python 计算器

import re
from functools import reduce

'''
分析：
  1.用户输入
  2.判断表达式是否正确
  3.计算
    1）通过正则获取（。。。）进行计算并替换（。。。）
    2）循环1）直到未找到则标记为最后一层，计算结果并返回
    计算中的注意事项：
        注意两个数的前后位置，不要颠倒了
        查找含有负数的符号，进行转换（查找负数符号最前面的-或+进行倒换，并去掉负数符号）
  4.显示结果
'''
def count(first,symbol,second):
    '''
    计算两个数的运算值
    :param first: 第一个数
    :param symbol: 运算符
    :param second: 第二个数
    :return:
    '''
    if symbol == '*':
        return float(first) * float(second)
    elif symbol=='/':
        return float(first) / float(second)
    elif symbol=='+':
        return float(first) + float(second)
    elif symbol == '-':
        return float(first) - float(second)

def count_express(group):

    '''
    计算一个字符表达式的值
    :param group: 要运算的字符
    :return:
    '''
    result = re.findall('[0-9]+[.0-9]*|[^0-9]', group)
    list1 = list(reversed(list(result)))
    first = ''
    symbol = '+'
    second = '0'
    first = list1.pop()
    stack =[]
    # '1+2*(2+3*4-4)'
    if first == '-':
        first += list1.pop()
    while len(list1) != 0:
        symbol = list1.pop()
        second = list1.pop()
        if symbol == '-' or symbol == '+':
            stack.append(first)
            stack.append(symbol)
            first = second
            continue
        first = str(count(first, symbol, second))
    while len(stack) > 1:
        symbol = stack.pop()
        second = stack.pop()
        first = count(second, symbol, first)
    return str(first)
def group_count(express):
    '''
    计算表达式值
    :param express: 字符表达式
    :return:
    '''

    while True:
        pattern = r'([^()]+)'
        pat = re.compile(pattern)
        group = pat.search(express)
        result = []
        # 判断是否找到（）
        flag = True
        if group != None:
            group = group.group()
            group = group.replace('(','').replace(')','')
        else:
            group = express
            flag = False
        result = count_express(group)
        # 如果是最后一层，直接返回值
        if flag is False:
            return result
        # 将express中找到(..)的运算结果进行替换
        express = pat.sub(result,express)
        express = replace_symbol(express)
        print(express)
def replace_symbol(express):
    '''
    将负数转正
    :param express:
    :return:
    '''
    split_list = re.split('[+*/-]-', express)
    b = split_list[0]
    # 字符反转
    c = reduce(lambda x, y: y + x, b)
    # 查找离要负号位最近是（+-）进行替换
    d = re.search('[+-]', c)
    f = d.group()
    m = '+' if f == '-' else '-'
    list_str = list(c)
    list_str[d.span()[0]] = m
    list_str.reverse()
    c = ''.join(list_str)
    return c +express[len(c)]+ split_list[1]
def check_express(express):
    '''
    校验输入的表达式是否正确
    :param express: 字符表达式
    :return:
    '''
    pattern1 = '[^0-9+*/.()-]|[^0-9][+/*.]|[0-9][(]|^[+/*.]|[+/*.]$'
    result = re.search(pattern1,express)
    return result

def input_express():
    # express = input("请输入（只能包含0-9+-*/.（）字符）：")
    express = '-1+2*3-4/(-10*2)-3'
    express = re.sub(' ', '', express)
    result = check_express(express)
    if result != None:
        span = str(result.span())
        group = str(result.group())
        print('输入了错误的字符或表达式格式不对，所在位置：%s，错误字符：%s' % (span, group))
        sys.exit(0)
    return express

if __name__ == '__main__':
    express = input_express()
    result = group_count(express)
    print('结果：%s' %result)