[Samuele Pedroni, on sizeof()]
it is very unlikely that we can implement this in Jython,
I'm sure that you could, but no better than CPython: return a lower bound that may or may not be within a factor of 1000 of reality. CPython could do C sizeof() on Python objects, but that's about it. There's no clear way to know how much overhead is due to pymalloc "per object", and no way at all to know how much overhead is imposed by the system malloc, or by the OS.
Continuing Jeremy's example: How much memory does an empty Python dict consume? It's certain that *some* empty dicts grab at least 256KB from the system malloc, because that's the minimum chunk size pymalloc uses for its object arenas. Telling a user that the unlucky empty dict triggering a new arena allocation consumed some 128 bytes is a factor of 2000 off from what was actually gotten from the C library.
There are many layers of memory, and we have a poor handle on most of them even in CPython. In a debug Python build, setting the envar PYTHONMALLOCSTATS displays a lot of detail about pymalloc's memory use, but that's only part of the story.
Implement the Jython sizeof() to always return 8, and who'd know the difference <wink>.