In what structure is a Python object stored in memory?

Say I have a class A:

class A(object):
    def __init__(self, x):
        self.x = x

    def __str__(self):
        return self.x

And I use sys.getsizeof to see how many bytes instance of A takes:

>>> sys.getsizeof(A(1))
64
>>> sys.getsizeof(A('a'))
64
>>> sys.getsizeof(A('aaa'))
64

As illustrated in the experiment above, the size of an A object is the same no matter what self.x is.

So I wonder how python store an object internally?