https://avva.livejournal.com/3098662.html

import operator as O
import re

ops = {}
ops["+"] = O.add
ops["-"] = O.sub
ops["*"] = O.mul
ops["/"] = O.div
ops["^"] = O.pow

re_ops = "[%s]" % "\\".join([" "]+ops.keys())[1:]


class OpIsNotSupported(Exception):
    pass


def calc(fst, op, snd):
    print "calc, %s %s %s" % (fst, op, snd)
    if op not in ops:
        raise OpIsNotSupported(op)
    return ops[op](fst, snd)

increment = lambda x, y: ((x or 0)*10) + int(y)


def expr_wo_brackets(s):
    (fst,  op, snd) = [None]*3
    (fstsign, sndsign) = (1, 1)
    for c in s:
        if c == "-" and op and snd is None:
            sndsign = -1
        elif c == "-" and fst is None:
            fstsign = -1
        elif '0' <= c <= '9':
            if op is None:
                fst = increment(fst, c)
            else:
                snd = increment(snd, c)
        elif re.match(re_ops, c):
            if snd is not None:
                fst = calc(fstsign * fst, op, sndsign * snd)
                op = c
                snd = None
                (fstsign, sndsign) = (1, 1)
            else:
                op = c
        else:
            raise OpIsNotSupported(c)
    return calc(fstsign * fst, op, sndsign*snd) if snd else fstsign * fst


def validate(s):
    diff = re.sub("\d+|%s|\(|\)" % re_ops, "", s)
    if diff:
        raise OpIsNotSupported(diff[0])


def calculator(s):
    s = s.replace("(-(", "(0-(")
    validate(s)
    while "(" in s:
        s = re.sub("\((\-?\d+(%s\-?\d+)*)\)" % re_ops, lambda m: `expr_wo_brackets(m.group(1))`, s)
    return expr_wo_brackets(s)


print calculator("100/(8^2+6^(-(-2)))-1")