[Python-Dev] PEP 334 - Simple Coroutines via SuspendIteration

Wed Sep 8 15:26:03 CEST 2004

On Wed, Sep 08, 2004 at 09:07:59AM -0400, Jp Calderone wrote:
| 
|     def somefunc():
|         raise SuspendIteration()
|         return 'foo'
| 
|     def genfunc():
|         yield somefunc()

Interesting, but:
 - somefunc is a function, thus SuspendIteration() should
   terminate the function; raising an exception
 - somefunc is not a generator, so it cannot be yielded.

However, perhaps something like...

   def suspend(*args,**kwargs):
       raise SuspendIteration(*args,**kwargs)
       # never ever returns

   def myProducer():
       yeild "one"
       suspend()
       yield "two"

Regardless, this is a side point.  The authors of iterators that
raise a SuspendIterator() will be low-level code, like a next()
which reads the next block from a socket or row from a database
query.  In these cases, the class style iterator is sufficient.

The real point, is that user-level generators, such as this example 
from the PEP (which is detailed as a class-based iterator), should
transparently handle SuspendIteration() by passing it up the generator
chain without killing the current scope.

|     def ListAlbums(cursor):
|         cursor.execute("SELECT title, artist FROM album")
|         yield '<html><body><table><tr><td>Title</td><td>Artist</td></tr>'
|         for (title, artist) in cursor:
|             yield '<tr><td>%s</td><td>%s</td></tr>' % (title, artist)
|         yield '</table></body></html>'

For those who say that this iterator should be invalidated when
cursor.next() raises SuspendIteration(), I point out that it is not
invalided when cursor.next() raises StopIteration().

Kind Regards,

Clark

-- 
Clark C. Evans                      Prometheus Research, LLC.
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