Defining magic methods on classes

Question

I would like to define a single object that can be iterated over without having to create a class and then an instance. Something like this:

class Thing(object): stuff = ["foo", "bar", "baz"] @classmethod def __iter__(cls): return iter(cls.stuff) for thing in Thing: print thing

However this doesn't actually work. Is there any way to do this?

Magic method lookup occurs on the class. So you'll either have to create an instance and class or class and metaclass. But it also seems like you could just subclass list (at least in your example). — jedwards
– jedwards, Commented Apr 20, 2015 at 22:12
@jedwards Ok, I was trying to avoid creating extra things in the namespace of my module but I guess I'll just have to deal with it. This is a much simplified example, in my actual use case there's more internal state than just a list. — DanielGibbs
– DanielGibbs, Commented Apr 20, 2015 at 22:16

nickie · Accepted Answer · 2015-04-20 22:26:06Z

What Ashwini correctly suggested in his comment is the following. This works in Python 2.

class ThingType(type): __stuff__ = ["foo", "bar", "baz"] @classmethod def __iter__(cls): return iter(cls.__stuff__) class Thing(object): __metaclass__ = ThingType for thing in Thing: print thing

And this works in Python 3:

class ThingType(type): __stuff__ = ["foo", "bar", "baz"] @classmethod def __iter__(cls): return iter(cls.__stuff__) class Thing(object, metaclass=ThingType): pass for thing in Thing: print(thing)

It will work because(even without classmethod) Python can find the attribute by looking up in the MRO. Earlier I misread it and thought you had defined stuff in Thing not ThingType. So... ;-)

Daniel Pryden · Accepted Answer · 2015-04-20 22:08:24Z

Does Thing actually need to be a type? You could make it an object that has type-like callable behavior, which would probably be simpler:

class RealThing(object): pass class ThingFactory(object): def __iter__(self): return iter(["foo", "bar", "baz"]) def __call__(self): return RealThing() Thing = ThingFactory()

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