Very minor point: be careful comparing IDs the way you do in the REPL (i.e., without keeping a reference to the previous object)--since the old one is released, it's possible that the memory address is reused and the ID of the new object ends up being the same, even though the object is different, which can have misleading results:

    Python 2.7.10 (default, Oct 23 2015, 19:19:21) 
    [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.5)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> class A:
    ...   def b(self): pass
    ... 
    >>> A.b
    <unbound method A.b>
    >>> hex(id(A.b))
    '0x104e2c0a0'
    >>> hex(id(A.b))
    '0x104daeb90'
    >>> hex(id(A.b))
    '0x104daeb90'

So the `x is y` approach is "safer." :)