Serhiy,
I don't understand what you are trying to tell me. "cannot serialize '%s' object" is used all over the interpreter, e.g. io, pickle, etree, and more. I feel it's the standard message.

Vitaly,
A lot of objects can't be copied. It's the general case for all kinds of objects that hold operating system resources (files, sockets) or wrap external C libraries (bz2, lzma, sqlite, ssl). We generally don't document that an object cannot be pickled, serialized, or copied. If documentation doesn't state that an object is copy-able or doesn't provide a dedicated copy method, than it can't be copied.