is implemented with id ?

[Hans Mulder]

On 5/09/12 17:09:30, Dave Angel wrote:
> But by claiming that id() really means address, and that those addresses
> might move during the lifetime of an object, then the fact that the id()
> functions are not called simultaneously implies that one object might
> move to where the other one used to be before the "move."

Whoa!  Not so fast!  The id() of an object is guaranteed to not
change during the object's lifetime.  So if an implementation
moves objects around (e.g. Jython), then it cannot use memory
addresses for the id() function.

> I think it much more likely that jython uses integer values for
> the id() function, and not physical addresses.

The id() function is guaranteed to return some flavour of integer.

In Jython, the return values are 1, 2, 3, 4, etc., except, of course,
if you invoke id() on an object you've id'd before, you get the same
number as before.

In current versions of CPython, you do get the (virtual) memory
address, converted to an int (or a long).  But then, CPython does
not move objects.

Maybe the next version of CPython should shift id values two or three
bits to the right, just to make sure people don't misinterpret ids as
memory addresses.


Hope this helps,

-- HansM