Antoine: in (my experience of) memory analysis, the size of a single object is mostly irrelevant. If you need to know how much memory something consumes, you typically want to know the memory of a set of objects. So this is the case that really must be supported.

For that, users will have to use libraries that know how to count memory. It's not asked to much that the authors of such libraries know about internals of Python (such as the existence of sys.getsizeof, or gc.get_referents). The question is: can such a library reasonably implemented? For that, it is important that getsizeof behaves uniformly across objects.

If you really don't like the proposed uniformity, please propose a different rule. However, don't give deviations in other places (OrderedDict) as a reason to break the rule here as well. Instead, if OrderedDict.__getsizeof__ is broken, it needs to be fixed as well.