Converting strings to int/float if user input is valid

Question

Hi I am curious if there is a way to convert user input to an specific variable type based off the content of the input (I'm using raw_input(), not sure if there's a better function for this). I know raw_input() always makes a string, but can said string be converted to an integer or a floating point if the input is valid? For example, if the user enters '1,' is there a way to have that variable stored as an integer rather than a string? Here is the code I am attempting to make work:

print "Subtraction Calculator (x-y)"

def calculator(x, y):
  if [isinstance(x, int) or isinstance(x, float)] and [isinstance(y, int) or 
isinstance(y, float)]: #This should be checking to see if x and y are both
numerical
  D = x - y 
  return D
 else:
  return "One or more inputs is not a number."

x = raw_input("x = ")
y = raw_input("y = ")

print calculator(x, y)

Obviously this code doesn't work as x and y are both strings due to raw_input(), but I don't seem to be getting the proper error message ("One or more inputs is not a number.") and am instead getting an error at (D = x - y). I believe that this is due to my 'if' statement always registering as True, but I am not sure why this is either.

you can change string to int by use of int() method like x= int(raw_input("x = ")) — mukesh kudi
– mukesh kudi, Commented Dec 22, 2017 at 19:16
[isinstance(x, int) or isinstance(x, float)] is a non-empty list and thus true. — Stefan Pochmann
– Stefan Pochmann, Commented Dec 22, 2017 at 19:25

enumaris · Accepted Answer · 2017-12-22 19:42:26Z

1

You can do this using exception handling. Basically, the idea is simply try to convert x and y into floats and do the calculation you want. If x or y are not able to be converted to floats, python will raise a ValueError which you can catch in a try and except statement. You could make a loop so that you keep asking the user to input x and y until it works like so:

    while(True):
        try:
            x = raw_input('x = ')
            y = raw_input('y = ')
            print(float(x)-float(y))
            break
        except ValueError:
            print('One of those was not a float! Try again!')

answered Dec 22, 2017 at 19:42

enumaris

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Julien Spronck · Accepted Answer · 2017-12-22 19:27:44Z

0

What about simply returning the result that you want and if one of the two cannot be converted to a number, it will raise a ValueError:

def calculator(x, y):
    return float(x) - float(y)

x = raw_input("x = ")
y = raw_input("y = ")

print calculator(x, y)

Note that if you do want to check if a variable is either an int or float you can use isinstance(x, (int, float)). Therefore, your code can be rewritten as:

def calculator(x, y):
    if isinstance(x, (int, float)) and isinstance(y, (int, float)):
        return x - y
    return "One or more inputs is not a number."

answered Dec 22, 2017 at 19:27

Julien Spronck

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