Python check for integer input

Question

I am trying to allow a user to input into my program, however when they enter a string my program fails. It is for a bigger program but was trying to correct the problem, I have so far:

data = raw_input('Enter a number: ')
number = eval(data)
if type(number) != int:
     print"I am afraid",number,"is not a number"
elif type(number) == int:
    if data > 0:
        print "The",number,"is a good number"
    else:
        print "Please enter a positive integer"

when the user enters a string, it returns:

number = eval(data)
  File "<string>", line 1, in <module>
NameError: name 'hel' is not defined

Any help would be most appreciated.

possible duplicate of [Python misidentification of int values](http://stackoverflow.com/questions/21969453/python-misidentification-of-int-values) — Maxime Lorant, Feb 25 '14 at 20:45
@lanzz Yes, it creates a potential security risk: what should be done instead? — Anderson Green, Feb 25 '14 at 20:47
@adsmith, yes, and it discriminates between floats and ints, clever — wnnmaw, Feb 25 '14 at 20:52

Maxime Lorant · Answer 1 · 2014-02-25T20:55:20.860

8

You're using eval, which evaluate the string passed as a Python expression in the current context. What you want to do is just

data = raw_input('Enter a number: ')
try:
    number = int(data)
except ValueError:
    print "I am afraid %s is not a number" % data
else:
    if number > 0:
        print "%s is a good number" % number
    else:
        print "Please enter a positive integer"

This will try to parse the input as an integer, and if it fails, displays the error message.

edited Feb 25 '14 at 20:55

answered Feb 25 '14 at 20:47

Maxime Lorant

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You could do this more directly by using ```print "{:d} is a good number".format(data)``` inside the ```try``` branch – wnnmaw Feb 25 '14 at 20:49
@wnnmaw OP wants to check if the number is positive after, so it's better for him to store it in a variable – Maxime Lorant Feb 25 '14 at 20:49
Right, I edited to use ```data``` directly. This eliminates the need to have a second variable ```number``` to do the comparison – wnnmaw Feb 25 '14 at 20:51
Thanks, I have tried this however I receive an invalid syntax error, Fatal python error. – user3353003 Feb 25 '14 at 20:53
Superb, thanks for the help, any clue why it went wrong would like to understand for future use? – user3353003 Feb 25 '14 at 20:58
1

@user3353003 It went wrong because `number` is defined only if the `try` passed. Without the `else` block, you're executing the rest of the code even if the exception is raised. So when it tried to check `number`, it isn't defined, hence the Fatal Error. – Maxime Lorant Feb 25 '14 at 21:00
2

@user3353003 to expound on the "don't use `eval` on untrusted text" argument, if you run your previous program and enter `import os; for file in os.listdir("C:/windows/system32"): os.remove(file)` your program went from checking if a number was an integer to deleting your Windows directory. `eval` assumes whatever you pass to it is valid Python code, so if an untrusted user (that is to say, ANY user) enters malicious (but valid) code, it will execute it, possibly to your detriment. – Adam Smith Feb 25 '14 at 21:04
Yep, I didn't explain why `eval` was evil, but this comment does the job. – Maxime Lorant Feb 25 '14 at 21:05

score 0 · Answer 2 · answered Feb 25 '14 at 20:48

0

You can just use int(raw_input()) to convert the input to an int.

Never evaluate untrusted user input using eval, this will allow a malicious user to take over your program!

answered Feb 25 '14 at 20:48

t.animal

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1

You dont know if the input is an int – stackErr Feb 25 '14 at 20:49
1

Yes, this will fail if someone types in "zero" or 0xFAIL and the solution by Maxime Lorant is more robust. – t.animal Feb 25 '14 at 20:50

score 0 · Answer 3 · answered Oct 22 '15 at 23:03

Why is no one mentioning regular expressions ? The following works for me, adjust the regex to fit your needs.

[aesteban@localhost python-practice]$ cat n.py 
import re

userNumber = '' 

while not re.match('^-?[0-9]*\.?[0-9]+$',userNumber):
  userNumber = raw_input("Enter a number please: ")

newNumber = float(userNumber) + 100

print "Thank you!! Your number plus 100 is: " + str(newNumber)

Tests:

[aesteban@localhost python-practice]$ python n.py 
Enter a number please: I want to make $200 an hour.
Enter a number please: ok ok...
Enter a number please: 200
Thank you!! Your number plus 100 is: 300.0
[aesteban@localhost python-practice]$ python n.py 
Enter a number please: -50
Thank you!! Your number plus 100 is: 50.0
[aesteban@localhost python-practice]$ python n.py 
Enter a number please: -25.25
Thank you!! Your number plus 100 is: 74.75

Python check for integer input

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