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

28 Jul 2020

      There's been a discussion of the "for ... else ..." construction in Python.
Here's a suggestion as to how we might improve matters, and perhaps
usefully extend the language. I hope it might benefit some, without harming
anyone else.

Aside: I've not read the whole of the previous discussion, and it may be
that some or all of what I say has already been expressed there. So
apologies for any missing credits to others. The errors, I'm responsible
for.

Here's what happens at present.
    for i in items:
        LOOP_BODY
    else:
        print('exit via StopIteration')

I find the following clearer (where 'if break' is a new language 'compound
keyword').
    for i in items:
        LOOP_BODY
    if break:
        pass
    else:
        print('exit via StopIteration')

This would now allow us to write
    for i in items:
        LOOP_BODY
    if break:
        print('exit via break')
    else:
        print('exit via StopIteration')

Sometimes a loop has multiple break commands, which have different
purposes. So a further extension might be
    for i in items:
        LOOP_BODY
    if break left:
        print('exit via break left')
    elif break right:
        print('exit via break right')
    elif break:
        print('exit via some other break')
    else:
        print('exit via StopIteration')

To allow this to work, we extend the language to allow commands such as
            break left
            break right
as well as the usual
            break

Here the identifiers 'left' and 'right' look like variable names, but in
fact they are labels. The compiler can, and I think should, produce a
hard-coded jump from command
        break right
to the code block under
        elif break right:

This is the basic idea, keeping the syntax close to Python as it now is
(but of course adding to the semantics). Some programmers, and I think I'm
one, might want a clearer distinction between
    for i in items:
        LOOP_BODY
    if break:
        do_something
and
    for i in items:
        LOOP_BODY
    if broke:
        do_something

For this reason I tentatively suggest instead
    for i in items:
        LOOP_BODY
    case:
        break left:
            print('exit via break left')
        break right:
            print('exit via break right')
        break:
            print('exit via some other break')
        else:
            print('exit via StopIteration')

Some people have asked a built-in means to detect, at the end of the
iteration, whether theitems was empty to begin with. The syntax could
easily provide this, for example via
     case:
        zero:
            print('no items to process')

However, if we adopt this perhaps we'd also like to retain the simpler for
... if break ... else ... construct.

I hope this helps.

-- 
Jonathan

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

Jonathan Fine