1，计算器的实现

def calc_multi_div(expression):
    if "*" in expression:
        a,b=expression.split(‘*‘)
        return float(a)*float(b)
    if "/" in expression:
        a, b = expression.split(‘/‘)
        return float(a)/float(b)
def calc_sub_add(expression):
    if "+" in expression:
        a, b = expression.split(‘+‘)
        return float(a) + float(b)
    if expression[0] == ‘-‘:  ##计算加减运算时，需要注意被减数为负值的减法运算
        a,b,c =expression.split(‘-‘)
        return -float(b)-float(c)
    if "-" in expression:
        a, b = expression.split(‘-‘)
        return float(a) - float(b)
def exp_fmt(expression):           ###注意表达式的格式化
    while re.search(‘[+-]{2,}‘,expression):
        expression = expression.replace(‘--‘,‘+‘)
        expression = expression.replace(‘++‘,‘+‘)
        expression = expression.replace(‘+-‘, ‘-‘)
        expression = expression.replace(‘-+‘, ‘-‘)
    return expression
def func(regx2,regx3,exp_child):
    while 1:
        res0 = re.search(regx2, exp_child)
        if res0:
            exp_child = exp_child.replace(str(res0.group()), str(calc_multi_div(res0.group())))
        else:break
    exp_child = exp_fmt(exp_child)
    while 1:
        res0 = re.search(regx3, exp_child)
        if res0:
            exp_child = exp_child.replace(str(res0.group()), str(calc_sub_add(res0.group())))
        else:break
    return exp_child
import re
def cal(exp):
    regx1 = re.compile(‘[(]([-+*/\d+.\d+]*?)[)]‘)  ###匹配最内层括号里面的表达式
    regx2 = re.compile(‘\d+(\.\d+)?[/*]-?\d+(\.\d+)?‘)  ###匹配从左到右第一个*/表达式
    regx3 = re.compile(‘[+-]?\d+(\.\d+)?[-+]\d+(\.\d+)?‘)  # 匹配加减运算左边第一个+-运算
    while 1:
        old_exp_child = re.search(regx1,exp)
        if old_exp_child == None:
            return func(regx2,regx3,exp)
        exp_child = old_exp_child.group()
        exp_child = func(regx2, regx3, exp_child)
        exp_child=exp_child.split(‘(‘)[1].split((‘)‘))[0]
        exp = exp_fmt(exp.replace(old_exp_child.group(),exp_child))
exp= ‘1 - 2 * ((60-30 +(-40/5) * (9-2*5/-3 + 7/3*99/4*2998 +10* 568/14)) - (-4*3)/(16-3*2))‘
# exp=‘1+2.238-++317+-428-5+6‘
exp = exp.replace(" ","")
exp = exp_fmt(exp)
print(exp)
print(cal(exp))
print(eval(exp))