Python: Error while making foolproof input method using for loop

Question

digits = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '0']

def inp():
    inp = input('Enter a value: ')

    for i in inp:
        if i not in digits:
            print('Please enter a value from 2 to 100')
            inp()

    n = int(inp)
    if not 2 <= n <= 100:
        print('Please enter a value from 2 to 100')
        inp()
    else:
        return n

number = inp()

I am unable to make it foolproof: user can enter an empty string "" or float, and it gives an error.

For example: If I enter 10.5 and then 10 then it shows error that 10.5 cannot be taken as integral value.

If there would be a simple function like val() from VB6 or something equivalent for python, I would be very grateful.

Edit: Expected behaviour: I want the code to take only integral value, 2 to 100 as input, and if incorrect value is entered then it should repeat until an appropriate value is received.

Please help.

I have to re-ask since https://stackoverflow.com/questions/69658320/error-while-making-foolproof-input-method-using-for-loop was closed and my problem was not solved. — Ishan Jindal, Oct 21 '21 at 12:00
What is the purpose of the full-proof test? Please describe the expected behavior (or your question might be closed again) — mozway, Oct 21 '21 at 12:01
Also, avoid the recursive approach, an infinite loop with a break on condition is much better — mozway, Oct 21 '21 at 12:03
"infinite loop with a break on condition" --- I am unaware of that, fellow coder Are you referring to "while True break"?? — Ishan Jindal, Oct 21 '21 at 12:08
the main problem is solved. anyways, is there an val() equivalent function from VB6 in Python, or in some library i can import? — Ishan Jindal, Oct 21 '21 at 13:23

score 0 · Accepted Answer · answered Oct 21 '21 at 12:47

I found the solution to be this:

def inp_w():
    while True:
        try:
            inp = round(float(input('Width of the maze (number of blocks): ')))
        except:
            print('Please enter a value from 2 to 100')
            continue
        else:
            break
    
    if not 2 <= inp <= 100:
        print('Please enter a value from 2 to 100')
        inp_w()
    else:
        return inp


a = inp_w()

Thanks to
"https://stackoverflow.com/users/16343464/mozway"
"https://stackoverflow.com/users/13000953/mahrkeenerh"
"https://stackoverflow.com/users/9635106/karl-wilhelm"
"https://stackoverflow.com/users/16601101/jeandemeusy"

jeandemeusy · Answer 2 · 2021-10-21T12:36:01.980

There is three errors you are making:

you named a method and a variable the same way. The interpreter doesn't know what to do when it comes to "inp()". Are you trying to call method "inp" or the variable "inp" ? One of them is impossible to do, so it crashes. As y good practice, never give the same name to different element in your code.
you are missing the character '.' in your list of digits. When parsing the string "10.5" it doesn't recognizes the dot, and thus crashes.
the conversion from string to int is not able to convert "10.5" to 10, because of the floating point part. So convert it to a float instead of an int.

Here is a working code:

valid_chars = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0", "."]


def input_val():
    inp = input("Enter a value: ")

    for i in inp:
        if i not in valid_chars:
            print("Please enter a value from 2 to 100")
            return input_val()

    n = float(inp)
    if not 2 <= n <= 100:
        print("Please enter a value from 2 to 100")
        return input_val()

    return n


number = input_val()

Edit: A better answer, without recursion could be something like this:

def is_float(string):
    try:
        float(string)
        return True
    except:
        return False


def input_val(min_val: int, max_val: int):

    while True:
        inp = input("Enter a value: ")

        if not is_float(inp):
            print("Please enter a number")
        else:
            n = float(inp)
            if not min_val <= n <= max_val:
                print(f"Please enter a value from {min_val} to {max_val}")
            else:
                break
    return n


number = input_val(2, 100)

And what for an empty string "" ? It still gives an error when I pass a condition for the same and seems to save the value from last iteration even after taking input. — Ishan Jindal, Oct 21 '21 at 12:16
Just add a condition after the input. something like "if len(inp) == 0: print("error") return input_val()" — jeandemeusy, Oct 21 '21 at 12:19
I know, but I kept his structure to make his code work. Personally I would have used a while loop with a flag to break out. — jeandemeusy, Oct 21 '21 at 12:23

Python: Error while making foolproof input method using for loop

2 Answers2