Added code to evaluate add,sub,mul,div expression

calculator_v5.py

def get_values_from_expr(expression):
    if "+" in expression:
        raw_values = expression.split("+")
        # raw_values is a list of operands like ['120 ',' 20'] with extra spaces
        values = list(map(str.strip, raw_values))
    elif "-" in expression:
        raw_values = expression.split("-")
        # raw_values is a list of operands like ['120 ',' 20'] with extra spaces
        values = list(map(str.strip, raw_values))
    elif "*" in expression:
        raw_values = expression.split("*")
        # raw_values is a list of operands like ['120 ',' 20'] with extra spaces
        values = list(map(str.strip, raw_values))
    else:
        raw_values = expression.split("/")
        # raw_values is a list of operands like ['120 ',' 20'] with extra spaces
        values = list(map(str.strip, raw_values))
    return values

def calc(expression):
    if '+' in expression:
        values = get_values_from_expr(expression)
        result = int(values[0]) + int(values[1])
    else:
        values = get_values_from_expr(expression)
        result = int(values[0]) - int(values[1])
    return result