calculations.py

import re
import sys

class ExpressionError(Exception):
    def __init__(self):
        super().__init__()
    def __str__(self):
        return "Error"

def add(a, b):
    return a+b

def sub(a, b):
    return a-b

def mult(a, b):
    return a*b

def div(a, b):
    try:
        return a/b
    except ZeroDivisionError:
        return float("inf")

operations = {
    "+": add,
    "-": sub,
    "*": mult,
    "/": div
}

def calculate(exp):
    ar_exp = re.split("\+|\-|\*|\/", exp)
    if not len(ar_exp) == 2:
        raise ExpressionError()
    oper = exp[1]
    func = operations.get(oper)
    if func is not None:
        try:
            return func(int(ar_exp[0]), int(ar_exp[1]))
        except:
            raise ExpressionError()
    else:
        raise ExpressionError()


if __name__ == "__main__":
    exp = sys.argv[1]
    try:
        print(calculate(exp))
    except ExpressionError:
        print("Error")