In article <8e66719c-5e90-4776-bba9-a11c29fba...@googlegroups.com>, bartolome.sin...@gmail.com wrote:
> In Python 3.3 for Windows, every list gets a different id when it is created: > > >>> id([3]) > 46555784 > >>> id([3]) > 47920192 > >>> id([4]) > 46532048 > > But if I write a tuple asking for the ids of two lists, each list seems to > get the same id: > > >>> id([3]), id([4]) > (43079000, 43079000) > > I was expecting a tuple with two different ids. What is the reason for this > behaviour? This is a classic little gotcha. Object ids are guaranteed to be unique, but only for the lifetime of an object. Or, to put it another way, it is guaranteed that no two objects that exist at the same time can have the same id. What's happening when you do: >>> id([3]), id([4]) Is that a list, [3], is created, its id is taken, and then the list is destroyed. In this particular implementation, object ids happen to be the address of the object in memory. Then, a new list, [4], is created. It just so happens that it gets created using the same memory block that was just freed by the destruction of the other list, so it ends up in the same place in memory. So it gets the same id! Kind of neat, huh? PS -- this makes a nice interview question to see if somebody really understands what's happening beneath the surface. -- http://mail.python.org/mailman/listinfo/python-list
