def update_command(self, command):
if command is None:<EOL><INDENT>self._command = lambda: None<EOL><DEDENT>else:<EOL><INDENT>self._command = command<EOL><DEDENT>
Updates the callback command which is called when the Combo
changes.

Setting to `None` stops the callback.

:param callback command:
    The callback function to call, it can accept 0 or 1 parameters.

    If it accepts 1 parameter the `value` of the Combo will be
    passed.