How to Multiply Decimals in Python

Question

I was writing a pounds to dollars converter program. I encountered some problems multiplying the two numbers.

pounds = input('Number of Pounds: ')
convert = pounds * .56
print('Your amount of British pounds in US dollars is: $', convert)

Can anyone tell me the code to correct this program?

Answer 1

In Python 3 input will return a string. This is basically equivalent of raw_input in Python 2. So, you need to convert that string to a number before performing any calculation. And be prepared for "bad input" (i.e: non-numeric values).

In addition, for monetary values, it is usually not a good idea to use floats. You should use decimal to avoid rounding errors:

>>> 100*.56
56.00000000000001
>>> Decimal('100')*Decimal('.56')
Decimal('56.00')

All of that lead to something like that:

import decimal

try:
    pounds = decimal.Decimal(input('Number of Pounds: '))
    convert = pounds * decimal.Decimal('.56')
    print('Your amount of British pounds in US dollars is: $', convert)
except decimal.InvalidOperation:
    print("Invalid input")

Producing:

sh$ python3 m.py
Number of Pounds: 100
Your amount of British pounds in US dollars is: $ 56.00

sh$ python3 m.py
Number of Pounds: douze
Invalid input