@abstractmethod<EOL><INDENT>def set_attribute(self, session_key, attribute_key, value):<DEDENT>
pass<EOL>
Binds the specified value to the associated session uniquely identified
by the attribute_key.  If there is already a session attribute
bound under the attribute_key, that existing object will be
replaced by the new value.

If the value parameter is None, it has the same effect as if the
remove_attribute(session_key, attribute_key) method was called.

raises InvalidSessionException if the specified session has stopped or
expired prior to calling this method