任意输入含加减乘除乘方开方的文本数学表达式，能检查语法错误，并计算结果

def str2float(s):
    def fn(x, y):
        return x * 10 + y
    def char2num(s):
        return {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}[s]
    def fun(s):
        return {'+':add, '-':sub, '*':mul, '/':div}[s]
    def calc(s):
        num, funs, n = 0, [], 0
        num = reduce(fn, map(char2num, s.replace('.', '')))
        for c in s:
            if c in '+-*/':
                funs.append(fun(c))
                n -= 1
        return reduce(lambda x, y: y(x, nums[n]), funs, num)
    return calc(s)