def purchase(self, customername, itemname, qty, phone_number=None):
## Find or create new customer
customer_idx = self.find_customer_in_store(customername)
if customer_idx == -1:
customer = Customer(customername, phone_number)
self.customers.append(customer)
else:
customer = self.customers[customer_idx]
## Find the item they want to buy
idx = self.find_item_in_store(itemname)
if idx != -1: # check if the itemname is already in stock, increment the quantity of the itemname
self.items[idx].purchase(qty)
transaction = Transaction(customer, self.items[idx], qty)
customer.add_dollars_spent(self.items[idx].price * qty)
self.transactions.append(transaction)
else: # not if stock, create the itemname object and add it to our stock.
raise ValueError("Trying to buy an itemname that is not in our store!")
