def trace_variable(self, mode, callback):
        """Define a trace callback for the variable.
        MODE is one of "r", "w", "u" for read, write, undefine.
        CALLBACK must be a function which is called when
        the variable is read, written or undefined.
        Return the name of the callback.
        This deprecated method wraps a deprecated Tcl method that will
        likely be removed in the future.  Use trace_add() instead.
        """
        # TODO: Add deprecation warning
    trace = trace_variable