Python嵌套字典循环缩进问题及TypeError报错求助

问题描述 #

我正在学习《Python编程快速上手：让繁琐工作自动化（第2版）》，尝试在for循环后添加缩进插入num_brought值时，持续报错：

TypeError: unsupported operand type(s) for +: 'int' and 'str'

书本参考代码 #

allGuests = {'Alice': {'apples': 5, 'pretzels': 12},
             'Bob': {'ham sandwiches': 3, 'apples': 2},
             'Carol': {'cups': 3, 'apple pies': 1}}

def totalBrought(guests, item):
    numBrought = 0
    for k, v in guests.items():
        numBrought = numBrought + v.get(item, 0)
    return numBrought
    
print('Number of things being brought:')
print(' - Apples         ' + str(totalBrought(allGuests, 'apples')))
print(' - Cups           ' + str(totalBrought(allGuests, 'cups')))
print(' - Cakes          ' + str(totalBrought(allGuests, 'cakes')))
print(' - Ham Sandwiches ' + str(totalBrought(allGuests, 'ham sandwiches')))
print(' - Apple Pies     ' + str(totalBrought(allGuests, 'apple pies')))

我的代码 #

all_guests = {
    'Alice': {'apple': 1, 'ham': 2},
    'Joe': {'potato': 7, 'beer': 7},
    'Carl': {'apple': 3, 'beer': 'none'}}

def total_brought(guest, item):
    num_brought = 0
    for k, v in guest.items():
        num_brought = num_brought + v.get(item, 0)
    return num_brought

print('Total brought: ')
print('Apples: ' + str(total_brought(all_guests, 'apple')))
print('Beer: ' + str(total_brought(all_guests, 'beer')))
print('Ham: ' + str(total_brought(all_guests, 'ham')))
print('Potato: ' + str(total_brought(all_guests, 'potato')))

问题根源 #

报错和缩进无关，核心是字典内数据类型不统一：

• 你的all_guests中，Carl的beer值是字符串'none'，但其他用户的beer是整数7
• 函数计算总数时，会尝试把整数7和字符串'none'相加，Python不允许整数与字符串直接执行加法操作，因此抛出TypeError

修复方案 #

方案1：统一字典数据类型（推荐）

将Carl的beer值改为整数0，代表未携带该物品：

all_guests = {
    'Alice': {'apple': 1, 'ham': 2},
    'Joe': {'potato': 7, 'beer': 7},
    'Carl': {'apple': 3, 'beer': 0}  # 替换'none'为0
}

方案2：在函数中兼容类型转换

如果需要保留字符串形式的“无”，可以在函数里添加类型判断与转换逻辑：

def total_brought(guest, item):
    num_brought = 0
    for k, v in guest.items():
        value = v.get(item, 0)
        # 尝试将字符串转为整数，转换失败则按0处理
        if isinstance(value, str):
            try:
                value = int(value)
            except ValueError:
                value = 0
        num_brought += value
    return num_brought

内容的提问来源于stack exchange，提问作者THIAGO SAMPAIO