Python深入了解defaultdict之轻松处理默认值与复杂数据结构

from collections import defaultdict

# 创建一个 defaultdict，默认值为整数 0
dd = defaultdict(int)

# 添加一些键值对
dd['apple'] = 10
dd['banana'] = 5

# 访问存在的键
print(dd['apple'])  # 输出: 10

# 访问不存在的键，自动创建键并设置默认值 0
print(dd['orange'])  # 输出: 0

# 打印 defaultdict 对象的内容
print(dd)  # 输出: defaultdict(<class 'int'>, {'apple': 10, 'banana': 5, 'orange': 0})

from collections import defaultdict

# 创建一个 defaultdict，默认值为空列表
dd = defaultdict(list)

# 添加一些键值对
dd['fruits'].append('apple')
dd['fruits'].append('banana')
dd['vegetables'].append('carrot')

# 打印 defaultdict 对象的内容
print(dd)  # 输出: defaultdict(<class 'list'>, {'fruits': ['apple', 'banana'], 'vegetables': ['carrot']})

from collections import defaultdict

text = "hello world"
char_count = defaultdict(int)

for char in text:
    char_count[char] += 1

# 打印字符计数结果
print(dict(char_count))  # 输出: {'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, 'r': 1, 'd': 1}

from collections import defaultdict

data = [
    ('fruit', 'apple'),
    ('fruit', 'banana'),
    ('vegetable', 'carrot'),
    ('fruit', 'orange'),
    ('vegetable', 'broccoli')
]

grouped_data = defaultdict(list)

for category, item in data:
    grouped_data[category].append(item)

# 打印分组后的数据
print(dict(grouped_data))  # 输出: {'fruit': ['apple', 'banana', 'orange'], 'vegetable': ['carrot', 'broccoli']}

from collections import defaultdict

# 创建一个嵌套的 defaultdict
nested_dd = defaultdict(lambda: defaultdict(int))

# 添加数据
nested_dd['2024']['January'] = 5
nested_dd['2024']['February'] = 8
nested_dd['2025']['January'] = 3

# 打印嵌套的 defaultdict 对象
print(dict(nested_dd))  
# 输出: {'2024': {'January': 5, 'February': 8}, '2025': {'January': 3}}

from collections import defaultdict

class CustomObject:
    def __init__(self, value):
        self.value = value

    def __repr__(self):
        return f"CustomObject(value={self.value})"

def default_value():
    return CustomObject("default")

# 创建一个 defaultdict，默认值为 CustomObject 对象
custom_dd = defaultdict(default_value)

# 访问不存在的键
print(custom_dd['key'])  # 输出: CustomObject(value=default)

# 打印 defaultdict 对象的内容
print(custom_dd)  # 输出: defaultdict(<function default_value at ...>, {'key': CustomObject(value=default)})