How can i display wrong input message to users if wrong value is entered in a field?

My code below

@api.onchange('normal_range', 'result', 'check_value')
    def onchange_result(self):
        if self.check_value == '<' and self.normal_range:
            no1,no2 = self.normal_range.split('<')
            if self.result:
                if int(self.result) >= int(no1) and int(self.result) <= int(no2):
                    self.remark = 'Average'
                elif int(self.result) <= int(no1):
                    self.remark = 'Below Average'
                elif int(self.result) >= int(no2):
                    self.remark = 'Above Average'

Error shown below:

File "/home/dev/Desktop/odoo-10.0/customs_mark/mark_result.py", line 418, in onchange_result if float(self.result) >= float(no1) and float(self.result) <= float(no2):
ValueError: could not convert string to float: AB