<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def conv(self,field_name,conv_func):

 """When a record is returned by a SELECT, ask conversion of
 specified field value with the specified function"""

        if field_name not in self.fields:
            raise NameError,"Unknown field %s" %field_name
        self.conv_func[field_name] = conv_func