> As a compromise, we could suppress the sharing for small int objects, since they are singletons, anyway.

This is implementation detail.

But we can use more efficient way to memoizating small int objects.

I also suppose than efficient C implementation of IdentityDict will significant decrease an overhead (may be 2 times). As far as this is probably a first case for which IdentityDict give significant benefit, I'll provide an implementation for 3.5.