Simple Reverse Polish notation calculator.

ReversePolishNotationCalculator.py

"""
Simple Reverse Polish notation calculator.
Uses a deque as a queue to push and pop operands from and eval to perform operations.

"""

from collections import deque

def isNumber(val):
	"""
	Validates input str is a real number.

	Args:
		val (str): Input str to validate.

	Returns:
		True if val is a real number.
		False otherwise.
	"""
	if val.isdigit():
		return True
	try:
		float(val)
	except ValueError:
		return False
	return True

def getValidInput():
	"""
	Validates user input. Prompts for new input if not valid.

	Returns:
		User input when it is valid.

	"""

	valid_ops = ('+', '-', '*', '/', '%')	
	inp = str(raw_input('> '))
	
	if (not isNumber(inp)) and (inp not in valid_ops) and (inp != 'done'):
		print "Please enter valid input."
		inp = getValidInput()
	return inp

def runCalc():
	"""
	Starts calculator and begins prompting user for input.
	
	"""
	print "Type 'done' to exit."
	# Initialize queue for operands
	queue = deque()
	# Get initial input
	inp = getValidInput()

	while inp != "done":
		# If it's a number push to queue...
		if isNumber(inp):
			queue.append(inp)
		# ... otherwise it is an operator so pop operands from queue
		elif len(queue) >= 2:
			op = inp
			cmd = "%s %s %s" % (queue.popleft(), op, queue.popleft())
			result = eval(cmd)
			print result
			queue.appendleft(result)
		# If there aren't enough operands in queue then ignore operator.
		else:
			print "Not enough operands in queue to perform operation."
		inp = getValidInput()