How to keep track of class instances?

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One way to keep track of instances is with a class variable:

class A(object):
    instances = []

    def __init__(self, foo):
        self.foo = foo
        A.instances.append(self)

At the end of the program, you can create your dict like this:

foo_vars = {id(instance): instance.foo for instance in A.instances}

There is only one list:

>>> a = A(1)
>>> b = A(2)
>>> A.instances
[<__main__.A object at 0x1004d44d0>, <__main__.A object at 0x1004d4510>]
>>> id(A.instances)
4299683456
>>> id(a.instances)
4299683456    
>>> id(b.instances)
4299683456

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