Loop until an input is of a specific type

Question

I'm trying to make a program that repeatedly asks an user for an input until the input is of a specific type. My code:

value = input("Please enter the value")

while isinstance(value, int) == False:
     print ("Invalid value.")
     value = input("Please enter the value")
     if isinstance(value, int) == True:
         break

Based on my understanding of python, the line

if isintance(value, int) == True
    break

should end the while loop if value is an integer, but it doesn't.

My question is:
a) How would I make a code that would ask the user for an input, until the input is an integer?
b) Why doesn't my code work?

what! input return int if given. try input()+1 and give 2 as input/ — FUD, Oct 23 '13 at 13:24
In Python3, `input()` returns string. And in Py2, `input()` is the same as `eval(input())` in Py3 — JadedTuna, Oct 23 '13 at 13:26

score 2 · Accepted Answer · answered Oct 23 '13 at 13:26

2

The reason your code doesn't work is because input() will always return a string. Which will always cause isinstance(value, int) to always evaluate to False.

You probably want:

value = ''
while not value.strip().isdigit():
     value = input("Please enter the value")

answered Oct 23 '13 at 13:26

Hoopdady

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Also, you may want `value.strip().isdigit()` – JadedTuna Oct 23 '13 at 13:26

Olav · Answer 2 · 2013-10-23T14:17:13.163

1

Be aware when using .isdigit(), it will return False on negative integers. So isinstance(value, int) is maybe a better choice.

I cannot comment on accepted answer because of low rep.

edited Oct 23 '13 at 14:17

answered Oct 23 '13 at 14:11

Olav

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score 0 · Answer 3 · answered Oct 23 '13 at 13:30

0

input always returns a string, you have to convert it to int yourself.

Try this snippet:

while True:
    try:
        value = int(input("Please enter the value: "))
    except ValueError:
        print ("Invalid value.")
    else:
        break

answered Oct 23 '13 at 13:30

OdraEncoded

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score 0 · Answer 4 · answered Oct 23 '13 at 14:32

0

If you want to manage negative integers you should use :

value = ''
while not value.strip().lstrip("-").isdigit():
    value = input("Please enter the value")

answered Oct 23 '13 at 14:32

mrlouhibi

121
4

Loop until an input is of a specific type

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