My calc solution following the multiplication stack

Calc

__author__ = 'gregrice'

from functools import reduce
from itertools import zip_longest
import operator

OPS = {"+": operator.add, "-": operator.sub, "*": operator.mul, "/": operator.truediv}

def eval_string(calc_string):
    parsed = calc_string.split(" ")
    ops = [OPS[n] for n in parsed if n in OPS]
    numbers = [int(n) for n in parsed if n not in OPS]
    add_stack = []
    add_ops = []
    mult_stack = []
    mult_ops = []
    for n,o in zip_longest(numbers, ops):
        if o in {operator.sub, operator.add}:
            if mult_stack:
                mult_stack.append(n)
                cleared_stack = clear_op_stack(mult_stack, mult_ops)
                add_stack.append(cleared_stack)
                mult_stack, mult_ops = [], []
            else:
                add_stack.append(n)
            add_ops.append(o)
        else:
            mult_stack.append(n)
            mult_ops.append(o)
    add_stack.append(clear_op_stack(mult_stack, mult_ops))
    return int(clear_op_stack(add_stack, add_ops))


def clear_op_stack(numbers, ops):
    if not numbers:
        return 0
    else:
        first = numbers[0]
        remaining = numbers[1:]
        return reduce(lambda x, y: y[0](x, y[1]), zip(ops, remaining), first)

if __name__ == "__main__":
    print(eval_string("1 + 2 * 3 * 4 / -2 + 1 + 0"))
    print(eval_string("1 + 2 + 3"))
    print(eval_string("1"))