Reverse Polish Notation in Python

stack = []

operations = {
    '+': lambda a, b: a + b,
    '-': lambda a, b: a - b,
    '*': lambda a, b: a * b,
    '/': lambda a, b: a / b
}

def calculate(a, b, sign):
    return operations.get(sign)(a, b)


def main():
    data = input("Dej mnie równanie: ").split()
    for item in data:
        if item in operations.keys():
            b = stack.pop()
            a = stack.pop()
            stack.append(calculate(a, b, item))
        else:
            try:
                stack.append(float(item))
            except ValueError:
                print("Co to za guwno")
                return
    print(stack.pop())

main()