And another message that was rejected (I sent from an unregistered email address)


On Sat, Jul 27, 2019 at 1:49 AM Serhiy Storchaka <storchaka@gmail.com> wrote:
26.07.19 21:52, Bruce Leban пише:

To put this in a simpler way: the proposal is to add an except clause that applies ONLY to the direct operation of the with or for statement and not to the block. That's an interesting idea.

The one thing I find confusing about your proposal is that the proposed syntax does not imply the behavior. In a try statement, the except appears at the end and after all possible statements that it could cover. The proposal mimics that syntax but with different semantics. Something like this would be much more clear what is going on:

    for VARIABLE in EXPRESSION:
        except EXCEPTION:
            BLOCK
        BLOCK

    with EXPRESSION as VARIABLE:
        except EXCEPTION:
            BLOCK
        BLOCK

    while EXPRESSION:
        except EXCEPTION:
            BLOCK
        BLOCK


Besides an unusual for Python layout (a clause has different indentation than the initial clause of the statement to which it belongs) there is other problem. The exception block is not the part of the "for" or "with" block. After handling an exception in the "for" clause you do not continue to execute the "for" block, but leave the loop. After handling an exception in the "with" clause you do not continue to execute the "with" block and do not call `__exit__` when leave it. To me, this syntax is much more confusing than my initial proposition.

And I find it less confusing. And neither of those is the standard to use. The goal is for syntax to imply semantics (which my proposal does and I do not think yours does, given several people commenting that they thought it applied to the entire loop) and to choose syntax that is more clear to more people (which requires more than two peoples' opinions).

Consider how you would write this if everything was an expression in Python and we had braces:

    for VAR in ( EXPR except EXCEPTION: { BLOCK; break; } ):
        BLOCK

I do agree that it is not obvious that the exception block breaks out of the loop. I think in actual code it will be fairly obvious what's happening as continuing into the loop when the loop expression through an expression doesn't make sense. I'm open to alternatives. On the other hand, an except clause at the bottom of the loop that does not apply to the loop body is going to catch me every time I see it.

--- Bruce