How to check if string input is a number?

Question

Simply try converting it to an int and then bailing out if it doesn't work.

try:
    val = int(userInput)
except ValueError:
    print("That's not an int!")

See Handling Exceptions https://docs.python.org/3/tutorial/errors.html#handling-exceptions

Apparently this will not work for negative values, but it will for positive numbers.

Use isdigit()

if userinput.isdigit():
    #do stuff

The method isnumeric() will do the job (Documentation for python3.x):

>>>a = '123'
>>>a.isnumeric()
True

But remember:

>>>a = '-1'
>>>a.isnumeric()
False

isnumeric() returns True if all characters in the string are numeric characters, and there is at least one character.

So negative numbers are not accepted.

For Python 3 the following will work.

userInput = 0
while True:
  try:
     userInput = int(input("Enter something: "))       
  except ValueError:
     print("Not an integer!")
     continue
  else:
     print("Yes an integer!")
     break 

EDITED: You could also use this below code to find out if its a number or also a negative

import re
num_format = re.compile("^[\-]?[1-9][0-9]*\.?[0-9]+$")
isnumber = re.match(num_format,givennumber)
if isnumber:
    print "given string is number"

you could also change your format to your specific requirement. I am seeing this post a little too late.but hope this helps other persons who are looking for answers :) . let me know if anythings wrong in the given code.