Question

I’m trying to assign a new value to a key in a dictionary. But getting “ValueError: invalid literal for int() with base 10:”
Here is what I do.

balance = {'beer':5, 'toast':2}
item = input("what did you eat?: ")
price = input("how much?: ")
if item not in balance:
   balance[item] = int(price)
else:
   balance[item] = balance[item] + int(price)

I’m puzzled, since I can do balance[‘beer’] = 5 in python shell, what am I missing?

Answer 1

Read the error message again:

ValueError: invalid literal for int() with base 10

This means, that you are passing an invalid argument to function int(), i.e. a string that does not represent a valid decimal number.

Check what price name holds. As it’s taken from the user, your program must be prepared to handle a situation when it’s garbage. I usually make a function to handle the boilerplate:

def input_int(prompt):
    while True:
        data = input(prompt)
        try:
            return int(data)
        except ValueError:
            print("That's not a valid number. Try again.")

You can also add some escape condition if it makes sense in your program.