[Python-ideas] Re: Idea: Extend "for ... else ..." to allow "for ... if break ..." else

5 Aug 2020

      On 05/08/2020 15:29, Mathew Elman wrote:
...
Being able to break multiple loops and having "labelled" breaks would 
be achievable using `except`, i.e. adding `except` to the loop 
statements before `else` like this:
for elem in iterable:
        ...
        if should_break(elem):
            raise SomeException
    except SomeException as e:
        handle_break_behaviour(e)
    else:
        print("Did not break")
would be sugar for:
try:
        for elem in iterable:
            ...
            if should_break(elem):
                raise SomeException
    except SomeException as e:
        handle_break_behaviour(e)
    else:
        print("Did not break")
I (and others) have suggested this before and no one has said it's a 
/bad /option, it's just been ignored, despite seeming to be an 
intuitive way to accomplish `else` clarity, "labelled" breaks and 
breaking from multiple loops. Is there a reason that this suggestion 
is worse / no better than adding special break syntax?
It's certainly a reasonable option.  Indeed, I would consider using the 
second (currently legal) version in my own code if it seemed appropriate.
Some pros and cons that occur to me (I may be biased, YMMV):
Pros:
     (1) It can cleanly separate the handling of break-type exceptions 
and other exceptions, if needed.
     (2) It actually clarifies what the dreaded "else" means!
     (3) it allows you to group your "break" cases using exception 
subclasses (although this is probably OTT for most use cases).
Cons:
     (4) It's more work.  You have to decide what exceptions to use and 
(most likely) create them.
     (5) It adds the runtime overhead of setting up the "try" and 
possibly raising the exception.
     (6) Putting (implicitly) try...except around a possibly long 
for-suite feels bad somehow, even though it wouldn't catch other 
unwanted exceptions.
     (7) It does not catch the "zero iterations" case.

If, contra your suggestion, special syntax were to be added, it could 
use "except" instead of "if ... elif".
For example (this is just doodling, it's not a fully thought-out proposal):

     for x in range(10):
         ...
         if <something happens>:
             break "oops" # under the hood this is a compiled jump, not 
the raising of an exception
         if <something else happens>:
             break 42
     except "oops":
         <handle something>
     except 42:
         <handle something else>
     except break:
         # catches all "break"s not already explicitly caught
         pass
     else:
         print("Did not break")

Rob Cliffe