return self.discount_price, self.discount_rate

Actually when you wanna get result you wanna return the values. so use given above code after

self.discount_price=float(self.price*self.discount_rate/100)-self.price

your full updated code is below:

class Shop:

def __init__(self,name,item_no,price,discount):

self.name=name

self.item_no=item_no

self.price=float(price)

def discount(self,discount_rate,discount_price):

self.discount_rate=float(0.15)

self.discount_price=float(self.price*self.discount_rate/100)-self.price

return self.discount_price, self.discount_rate

item1 = Shop('bag','9988','800','0.15')

print(item1.discount("",""))