steven D'Aprano]

On Sun, May 12, 2019 at 11:16:21AM +0200, Oleg Broytman wrote:
> On Sun, May 12, 2019 at 01:36:28AM -0700, Elias Tarhini <eltrhn at gmail.com> wrote:
> > If I may propose `break n` as a replacement for the original message's
> > `break break ... break`, where n>0 is the number of contiguous loops to
> > break out of and `break 1` is synonymous with `break`. Seems easier on my
> > eyes/noggin than counting out the individual `break` statements.
> 
>    This is very much error-prone because on any refactoring (increasing
> or decreasing the number of loop levels) one must increase/decrease all
> numbers in internal loops.
> 
>    Labels are at least stable.

Indeed.

Go has labels for loops:

    Label:
        for { ...
              break Label
            }

as does Rust:

    'label: while condition:
           {
            ...
            break label
            }

We can't use the same syntax in Python, but we might write it like this:

    @label while condition:
        block
        break label

and similar with for loops:

    @outer for x in range(100):
        @inner for y in range(3):
            if x == y == 1: break  # like ``break inner``
            if x == y == 2: break outer
            print(x, y)
        # break inner jumps to here
    # break outer jumps to here


-- 
Steven