calc3.py

#!/usr/bin/env python3

def sw(op):
    return  [lambda a, b: a + b,
             lambda a, b: a - b,
             lambda a, b: a * b,
             lambda a, b: a / b,
             lambda a, b: "err: operação inválida!"]['+-*/'.find(op)]

from sys import argv

if len(argv) != 4:
    print(
"""Usage: 
    {0} num_1 op num_2

Ex.: 
    {0} 2 + 2
    4.0""".format(argv[0]))
    exit(-1)

num_1, op, num_2 = argv[1:]
num_1 = float(num_1)
num_2 = float(num_2)

print(sw(op)(num_1, num_2))