calc.py

from decimal import Decimal
def apply_high_order(ind, element, expression_elements):
    if element in ("*", "/"):
        if element == "*":
            evaluated_num = str(float(expression_elements[ind - 1]) * float(expression_elements[ind + 1]))
        else:
            evaluated_num = str(float(expression_elements[ind - 1]) / float(expression_elements[ind + 1]))
        expression_elements = expression_elements[:ind - 1] + [evaluated_num] + expression_elements[ind + 2:]
    elif element == "(":
        if expression_elements[ind + 2] == "+":
            evaluated_num = str(float(expression_elements[ind + 1]) + float(expression_elements[ind + 3]))
        elif expression_elements[ind + 2] == "-":
            evaluated_num = str(float(expression_elements[ind + 1]) - float(expression_elements[ind + 3]))
        elif expression_elements[ind + 2] == "*":
            evaluated_num = str(float(expression_elements[ind + 1]) * float(expression_elements[ind + 3]))
        elif expression_elements[ind + 2] == "/":
            evaluated_num = str(float(expression_elements[ind + 1]) / float(expression_elements[ind + 3]))
        expression_elements = expression_elements[:ind] + [evaluated_num] + expression_elements[ind + 5:]
    return expression_elements

def calc(expression):
    expression_elements = expression.split(" ")
    while "*" in expression_elements or "/" in expression_elements or "(" in expression_elements:
        for ind, element in enumerate(expression_elements):
            if element in ("(", "*", "/"):
                expression_elements = apply_high_order(ind, element, expression_elements)
                break
    operators = []
    for element in expression_elements:
        try:
            float(element)
        except:
            operators.append(element)
            expression_elements.remove(element)

    expression_elements.reverse()
    operators.reverse()

    while len(expression_elements) != 1:
        ln = len(expression_elements)
        a = expression_elements.pop()
        b = expression_elements.pop()
        opr = operators.pop()
        if opr == "+":
            print("{}{}{}".format(a, "+", b))
            expression_elements.append(str(Decimal(a) + Decimal(b)))
            print expression_elements
        else:
            print("{}{}{}".format(a, "-", b))
            expression_elements.append(str(Decimal(a) - Decimal(b)))

    return expression_elements[0]

# example:
# calc("( 5 + 5 ) * 2 + 10 * 2")