Simple calculator using python

calcu.py

import sys

priority = {
        '+': 1,
        '-': 1,
        '*': 2,
        '/': 2,
        '^': 3,
        '(': 0
}

opsfun = {
    "+" : lambda x, y: x + y,
    "-" : lambda x, y: x - y,
    "*" : lambda x, y: x * y,
    "/" : lambda x, y: x / y,
    "^" : lambda x, y: x ** y
}


def read_number(s, i):
    return next((k for k,e in enumerate(s[i+1:]) if e in " +-*/^()"), len(s[i:]) - 1) + i + 1


def read_string(s):
    tokens = []
    last = "^"
    i = 0

    while i < len(s):

        if s[i] == " ":
            i = i + 1
            pass

        elif s[i] in "+*/^()" or (s[i] == "-" and last in "0123456789)"):
            tokens.append(s[i])
            last = s[i]
            i = i + 1

        else:
            ix = read_number(s,i)
            tokens.append(s[i:ix])
            last = s[ix-1]
            i = ix

    return tokens


def reduceWhile(ops, vals, condition):
    while ops and condition(ops[-1]):
        vals[-1] = opsfun[ops.pop()](vals[-2], vals.pop())


def calcu(tokens):
    ops, vals = [], []
    expr = read_string(tokens.strip())

    for c in expr:
        if (c == '('):
            ops.append(c)

        elif (c == ')'):
            reduceWhile(ops, vals, lambda last: last != '(')
            ops.pop()

        elif c not in ["+", "-", "*", "/", "^"]:
            vals.append(float(c))

        else:
            reduceWhile(ops, vals, lambda last: priority[c] <= priority[last])
            ops.append(c)

    reduceWhile(ops, vals, lambda x: True)
    return vals[-1]


if __name__ == "__main__":
    for line in sys.stdin:
        print(calcu(line))