singleton ... again

[Roy Smith]

In article <mailman.6850.1392313443.18130.python-list at python.org>,
 Ethan Furman <ethan at stoneleaf.us> wrote:

> Say you have a class that represents serial ports or your computer.  You 
> should get the same object every time you ask 
> for SerialPort(2).

Why?  Certainly, you should get objects which refer to the same physical 
port.  So:

port_a = SerialPort(2)
port_b = SerialPort(2)

port_a.enable()
assert port_b.is_shutdown() == False

port_a.shutdown()
assert port_b.is_shutdown() == True

But, why do they have to be the same object?  Why should I care if

port_a is port_b

is False, as long as all operations I perform on either are reflected in 
correct state changes on the other one?