Can I iterate over a class in Python?

Question

I have a class that keeps track of its instances in a class variable, something like this:

class Foo:     by_id = {}      def __init__(self, id):         self.id = id         self.by_id[id] = self 

What I'd like to be able to do is iterate over the existing instances of the class. I can do this with:

for foo in Foo.by_id.values():     foo.do_something() 

but it would look neater like this:

for foo in Foo:     foo.do_something() 

is this possible? I tried defining a classmethod __iter__, but that didn't work.

Answer 1

If you want to iterate over the class, you have to define a metaclass which supports iteration.

x.py:

class it(type):     def __iter__(self):         # Wanna iterate over a class? Then ask that class for iterator.         return self.classiter()  class Foo:     __metaclass__ = it # We need that meta class...     by_id = {} # Store the stuff here...      def __init__(self, id): # new isntance of class         self.id = id # do we need that?         self.by_id[id] = self # register istance      @classmethod     def classiter(cls): # iterate over class by giving all instances which have been instantiated         return iter(cls.by_id.values())  if __name__ == '__main__':     a = Foo(123)     print list(Foo)     del a     print list(Foo) 

As you can see in the end, deleting an instance will not have any effect on the object itself, because it stays in the by_id dict. You can cope with that using weakrefs when you

import weakref 

and then do

by_id = weakref.WeakValueDictionary() 

. This way the values will only kept as long as there is a "strong" reference keeping it, such as a in this case. After del a, there are only weak references pointing to the object, so they can be gc'ed.

Due to the warning concerning WeakValueDictionary()s, I suggest to use the following:

[...]     self.by_id[id] = weakref.ref(self) [...] @classmethod def classiter(cls):     # return all class instances which are still alive according to their weakref pointing to them     return (i for i in (i() for i in cls.by_id.values()) if i is not None) 

Looks a bit complicated, but makes sure that you get the objects and not a weakref object.