A humble calculator

calculator.py

import re
from copy import deepcopy
import operator

RE_NUM = re.compile('\s*([-+]?\d+(\.\d+)?)')


def number(word):
    result = RE_NUM.match(word)
    groups = result.groups()
    num_str = result.groups[0]
    
    if groups[1] is not None:
        return float(num_str)
    else:
        return int(num_str)


class Chain:
    
    _dict = {
        '+': {'w':2, 'func': operator.add},
        '-': {'w':2, 'func': operator.sub},
        '*': {'w':4, 'func': operator.mul},
        '/': {'w':4, 'func': operator.truediv},
        '(': {'w':5, 'func': None},
        ')': {'w':-5, 'func': None}
    }

    _border = {'w': -100000, 'func': None}

    def __init__(self, raw_string):
        self._operators = []
        self._num = []
        
        base = 0
        
        for i in raw_string.split():
            if i in set(Chain._dict.keys()):
                if i in set('()'):
                    base = base + Chain._dict[i]['w']
                else:
                    oper = deepcopy(Chain._dict[i])
                    oper['w'] += base
                    self._operators.append(oper)
            else:
                self._num.append(number(i))

        if base != 0:
            pass #error

    def __len__(self):
        return len(self._operators)

    def __getitem__(self, i):
        if i < 0 or i >= len(self):
            return Chain._border
        else:
            return self._operators[i]

    def reduce_chain(self, i):
        self._num[i] = \
            self._operators[i]['func'](self._num[i], self._num[i+1])

        del(self._operators[i])
        del(self._num[i+1])

    def result(self):
        return self._num


i = 0
raw_string = '1 + 2 + ( 2 * 3 + 1 ) * 2 + 2 * 3 + ( 1 + 2 * 8 )'
chain = Chain(raw_string)

while len(chain) != 0:
    j = max((i, i-1, i+1), key=lambda a: chain[a]['w'])
    
    if i == j:
        chain.reduce_chain(i)
        if i == len(chain):  # not necessary but ...
            i = i-1
    else:
        i = j

    print(chain.result())