Re: [Python-Dev] Re: anonymous blocks

[Tim Delaney]

Guido van Rossum wrote:
- temporarily sidestepping the syntax by proposing 'block' instead of
'with'
- __next__() argument simplified to StopIteration or
ContinueIteration instance
- use "continue EXPR" to pass a value to the generator
- generator exception handling explained
+1
A minor sticking point - I don't like that the generator has to re-raise any 
``StopIteration`` passed in. Would it be possible to have the semantics be:

   If a generator is resumed with ``StopIteration``, the exception is 
raised
   at the resumption point (and stored for later use). When the generator
   exits normally (i.e. ``return`` or falls off the end) it re-raises the
   stored exception (if any) or raises a new ``StopIteration`` exception.

So a generator would become effectively::
   try:
       stopexc = None
       exc = None
       BLOCK1
   finally:
       if exc is not None:
           raise exc
       if stopexc is not None:
           raise stopexc
       raise StopIteration
where within BLOCK1:
   ``raise <exception>`` is equivalent to::
       exc = <exception>
       return
   The start of an ``except`` clause sets ``exc`` to None (if the clause is
   executed of course).
   Calling ``__next__(exception)`` with ``StopIteration`` is equivalent 
to::

       stopexc = exception
       (raise exception at resumption point)
   Calling ``__next__(exception)`` with ``ContinueIteration`` is equivalent 
to::

       (resume exception with exception.value)
   Calling ``__next__(exception)__`` with any other value just raises that 
value
   at the resumption point - this allows for calling with arbitrary 
exceptions.

Also, within a for-loop or block-statement, we could have ``raise 
<exception>`` be equivalent to::

   arg = <exception>
   continue
This also takes care of Brett's concern about distinguishing between 
exceptions and values passed to the generator. Anything except StopIteration 
or ContinueIteration will be presumed to be an exception and will be raised. 
Anything passed via ContinueIteration is a value.

Tim Delaney 

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