flow control and nested loops

[Terry Reedy]

kj wrote:
> 
> In Perl, one can label loops for finer flow control.  For example:
> 
> X: for my $x (@X) {
>   Y: for my $y (@Y) {
>     for my $z (@Z) {
>       next X if test1($x, $y, $z);
>       next Y if test2($x, $y, $z);
>       frobnicate($x, $y, $z);
>     }
>     glortz($x, $y); 
>   }
>   splat($x); 
> }
> 
> What's considered "best practice" in the Python world for this sort
> of situation?  The only approach I can think of requires setting
> up indicator variables that must be set and tested individually;
> e.g.
> 
> for x in X:
>     next_X = False
>     for y in Y:
>         next_Y = False
>         for z in Z:
>             if test1(x, y, z):
>                 next_X = True
>                 break
>             if test2(x, y, z):
>                 next_Y = True
>                 break
>             frobnicate(x, y, z)
>         if next_X:
>             break
>         if next_Y:
>             continue
>         glortz(x, y) 
>     if next_X:
>         continue
>     splat(x) 
> 
> Whereas I find the Perl version reasonably readable, the Python
> one I find nearly incomprehensible.  In fact, I'm not even sure
> that the Python version faithfully replicates what the Perl one is
> doing!
> 
> Is there a better approach?

1. Put inner loops in a function and return instead of break.

2. Put inner loops in
try:
   for..
     for
       if cond: raise Something
       # or do operation that raises exception if cond is true
except e:
   whatever

tjr