You must be using old-style Python classes.
You might find that things work much better with new-style
The problem with old-style classes and their constructors is
that there's no easy way to let jsonpickle know how to construct
it. We could support an extension mechanism wherein the
object returns the *args + **kwargs needed for class
construction. jsonpickle could save it and provide it at
object construction time..
I could imagine it being a __getdefault__(self) function that
classes would implement by doing return (args, kwargs) for use
by jsonpickle at object-construction time.
Let me know what you think.
Nonetheless, that's a lot of machinery when it might be simpler
to just use new-style classes.
Hopefully the code's under your control.
Anyways, the unpickler definitely needs a better error message
for the time being. Thanks for reporting it, I'll fix that.