How to check if string input is a number? [duplicate]

Answer 1

I also ran into problems this morning with users being able to enter non-integer responses to my specific request for an integer.

This was the solution that ended up working well for me to force an answer I wanted:

player_number = 0
while player_number != 1 and player_number !=2:
    player_number = raw_input("Are you Player 1 or 2? ")
    try:
        player_number = int(player_number)
    except ValueError:
        print "Please enter '1' or '2'..."

I would get exceptions before even reaching the try: statement when I used

player_number = int(raw_input("Are you Player 1 or 2? ") 

and the user entered "J" or any other non-integer character. It worked out best to take it as raw input, check to see if that raw input could be converted to an integer, and then convert it afterward.

Answer 2

I've been using a different approach I thought I'd share. Start with creating a valid range:

valid = [str(i) for i in range(-10,11)] #  ["-10","-9...."10"] 

Now ask for a number and if not in list continue asking:

p = input("Enter a number: ")

while p not in valid:
    p = input("Not valid. Try to enter a number again: ")

Lastly convert to int (which will work because list only contains integers as strings:

p = int(p)

Answer 3

while True:
    b1=input('Type a number:')
    try:
        a1=int(b1)
    except ValueError:
        print ('"%(a1)s" is not a number. Try again.' %{'a1':b1})       
    else:
        print ('You typed "{}".'.format(a1))
        break

This makes a loop to check whether input is an integer or not, result would look like below:

>>> %Run 1.1.py
Type a number:d
"d" is not a number. Try again.
Type a number:
>>> %Run 1.1.py
Type a number:4
You typed 4.
>>> 

Answer 4

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!")

Answer 5

the most elegant solutions would be the already proposed,

a=123
bool_a = a.isnumeric()

Unfortunatelly it doesn't work both for negative integers and for general float values of a. If your point is to check if 'a' is a generic number beyond integers i'd suggest the following one, which works for every kind of float and integer :). Here is the test:

def isanumber(a):

    try:
        float(repr(a))
        bool_a = True
    except:
        bool_a = False

    return bool_a


a = 1 # integer
isanumber(a)
>>> True

a = -2.5982347892 # general float
isanumber(a)
>>> True

a = '1' # actually a string
isanumber(a)
>>> False

I hope you find it useful :)

Answer 6

This works with any number, including a fraction:

import fractions

def isnumber(s):
   try:
     float(s)
     return True
   except ValueError:
     try: 
       Fraction(s)
       return True
     except ValueError: 
       return False