从基础到高级应用解析Python中的字典(dict)

# 创建一个简单的字典
user_profile = {
    "username": "python_lover",
    "age": 28,
    "skills": ["Python", "Django", "Flask"],
    "is_active": True
}

# 遍历键
for key in user_profile:
    print(f"Key: {key}")

# 遍历值
for value in user_profile.values():
    print(f"Value: {value}")

# 遍历键值对
for key, value in user_profile.items():
    print(f"{key}: {value}")

# 传统方式可能引发KeyError
age = user_profile["age"]

# 更安全的方式
age = user_profile.get("age", 0)  # 如果age不存在，返回默认值0

# 统计单词频率的经典用法
word_counts = {}
for word in ["apple", "banana", "apple", "orange"]:
    word_counts.setdefault(word, 0)
    word_counts[word] += 1

# 合并两个字典
default_settings = {"theme": "light", "notifications": True}
user_settings = {"theme": "dark", "language": "en"}

default_settings.update(user_settings)
# 结果: {'theme': 'dark', 'notifications': True, 'language': 'en'}

# 删除指定键并返回其值
removed_value = user_profile.pop("age")

# 删除并返回最后一个键值对(3.7+版本有序)
last_item = user_profile.popitem()

# 创建一个数字到其平方的映射
squares = {x: x*x for x in range(1, 6)}
# 结果: {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# 条件过滤
even_squares = {x: x*x for x in range(10) if x % 2 == 0}

from collections import defaultdict

# 自动为不存在的键初始化默认值
word_counts = defaultdict(int)
for word in ["apple", "banana", "apple"]:
    word_counts[word] += 1

from collections import OrderedDict

# 记住元素插入顺序(在Python 3.7+中普通dict也有序)
ordered_dict = OrderedDict()
ordered_dict["first"] = 1
ordered_dict["second"] = 2

# Python 3.9+ 新特性
dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
merged = dict1 | dict2  # {'a': 1, 'b': 3, 'c': 4}

# 传统方式
merged = {**dict1, **dict2}

class LRUCache:
    def __init__(self, capacity: int):
        self.cache = OrderedDict()
        self.capacity = capacity

    def get(self, key):
        if key not in self.cache:
            return -1
        self.cache.move_to_end(key)
        return self.cache[key]

    def put(self, key, value):
        if key in self.cache:
            self.cache.move_to_end(key)
        self.cache[key] = value
        if len(self.cache) > self.capacity:
            self.cache.popitem(last=False)