calc2.py

#!/usr/bin/env python3

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

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))