python input int only from input statement

Question

I'm new to python and am trying to check if startmiles and endmiles are input as integer only and gallons are input as float. When I enter alpha the program crashes. Thank You

#!/usr/bin/env python

import os
import sys
import subprocess
import datetime 
clear = lambda: os.system('clear')

clear()

t = datetime.datetime.now()
from time import gmtime, strftime
strftime("%a, %d %b %Y %X +0000", gmtime())
today = strftime("%a, %d %b %Y ")
print(today,'\n')

def main():
    startmiles = input("Enter Your Start Miles = ")
    endmiles = input("Enter Your Ending Miles = ")
    gallons = input("Enter Your Gallons = ")
    mpg = (int(endmiles) - int(startmiles)) / float(gallons)
    print("Miles Per Gallon =", round(mpg, 2))
    answer = input("\n Go Again Y or n ").lower()
    if answer == 'y': print('\n'), main()
    if answer != 'y': print('\n Thank You Goodbye')



if __name__=="__main__":
    main() # must be called from last line in program to load all the code

`when I enter alpha the program crashes` It would be much easier to help if you posted the exact error message, instead of vaguely telling us "the program crashes". — John Gordon, May 21 '17 at 03:36

score 0 · Answer 1 · edited May 23 '17 at 11:47

In Python 2.x the input should result in an integer anyways, unless you use raw_input. It looks like your mpg variable is taking the integer of the endmiles anyways.

In Python 3.x

startmiles = int(input("Enter Your Start Miles = "))

It sounds like you're saying certain characters crash the program, which means you need to do some exception handling. See this question response for more detail, but you need to find a way to handle your exceptions instead of crashing. Here's an example below forcing proper entry of the variable.

while True:
    try:
        # Note: Python 2.x users should use raw_input, the equivalent of 3.x's input
        age = int(input("Please enter your age: "))
    except ValueError:
        print("Sorry, I didn't understand that.")
        #better try again... Return to the start of the loop
        continue
    else:
        #age was successfully parsed!
        #we're ready to exit the loop.
        break
    if age >= 18: 
        print("You are able to vote in the United States!")
    else:
        print("You are not able to vote in the United States.")

John Gordon · Accepted Answer · 2017-05-21T03:46:56.060

Convert your inputs to int/float as soon as they are entered, wrapped in a try/except block in a loop to catch exceptions and immediately prompt again for more input:

while True:
    try:
        startmiles = int(input("Enter Your Start Miles = "))
        break
    except ValueError:
        print('that is not an integer, please try again')

Or just wrap your final calculation in a try/except:

try:
    mpg = (int(endmiles) - int(startmiles)) / float(gallons)
except ValueError:
    print('some error message')

But this way you can't easily ask for more input, and you don't know exactly which value caused the error.

Thank You this fixed my problem – marvin May 21 '17 at 14:50 — marvin, May 21 '17 at 14:50

python input int only from input statement

2 Answers2