Question

class A(object):
    _all = set()

    def __new__(cls):
        obj = super(A,cls).__new__(cls)
        cls._all.add(obj)
        return obj

class B(A):
    pass

class C(A):
    pass

b = B()
c = C()

This will create all instances and put them in A._all. What I want is for the instances of C to be placed in C._all (and similarly for C).

I could define B and C like this:

class B(A):
    _all = set()

class C(A):
    _all = set()

Which would work. However, this feels like repeating code, and if I forget to add _all when I subclass A next, or misspell _alll I’m likely to get hard to debug errors.

How would I go about doing this well?

Answer 1

Here’s a metaclass for it:

class make_all_meta(type):
   def __init__(cls, name, bases, dic):
       type.__init__(cls, name, bases, dic)
       cls._all = set()

class A(object):
    __metaclass__ = make_all_meta

In Python 3, the definition for A must be:

class A(metaclass=make_all_meta):
    pass