Doing Math on User Inputs

Our script works, but now suppose that instead of converting a text string to uppercase, we want to do some math with numeric input—squaring it, for example, perhaps in some misguided effort to discourage users who happen to be obsessed with youth. We might try statements like these to achieve the desired effect:

>>> reply = '20'
>>> reply ** 2
...error text omitted...
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'

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This won’t quite work in our script, though, because (as discussed in the prior part of the book) Python won’t convert object types in expressions unless they are all numeric, and input from a user is always returned to our script as a string. We cannot raise a string of digits to a power unless we convert it manually to an integer:

>>> int(reply) ** 2
400

Armed with this information, we can now recode our loop to perform the necessary math. Type the following in a file to test it:

while True:
    reply = input('Enter text:')
    if reply == 'stop': break
    print(int(reply) ** 2)
print('Bye')

This script uses a single-line if statement to exit on “stop” as before, but it also converts inputs to perform the required math. This version also adds an exit message at the bottom. Because the print statement in the last line is not indented as much as the nested block of code, it is not considered part of the loop body and will run only once, after the loop is exited:

Enter text:2
4
Enter text:40
1600
Enter text:stop
Bye

One note here: I’m assuming that this code is stored in and run from a script file. If you are entering this code interactively, be sure to include a blank line (i.e., press Enter twice) before the final print statement, to terminate the loop. The final print doesn’t quite make sense in interactive mode, though (you’ll have to code it after interacting with the loop!).