Re: [Python-ideas] for/except/else

March 1, 2017


      On 01.03.2017 12:56, Steven D'Aprano wrote:
...
On Wed, Mar 01, 2017 at 10:37:17AM +0100, Wolfgang Maier wrote:
...
Now here's the proposal: allow an except (or except break) clause to
follow for/while loops that will be executed if the loop was terminated
by a break statement.
Let me see if I understand the proposal in full. You would allow:
for i in (1, 2, 3):
        print(i)
        if i == 2:
            break
    except break:  # or just except
        assert i == 2
        print("a break was executed")
    else:
        print("never reached")  # this is never reached
    print("for loop is done")
as an alternative to something like:
broke_out = False
    for i in (1, 2, 3):
        print(i)
        if i == 2:
            broke_out = True
            break
    else:
        print("never reached")  # this is never reached
    if broke_out:
        assert i == 2
        print("a break was executed")
    print("for loop is done")
correct.
...
I must admit the suggestion seems a little bit neater than having to
manage a flag myself, but on the other hand I can't remember the last
time I've needed to manage a flag like that.
And on the gripping hand, this is even simpler than both alternatives:
for i in (1, 2, 3):
        print(i)
        if i == 2:
            assert i == 2
            print("a break was executed")
            break
    else:
        print("never reached")  # this is never reached
    print("for loop is done")
Right, that's how you'd likely implement the behavior today, but see my 
argument about the two alternative code branches not ending up together 
at the same level of indentation.
...
There are some significant unanswered questions:
- Does it matter which order the for...except...else are in?
  Obviously the for block must come first, but apart from that?
Just like in try/except/else, the order would be for (or 
while)/except/else with the difference that both except and else would 
be optional.
...
- How is this implemented? Currently "break" is a simple
  unconditional GOTO which jumps past the for block. This will
  need to change to something significantly more complex.
Yeah, I know that's why I listed this under cons.
...
- There are other ways to exit a for-loop than just break. Which
  of them, if any, will also run the except block?
None of them (though, honestly, I cannot think of anything but 
exceptions here; what do you have in mind?)
...