How can I convert a string to an int in Python?

Since you're writing a calculator that would presumably also accept floats (1.5, 0.03), a more robust way would be to use this simple helper function:

def convertStr(s):
    """Convert string to either int or float."""
    try:
        ret = int(s)
    except ValueError:
        #Try float.
        ret = float(s)
    return ret

That way if the int conversion doesn't work, you'll get a float returned.

Edit: Your division function might also result in some sad faces if you aren't fully aware of how python 2.x handles integer division.

In short, if you want 10/2 to equal 2.5 and not 2, you'll need to do from __future__ import division or cast one or both of the arguments to float, like so:

def division(a, b):
    return float(a) / float(b)

While calling your sub functions from your main functions you can convert the variables into int and then call. Please refer the below code:

import sys

print("Welcome to Calculator\n")
print("Please find the options:\n" + "1. Addition\n" + "2. Subtraction\n" + 
"3. Multiplication\n" + "4. Division\n" + "5. Exponential\n" + "6. Quit\n")

def calculator():
    choice = input("Enter choice\n")

    if int(choice) == 1:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        add(int(a), int(b))

    if int(choice) == 2:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        diff(int(a), int(b))

    if int(choice) == 3:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        mult(int(a), int(b))

    if int(choice) == 4:
        a = input("Enter first number\n")
        b = input("Enter second number\n")
        div(float(a), float(b))

    if int(choice) == 5:
        a = input("Enter the base number\n")
        b = input("Enter the exponential\n")
        exp(int(a), int(b))

    if int(choice) == 6:
        print("Bye")
        sys.exit(0)



def add(a, b):
    c = a+b
    print("Sum of {} and {} is {}".format(a, b, c))

def diff(a,b):
    c = a-b
    print("Difference between {} and {} is {}".format(a, b, c))

def mult(a, b):
    c = a*b
    print("The Product of {} and {} is {}".format(a, b, c))

def div(a, b):
    c = a/b
    print("The Quotient of {} and {} is {}".format(a, b, c))

def exp(a, b):
    c = a**b
    print("The result of {} to the power of {} is {}".format(a, b, c))

calculator()

Here what I did is I called each of the function while converting the parameters inputted to int. I hope this has been helpful.

In your case it could be changed like this:

 if choice == "0":
        numberA = raw_input("Enter your first number: ")
        numberB = raw_input("Enter your second number: ")
        print "Your result is:"
        print addition(int(numberA), int(numberB))

>>> a = "123"
>>> int(a)
123

Here's some freebie code:

def getTwoNumbers():
    numberA = raw_input("Enter your first number: ")
    numberB = raw_input("Enter your second number: ")
    return int(numberA), int(numberB)