On Thu, 20 Nov 2014 12:52:25 +1100, Chris Angelico wrote: > While poking about the cpython source tree with reference to PEP 479, I > came across this: > > https://docs.python.org/3.5/library/marshal.html > > """singletons None, Ellipsis and StopIteration can also be marshalled > and unmarshalled.""" > > StopIteration is a type. I suppose it's still kinda true that there's > only one StopIteration class (otherwise exception handling would get > very messy), but it seems unusual that this one particular exception > class should be special-cased. (And yes, the code does correspond - > marshal.c:293 handles StopIteration with its own byte code.)
StopIteration is not a singleton class. py> e1 = StopIteration() py> e2 = StopIteration() py> e1 is e2 False It it were, you couldn't support "return value" from generators, since the return value gets turned into the argument for StopIteration. Nor is it a singleton itself: StopIteration (the class) is an instance of type, and there are *many* instances of type: py> all(isinstance(T, type) for T in [int, float, str, bool, ... Exception, StopIteration, dict, type]) True I cannot imagine what they mean by calling StopIteration a singleton. > Why is StopIteration marshallable but no other exception is? Or, if they > are marshallable, why does this one get special handling? No idea. But you can marshal StopIteration itself, but not StopIteration instances: py> marshal.dumps(StopIteration) 'S' py> marshal.dumps(StopIteration()) Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: unmarshallable object -- Steven -- https://mail.python.org/mailman/listinfo/python-list
