[Python-Dev] Re: PEP 279 (original) (raw)

jepler@unpythonic.dhs.org jepler@unpythonic.dhs.org
Sat, 30 Mar 2002 08:56:19 -0600

On Sat, Mar 30, 2002 at 08:55:24AM -0500, Raymond Hettinger wrote:

Hmm, the client code needs to maintain TWO instance variables, the generator object and the instance of Whatever. It works but doesn't smell right to me:

w = Whatever() wg = w.generator() wg.next() wg.next() w.stop(); wg.next() # trigger clean-up

I think this example can be embellished a bit to avoid this problem. HaltingIterator subclasses define the .generator() method, but instantiates it in the init method, calling it at the right time from the .next() method. Cleanup is moved into a separate method, called from .stop() with an optional argument. Calling stop also makes the next .next() call raise StopIteration (unless you happen to call .stop() with a false argument, though).

I had a bit of confusion that there's not StopIteration traceback printed, and neither is "after that". Apparently an uncaught StopIteration call just exits silently? (I'm using 2.3a0 here, in the case that it matters---cvs from 2-3 weeks ago)

Jeff Epler

from future import generators

class HaltingIterator: def init(self, *args, **kw): self._stop = 0 self._generator = self.generator(*args, **kw)

def stop(self, arg=1):
self._stop = arg
self.cleanup(arg)

def next(self):
if self._stop:
    raise StopIteration
return self._generator.next()

def cleanup(self, arg): pass

class ExampleHaltingIterator(HaltingIterator): def generator(self): a, b = 1, 1 while 1: ret = a ret, a, b = a, b, a+b yield ret

def cleanup(self, arg):
print "halted with", arg

x = ExampleHaltingIterator()

for i in range(10): print x.next() x.stop("76 trombones") print x.next() print "after that"