[Python-Dev] PEP 340: Else clause for block statements

[Anders J. Munch]

GvR wrote:
> [Nick Coghlan]
> > Option 2: mimic if semantics
> >    An 'else' clause on a block statement behaves vaguely like the else
clause on
> > an if statement - the clause is executed only if the first suite is
never
> > entered, but no exception occurs (i.e. StopIteration is raised by the
first call
> > to next).
> 
> Strange because it's different from the behavior of a for loop, and
> the block-statement doesn't feel like an if-statement at all. But I
> could actually imagine a use case: when acquiring a lock with a
> time-out, the else-clause could be executed when the acquisition times
> out.
> 
>   block locking(myLock, timeout=30):
>       ...code executed with lock held...
>   else:
>       ...code executed if lock not acquired...

A file-closing block function has the same need, as does any block
function that manages a resource, whose acquisition might fail.

A surrounding try/except doesn't quite cut it; the problem is that the
try-clause will then also cover the suite.

Example:

    try:
        in opening('file1') as f1:
            ...
            in opening('file2') as f2:
                ...
    except IOError:
        print "file1 not available, I'll try again later"

How do I tell try/except that I really only meant to trap
opening('file1'), but opening 'file2' is not supposed to fail so I
want any exception from that propagated?  Better if I could write:

    in opening('file1') as f1:
        ...
        in opening('file2') as f2:
            ...
    else:
        print "file1 not available, I'll try again later"

or even

    in opening('file1') as f1:
        ...
        in opening('file2') as f2:
            ...
    except IOError:
        print "file1 not available, I'll try again later"

I rather like this version, because it is patently clear what should
happen if there is no except-clause: The exception propagates
normally.

- Anders