calculator.py

#basic calculator
import re
import operator

ops = { "1": operator.add, "2": operator.sub, "3": operator.div, "4": operator.mul }

def error():
	print "Sorry you didn't input a valid command."

def exit_func():
	for i in (range(1,4)[::-1]):
		print "Exiting in "+str(i)+"..."
	exit()

def check_for_continue(answer):
	if answer.upper()=="Y":
		print "Continuing..."
		return True
	elif answer.upper()=="N":
		exit_func()
	else:
		check_for_continue(raw_input("Let's try that again... Y to continue, N to exit: "))

def input1(user_input):
	if not check(user_input): #if check returns False, i.e. it's not a number...
		error()
		if check_for_continue(raw_input("Y to continue, N to exit: "))==True: #will loop as long as you choose to continue
			return input1(raw_input("Choose your first number again: "))
	else:
		print "First number chosen was: " + str(user_input)
		return user_input

def input2(user_input):
	if not check(user_input):
		error()
		if check_for_continue(raw_input("Y to continue, N to exit: "))==True:
			return input2(raw_input("Choose your second number again: "))
	else:
		print "Second number chosen was: " + str(user_input)
		return user_input

def operator(user_input):
	if user_input == "+":
		return ops["1"]
	elif user_input == "-":
		return ops["2"]
	elif user_input == "/":
		return ops["3"]
	elif user_input == "*":
		return ops["4"]
	else:
		error()
		if (check_for_continue(raw_input("Y to continue, N to exit: ")))==True:
			return operator(raw_input("Choose your operator again: "))

def check(user_input):
	match = re.search("\D", user_input) #re is a regex module, and \D returns True for non-digits
	if not match:
		return True
	else:
		return False

def in_use(user_input):
	if user_input.upper()=="Y":
		print "\nCalculator 1.0 up and running, note only integers and basic mathematical operations allowed."
		a=int(input1(raw_input("First number: ")))	
		b=int(input2(raw_input("Second number: ")))
		print operator(raw_input("Choose an operator: "))(a,b)
		in_use(raw_input("Y to use it again, N to quit: "))
	if user_input.upper()=="N":
		exit_func()
	else:
		print "I didn't quite catch that... Again?"
		in_use(raw_input("Y to use it, N to quit: "))

in_use(raw_input("Y to turn on the calculator, N to quit: "))