How to find type of user input and print different values depending on the type of input in Python 3.x

Question

Develop a Python function which either returns the float square of its parameter x if the parameter is a number, or prints the string "Sorry Dave, I'm afraid I can't do that" if the parameter is a string, and then returns 0.0.

What am I doing wrong? I'm a first year CS student and I have no previous programming background.

I created a function that takes user input, evaluates what type of input it is and print different out puts for number and strings.

For that I used eval(var) func. I also the type(var) == type to verify the type and a if-else loop.

def findt():
    userin = input("Input: ")       # Takes user input
    inpeval = eval(userin)          # Evaluates input type    

    if type(inpeval) == int:        # If input is an int
        userfloat = float(inpeval)  # Modifies into a float
        print(userfloat ** 2)       # Prints the square of the value
    elif type(inpeval) == float:    # If input is a float
        print(inpreval ** 2)        # Prints the square of the value
    elif type(userin) == str:       # If input is a string
        print("Sorry Dave, I'm afraid I can't do that")   # Print a string
        return 0.0                  # Return 0.0
    else:
        print("Invalid Input")    
findt()

When I run my code it works well when input is an int, a float or a char. But if I write more than one char it returns me an error:

NameError: name 'whateverinput' is not defined.

Answer 1

You're trying to eval input before you know it's needed. Get rid of it entirely:

def findt():
    userin = input("Input: ")       # Takes user input

    if type(userin) == int:        # If input is an int
        userfloat = float(userin)  # Modifies into a float
...

The root problem is that you can't evaluate an undefined name. If your input is a string that is not the name of an object in your program, it will fail. eval requires everything you feed it to be defined.

Answer 2

Here is a better way to achieve your goal by using the string method isnumeric() to test if the input is numeric or not.

def findt():
    userin = input("Input: ")

    if userin.isnumeric():
        # userin is numeric
        result = float(userin) ** 2
        print(result)

    else:
        try:
            # userin is a float
            result = float(userin) ** 2
            print(result)
        except ValueError:
            # userin is a string
            print("Sorry Dave, I'm afraid I can't do that")
            return 0.0

findt()

Update: a concise version:

def findt():
    userin = input("Input: ")

    try:
        # userin is an int or float
        result = float(userin) ** 2
        print(result)
    except ValueError:
        # userin is a string
        print("Sorry Dave, I'm afraid I can't do that")
        return 0.0

findt()

Answer 3

I found the solution for my problem. The way I did it I take the input from the user and i try to convert it to a float. If it is a number it will convert and print a float that is the square of the input. If the input is a string it cannot be converted to a float and will give me an error so I use an except ValueError: to print the string I want and return 0.0.

def whattype():
    user_input = input(">>> ")
    try:                                                
            user_input = float(user_input)                      
            print(user_input ** 2)
    except ValueError:
            print("Sorry Dave, I'm afraid I can't do that")
            return 0.0

whattype()

Thank you all for the suggestions and help