Python Cookbook

Testing Whether a String Represents an Integer

Credit: Robin Parmar

Problem

You want to know whether the contents of a string represent an integer (which is not quite the same thing as checking whether the string contains only digits).

Solution

try/except is almost invariably the best approach to such validation problems:

def isInt(astring):
    """ Is the given string an integer? """
    try: int(astring)
    except ValueError: return 0
    else: return 1

Testing if a string contains only digits is a different problem:

def isAllDigits(astring):
    """ Is the given string composed entirely of digits? """
    # In Python 2.0 and later, "astring.isdigit(  ) or not astring" is faster
    import string
    acceptable_characters = string.digits
    for acharacter in astring:
        if acharacter not in acceptable_characters:
             return 0
    return 1

Discussion

It’s always a good idea to make a Python source file runnable as a script, as well as usable via import. When run as a script, some kind of unit test or demo code executes. In this case, for example, we can finish up the little module containing this recipe’s functions with:

if _ _name_ _ == '_ _main_ _':
    print isInt('23')
    print isInt('sd')
    print isInt('233835859285')
    print isAllDigits('233835859285')

Running the module as a script will now confirm that 23 represents an integer, sd does not, and neither does 233835859285 (because it’s too large—it would need an integer greater than sys.maxint, which is impossible by definition). However, as the fourth and last print statement shows, even ...