def __init__(self, base, obj, msg=None):
        if not msg:
            try:
                base.class_mapper(type(obj))
                name = _safe_cls_name(type(obj))
                msg = (
                    "Class %r is mapped, but this instance lacks "
                    "instrumentation.  This occurs when the instance "
                    "is created before sqlalchemy.orm.mapper(%s) "
                    "was called." % (name, name)
                )
            except UnmappedClassError:
                msg = _default_unmapped(type(obj))
                if isinstance(obj, type):
                    msg += (
                        "; was a class (%s) supplied where an instance was "
                        "required?" % _safe_cls_name(obj)
                    )
        UnmappedError.__init__(self, msg)