def check_int(self, param, error_msg):
        """
        This function check if the parameter is int.
        If yes, the function returns the parameter,
        if not, it raises error message.
        
        **Args:**
        
        * `param` : parameter to check (int or similar)

        * `error_ms` : lowest allowed value (int), or None        
        
        **Returns:**
        
        * `param` : parameter (int)
        """
        if type(param) == int:
            return int(param)
        else:
            raise ValueError(error_msg)