计算器

import re

num =input('please input your equation:').strip()
num =num.replace(' ','')

def add(s): #计算加减,s是字符串,如s='4+5-4'
    add_num =f(s)
    add_num =add_num.split('+')
    sum = 0
    sum_1 = 0
    for i in add_num:
        if '-' in i:
            s2 = i.split('-')
            for i in s2:
                if  i:
                    sum_1 = sum_1-float(i)
            if  s2[0]:
                sum_1=float(s2[0])*2+sum_1
        else:
            sum += float(i)

    sum = sum + sum_1
    return str(sum)

def muldiv(s):#进行乘除
    if not re.search('d+.?d*[*/]-?d+.?d*',s):
        return s
    res =re.search('d+.?d*[*/]-?d+.?d*',s)
    res =res.group()
    if '*'in res:
        a,b=res.split('*')
        res_num =float(a)*float(b)
    else:
        a,b=res.split('/')
        res_num =float(a)/float(b)

    s =s.replace(res,str(res_num))
    return muldiv(s)

def f(s): #把'+-'变成'-'
    res=s.replace('+-','-')
    res=res.replace('--','+')
    return res

while re.search('([^()]+)',num):
    res = re.search('([^()]+)', num)
    res1 = res.group()
    res =res1[1:-1]
    res_num = muldiv(res)  # 把乘除进行计算了
    res_num = add(res_num)  # 把加减进行计算了
    num =num.replace(res1, res_num)
else:
    res_num =muldiv(num)
    res_num =float(add(res_num))
print('%.2f'%res_num)