Python how to only accept numbers as a input

Question

mark= eval(raw_input("What is your mark?"))
try:
    int(mark)
except ValueError:
    try:
        float(mark)
    except ValueError:
        print "This is not a number"

So I need to make a python program that looks at your mark and gives you varying responses depending on what it is.

However I also need to add a way to stop random text which isn't numbers from being entered into the program.

I thought I had found a solution to this but it won't make it it past the first statement to the failsafe code that is meant to catch it if it was anything but numbers.

So pretty much what happens is if I enter hello instead of a number it fails at the first line and gives me back an error that says exceptions:NameError: name 'happy' is not defined.

How can I change it so that it can make it to the code that gives them the print statement that they need to enter a number?

Why do you even need ``eval`` in this code? It should work as it is without it. — fjarri, Dec 16 '14 at 23:55
@Bogdan because user can put command, e.g. rm -rf / using system command in python, and if python runs with root privileges, big problem. — Marcin, Dec 17 '14 at 00:12
@Marcin: this is a concern if he *uses* ``eval``, and my comment says that he *does not need to use* it. — fjarri, Dec 17 '14 at 00:46

score 11 · Answer 1 · edited May 11 '21 at 02:27

11

remove eval and your code is correct:

mark = raw_input("What is your mark?")
try:
    int(mark)
except ValueError:
    try:
        float(mark)
    except ValueError:
        print("This is not a number")

Just checking for a float will work fine:

try:
    float(mark)
except ValueError:
    print("This is not a number")

edited May 11 '21 at 02:27

Community

1
1

answered Dec 16 '14 at 23:55

Padraic Cunningham

176,452
29
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321

no worries, you're welcome, really be careful with eval, there is very rarely a good reason to use it – Padraic Cunningham Dec 16 '14 at 23:58

Arkadio · Answer 2 · 2021-03-01T22:51:41.747

4

Is it easier to declare a global value than to pass an argument, In my case it's also gives an error.

def getInput():
    global value
    value = input()
    while not value.isnumeric():
        print("enter a number")
        value = input("enter again")
    return int(value)

getInput()
print(value)

#can't comment :)

edited Mar 01 '21 at 22:51

answered Feb 28 '21 at 10:26

Arkadio

55
4

score 0 · Answer 3 · answered Dec 16 '14 at 23:55

You can simply cae to float or int and catch the exception (if any). Youre using eval which is considered poor and you add a lot of redundant statements.

try:
    mark= float(raw_input("What is your mark?"))
except ValueError:
    print "This is not a number"

"Why not use eval?" you ask, well... Try this input from the user: [1 for i in range (100000000)]

score 0 · Answer 4 · edited Nov 24 '19 at 06:50

0

import re

pattern = re.compile("^[0-9][0-9]\*\\.?[0-9]*")

status = re.search(pattern, raw_input("Enter the Mark : "))

if not status:

        print "Invalid Input"

edited Nov 24 '19 at 06:50

Abdul Hadi

1,229
1
11
20

answered Dec 17 '14 at 01:25

hariK

2,722
13
18

score 0 · Answer 5 · answered Apr 05 '19 at 08:20

you can use the String object method called isnumeric. it's more efficient than try- except method. see the below code.

def getInput(prompt):
    value = input(prompt)
    while not value.isnumeric():
        print("enter a number")
        value = input("enter again")
    return int(value)

score 0 · Answer 6 · answered Dec 01 '22 at 22:42

Might be a bit too late but to do this you can do this:

from os import system
from time import sleep
while True:
    try:
        numb = float(input("Enter number>>>"))
        break

    except ValueError:
        system("cls")
        print("Error! Numbers only!")
        sleep(1)
        system("cls")

but to make it within a number range you can do this:

from os import system
from time import sleep
while True:
    try:
        numb = float(input("Enter number within 1-5>>>"))
        if numb > 5 or numb < 1:
            raise ValueError
        else:
            break

    except ValueError:
        system("cls")
        print("Error! Numbers only!")
        sleep(1)
        system("cls")

score -1 · Answer 7 · answered Dec 16 '14 at 23:59

Actually if you going to use eval() you have to define more things.

acceptables=[1,2,3,4,5,6,7,8,9,0,"+","*","/","-"]
try:
    mark= eval(int(raw_input("What is your mark?")))
except ValueError:
    print ("It's not a number!")
if mark not in acceptables:
    print ("You cant do anything but arithmetical operations!")

It's a basically control mechanism for eval().

Python how to only accept numbers as a input

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