[Python-ideas] Break multiple loop levels

[Terry Reedy]

On 5/11/2019 1:20 PM, haael wrote:
> 
> Python allows for breaking out from a loop through 'break' and 
> 'continue' keywords. It would be nice if it was possible to break many 
> loop levels using one command.
> 
> I propose two constructions for that:
> 
> break
> break break
> break break break
> ...
> 
> continue
> break continue
> break break continue
> ...
> 
> And so on.
> 
> Example:
> 
> for i in range(10):
>      for j in range(10):
>          if i == 2 and j == 3:
>              break break
> 
> for i in range(10):
>      for j in range(10):
>          if i == 2 and j == 3:
>              break continue
>          if i == 7:
>              break break

> Breaking out from many loops at once is a common practice, currently 
> implemented through boolean testing and exceptions.

Python exceptions generalize both 'return' and 'break' as methods for 
escaping nested contexts and are intended for generalized flow control.

> This proposal would make it cleaner.

I actually think that the general method is 'cleaner', be separating 
'leave here' and 'arrive here'.  This proposal would mean typing fewer 
words in the special case where there are nested loops within one function.

The cost is a) more syntax to learn and b) tightly tying together the 
two loops more tightly than might be necessary, and thereby inhibiting 
re-factoring.  In the examples above, where the loops are inherently 
tied together, the two loops can, as noted by others, be reduced to one. 
  If they are not inherently tied together, one might want to reduce the 
inner loop to a comprehension or wrap it in a new function so it can 
also be used elsewhere.  If the inner loop raises, wrapping or 
unwrapping it, as needed for reusability or efficiency, does not affect 
the logic.

-- 
Terry Jan Reedy