Convert SQLite Fetchone() Result to float for Math

[Extra]

So I can do this:

                        print('Selcted Item Quantity: ', SelectedItemQuantity)

                        x = float(SelectedItemQuantity)
                        y = SellPrice[0]
                        z = float(y)
                        print('Float Value = ', x)
                        print('x*z= ', x*z)
                        MaterialUsedPrice2 = x*z

                        #Calculate the Price of Material Used
                        # MaterialUsedPrice = SelectedItemQuantity * SellPrice
                        print('The Total Price for the ', '(',x,')', SelectedItemName, ' Used is: $', MaterialUsedPrice2)

And I get:

Output:
Selcted Item Quantity:  2
Float Value =  2.0
x*z=  3.0
The Total Price for the  ( 2.0 ) 1/2" PVC Coupling  Used is: $ 3.0

Which is what I want,
But then I get this error and the program crashes:

Error:
    y = SellPrice[0]
TypeError: 'NoneType' object is not subscriptable

Any way to get around/fix this?

[deanhystad]

As previously mentioned, fetchone() can return None. You need to check the return value before trying to use it.

[rob101]

As @deanhystad says; if there's a possibility that the sell price is Null, then you need to deal with that by catching it, to stop your script from crashing out.

You could use a try / except or a if branch. I'm guessing that the sell price should never be Null or even zero, so you may want to look in to why that condition occurs.

[deanhystad]

I think it is a bad idea to verify actual values returned from a database query. Maybe 0 can be, or worse is a valid sell price. The future maintainer of your code will be confused that database queries for free items always fail and they will curse you when they find the reason is buried deep in some method.

Verify values before you put them in the database. Verify that a query found a match. Assume values returned by the query are correct.