[Python-ideas] Possible PEP 380 tweak (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Oct 30 06:05:20 CEST 2010

On Sat, Oct 30, 2010 at 1:47 PM, Guido van Rossum <guido at python.org> wrote:

I agree that you've poked a hole in my proposal. If we can change the expansion of yield-from to restore the equivalency between gtally() and the simplest gtally2(), thereby restoring the original refactoring principle, we might be able to save it. Otherwise I declare defeat. Right now I am too tired to think of such an expansion, but I recall trying my hand at one a few nights ago and realizing that I'd introduced another problem. So this does not look too hopeful, especially since I really don't like extending GeneratorExit for the purpose.

I tried to make the original version work as well, but always ran into one of two problems:

Here's a crazy idea though. What if gtally2() could be written as follows:

def gtally2(): return from gtally()

If we make the generator tail call explicit, then the interpreter can do the right thing (i.e. raise StopIteration-with-a-value instead of reraising GeneratorExit) and we don't need to try to shoehorn two different sets of semantics into the single yield-from construct.

To give some formal semantics to the new statement:

# RETURN FROM semantics
_i = iter(EXPR)
_m, _a = next, (_i,)
# _m is a function or a bound method;
#  _a is a tuple of arguments to call _m with;
# both are set to other values further down
while 1:
    # Move the generator along
    # Unlike YIELD FROM, we allow StopIteration to
    # escape, since this is a tail call
    _y = _m(*_a)

    # Yield _y and process what came back in
    try:
        _s = yield _y
    except GeneratorExit as _e:
        # Request to exit
        try:
            # Don't reuse _m, since we're bailing out of the loop
            _close = _i.close
        except AttributeError:
            pass
        else:
            # Unlike YIELD FROM, we use StopIteration
            # to return the value of the inner close() call
            raise StopIteration(_close())
        # If there is no inner close() attribute, return None
        raise StopIteration
    except BaseException as _e:
        # An exception was thrown in; pass it along
        _a = sys.exc_info()
        try:
            _m = _i.throw
        except AttributeError:
            # Can't throw it in; throw it back out
            raise _e
    else:
        # A value was sent in; pass it along
        if _s is None:
            _m, _a = next, (_i,)
        else:
            _m, _a = _i.send, (_s,)
# Unlike YIELD FROM, this is a statement, so there is no RESULT

Summary of the differences between return from and yield from:

Cheers, Nick.

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

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