Solution: Calculator

Here I used the format method of the strings to insert the value of op in the {} placeholder. We'll learn about this later on.

def main():
    a = float(input("Number: "))
    b = float(input("Number: "))
    op = input("Operator (+-*/): ")

    if op == '+':
        res = a+b
    elif op == '-':
        res = a-b
    elif op == '*':
        res = a*b
    elif op == '/':
        res = a/b
    else:
        print(f"Invalid operator: '{op}'")
        return

    print(res)
    return


main()

• For historical reasons we also have the solution in Python 2

from __future__ import print_function

a = float(raw_input("Number: "))
b = float(raw_input("Number: "))
op = raw_input("Operator (+-*/): ")

if op == '+':
    res = a+b
elif op == '-':
    res = a-b
elif op == '*':
    res = a*b
elif op == '/':
    res = a/b
else:
    print("Invalid operator: '{}'".format(op))
    exit()


print(res)