from pprint import pprint
import re

INTEGER, ADD, SUBTRACT, MULTIPLY, DIVIDE = "INTEGER, ADD, SUBTRACT, MULTIPLY, DIVIDE".split(", ")
ORDER_OF_OPERATIONS = [MULTIPLY, DIVIDE, ADD, SUBTRACT]

scanner = re.Scanner([
    (r"([0-9]+)", lambda x, y: int(y)),
    (r"\+", lambda x, y: ADD),
    (r"-", lambda x, y: SUBTRACT),
    (r"\*", lambda x, y: MULTIPLY),
    (r"/", lambda x, y: DIVIDE),
    (r"\s+", lambda x, y: None),
], flags=re.DOTALL)

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

def operation_divide(a, b):
    return a / b

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

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

def interpret(results):
    assert isinstance(results[0], int) and isinstance(results[-1], int), "Trailing operators were encoutered!"
    if len(results) == 1:
        return results[0]
    for operation in reversed(ORDER_OF_OPERATIONS):
        try:
            boundary = results.index(operation)
            return globals()["operation_%s" % operation.lower()](
                interpret(results[:boundary]),
                interpret(results[boundary+1:]),
            )
        except ValueError:
            pass
    assert False, "Missing an operator"

test = "14 + 2 * 3 - 6 / 2"
results, remainder = scanner.scan(test)
assert not remainder, "Some of the input couldn't be interpreted"
print test, "=", interpret(results)