Re: [Python-ideas] Yield-From: Revamped expansion

21 Mar 2009

      Greg Ewing wrote:
...
I'm thinking about replacing the expansion with the
following, which hopefully fixes a couple of concerns
that were raised recently without breaking anything else.
Can anyone see any remaining ways in which it doesn't
match the textual description in the Proposal section?
(It still isn't *quite* right, because it doesn't
distinguish between a GeneratorExit explicitly thrown
in and one resulting from calling close() on the
delegating generator. I may need to revise the text
and/or my implementation on that point, because I want
the inline-expansion interpretation to hold.)
_i = iter(EXPR)
    try:
        _u = _i.next()
    except StopIteration, _e:
        _r = _e.value
    else:
        while 1:
            try:
                _v = yield _u
            except GeneratorExit:
                _m = getattr(_i, 'close', None)
                if _m is not None:
                    _m()
                raise
            except BaseException, _e:
                _m = getattr(_i, 'throw', None)
                if _m is not None:
                    _u = _m(_e)
                else:
                    raise
            else:
                try:
                    if _v is None:
                        _u = _i.next()
                    else:
                        _u = _i.send(_v)
                except StopIteration, _e:
                    _r = _e.value
                    break
    RESULT = _r
I'd adjust the inner exception handlers to exploit the fact that
SystemExit and GeneratorExit don't inherit from BaseException:

    _i = iter(EXPR)
    try:
        _u = _i.next()
    except StopIteration, _e:
        _r = _e.value
    else:
        while 1:
            try:
                _v = yield _u
            except Exception, _e:
                _m = getattr(_i, 'throw', None)
                if _m is not None:
                    _u = _m(_e)
                else:
                    raise
            except:
                # Covers SystemExit, GeneratorExit and
                # anything else that doesn't inherit
                # from Exception
                _m = getattr(_i, 'close', None)
                if _m is not None:
                    _m()
                raise
            else:
                try:
                    if _v is None:
                        _u = _i.next()
                    else:
                        _u = _i.send(_v)
                except StopIteration, _e:
                    _r = _e.value
                    break
    RESULT = _r

I think Antoine and PJE are right that the PEP needs some more actual
use cases though.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia
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