[Python-ideas] Proto-PEP on a 'yield from' statement

[Bruce Frederiksen]

I would think that in addition to forwarding send values to the 
subgenerator, that throw exceptions sent to the delegating generator 
also be forwarded to the subgenerator.  If the subgenerator does not 
handle the exception, then it should be re-raised in the delegating 
generator.  Also, the subgenerator close method should be called by the 
delegating generator.  Thus, the new expansion code would look something 
like:

    _i = iter(iterable)
    try:
        _value_to_yield = next(_i)
        while True:
            try:
                _value_to_send = yield _value_to_yield
            except Exception as _throw_from_caller:
                if hasattr(_i, 'throw'):
                    _value_to_yield = _i.throw(_throw_from_caller)
                else:
                    raise
            else:
                if hasattr(_i, 'send'):
                    _value_to_yield = _i.send(_value_to_send)
                else:
                    _value_to_yield = next(_i)
    except StopIteration as _exc_from_i:
        _a = _exc_from_i.args
        if len(_a) > 0:
            result = _a[0]
        else:
            result = None
    finally:
        if hasattr(_i, 'close'):
            _i.close()

I'm also against using return as the syntax for a final value from the 
subgenerator.  I've accidentally used return inside generators many 
times and appreciate getting an error for this.  I would be OK with 
removing the ability to return a final value from the subgenerator.  Or 
create a new syntax that won't be used accidentally.  Perhaps: finally 
return expr?

-bruce frederiksen

Greg Ewing wrote:
> Comments are invited on the following proto-PEP.