Implement IsNumber() function in Python

In this article we will see how to implement isNumber() method using Python. This method takes in a string as input and returns True or False according to whether the string is a number or not.
Examples:

Input :  "12345"
Output :  True

Input : "-12345" 
Output : True 

Input : "abc"
Output : False

Approach :
Here we will take advantage of the int() built-in function available in Python. Also with this approach we will see how Exception Handling comes to our aid. Using try-catch construct we try to convert the string to integer. In case the string cannot be converted the handler catches the exception that is thrown.
Below is the Implementation:

Python3

# Implementation of isNumber() function

def isNumber(s):

    # handle for negative values

    negative = False

    if(s[0] =='-'):

        negative = True

    if negative == True:

        s = s[1:]

    # try to convert the string to int

    try:

        n = int(s)

        return True

    # catch exception if cannot be converted

    except ValueError:

        return False

s1 = "9748513"

s2 = "-9748513"

s3 = "GeeksforGeeks"
 
print("For input '{}' isNumber() returned : {}".format(s1, isNumber(s1)))

print("For input '{}' isNumber() returned : {}".format(s2, isNumber(s2)))

print("For input '{}' isNumber() returned : {}".format(s3, isNumber(s3)))

Output :

For input '9748513' isNumber() returned : True
For input '-9748513' isNumber() returned : True
For input 'GeeksforGeeks' isNumber() returned : False

Note : This is not the only way to implement the isNumber() function but it is arguably the fastest way of doing so. Try/Catch doesn’t introduce much overhead because the most common exception is caught without an extensive search of stack frames.

Article Tags :

Python