On 04/20/2012 11:25 PM, Rotwang wrote: > On 21/04/2012 01:01, Roy Smith wrote: >> In article<877gxajit0....@dpt-info.u-strasbg.fr>, >> Alain Ketterlin<al...@dpt-info.u-strasbg.fr> wrote: >> >>> Tuples are immutable, while lists are not. >> >> If you really want to have fun, consider this classic paradox: >> >>>>> [] is [] >> False >>>>> id([]) == id([]) >> True > > Huh. This is not what I would have expected. What gives? >
It'd be easier to respond if you would say what's confusing to you about that result. The first is exactly what I'd expect, and the second is implementation dependent. [] is [] produces two independent lists, then compares their ID, then discards them. So of course they must be different. id([]) prepares a list object and figures out its id. Then discards the object. You do it a second time, and you *might* get a new object with the same id. After all, the first one is gone now, so there's no harm in re-using the id. In particular, the CPython implementation currently uses the address of the object as the ID, so if the memory is available, the next allocation just might get the same memory. Remember that the only guarantee on id() is that it won't assign the same number to two objects that exist at the same time. Once an object is gone, its ID may be reused right away, or after a while, or never. -- DaveA -- http://mail.python.org/mailman/listinfo/python-list
