Python Dictionary (字典)

Dictionary（簡稱 dict）是一種鍵值對 (key-value pair) 的資料結構，用來儲存有關聯性的資料。

建立 Dictionary

使用大括號 {} 並用冒號 : 分隔 key 和 value：

# 建立 dictionary
person = {
    "name": "Alice",
    "age": 25,
    "city": "Taipei"
}

# 空的 dictionary
empty_dict = {}

# 使用 dict() 函數
person = dict(name="Alice", age=25, city="Taipei")

存取元素

person = {"name": "Alice", "age": 25, "city": "Taipei"}

# 使用 key 存取
print(person["name"])  # Alice
print(person["age"])   # 25

# 使用 get() 方法（key 不存在時不會報錯）
print(person.get("name"))      # Alice
print(person.get("country"))   # None
print(person.get("country", "Unknown"))  # Unknown（指定預設值）

新增和修改元素

person = {"name": "Alice", "age": 25}

# 新增元素
person["city"] = "Taipei"
print(person)  # {'name': 'Alice', 'age': 25, 'city': 'Taipei'}

# 修改元素
person["age"] = 26
print(person)  # {'name': 'Alice', 'age': 26, 'city': 'Taipei'}

# update() - 一次更新多個元素
person.update({"age": 27, "country": "Taiwan"})
print(person)

刪除元素

person = {"name": "Alice", "age": 25, "city": "Taipei"}

# del - 刪除指定 key
del person["city"]
print(person)  # {'name': 'Alice', 'age': 25}

# pop() - 刪除並回傳值
age = person.pop("age")
print(age)     # 25
print(person)  # {'name': 'Alice'}

# popitem() - 刪除並回傳最後一個元素
person = {"name": "Alice", "age": 25, "city": "Taipei"}
item = person.popitem()
print(item)    # ('city', 'Taipei')
print(person)  # {'name': 'Alice', 'age': 25}

# clear() - 清空 dictionary
person.clear()
print(person)  # {}

檢查 key 是否存在

person = {"name": "Alice", "age": 25}

print("name" in person)      # True
print("city" in person)      # False
print("city" not in person)  # True

Dictionary 長度

person = {"name": "Alice", "age": 25, "city": "Taipei"}
print(len(person))  # 3

遍歷 Dictionary

person = {"name": "Alice", "age": 25, "city": "Taipei"}

# 遍歷 keys
for key in person:
    print(key)

for key in person.keys():
    print(key)

# 遍歷 values
for value in person.values():
    print(value)

# 遍歷 key-value pairs
for key, value in person.items():
    print(f"{key}: {value}")

輸出：

name: Alice
age: 25
city: Taipei

取得所有 keys 和 values

person = {"name": "Alice", "age": 25, "city": "Taipei"}

keys = person.keys()
values = person.values()
items = person.items()

print(list(keys))    # ['name', 'age', 'city']
print(list(values))  # ['Alice', 25, 'Taipei']
print(list(items))   # [('name', 'Alice'), ('age', 25), ('city', 'Taipei')]

Dictionary 複製

original = {"a": 1, "b": 2}

# 直接賦值是參考
ref = original
ref["c"] = 3
print(original)  # {'a': 1, 'b': 2, 'c': 3}

# 用 copy() 建立副本
original = {"a": 1, "b": 2}
copy = original.copy()
copy["c"] = 3
print(original)  # {'a': 1, 'b': 2}
print(copy)      # {'a': 1, 'b': 2, 'c': 3}

合併 Dictionary

dict1 = {"a": 1, "b": 2}
dict2 = {"c": 3, "d": 4}

# Python 3.9+ 使用 |
merged = dict1 | dict2
print(merged)  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

# 使用 ** 解包
merged = {**dict1, **dict2}
print(merged)  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

# 使用 update()（會修改原 dict）
dict1.update(dict2)
print(dict1)  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Dictionary Comprehension

# 建立數字對應其平方的字典
squares = {x: x ** 2 for x in range(5)}
print(squares)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# 加上條件
evens = {x: x ** 2 for x in range(10) if x % 2 == 0}
print(evens)  # {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

# 轉換現有的 dictionary
original = {"a": 1, "b": 2, "c": 3}
upper_keys = {k.upper(): v * 2 for k, v in original.items()}
print(upper_keys)  # {'A': 2, 'B': 4, 'C': 6}

巢狀 Dictionary

users = {
    "user1": {
        "name": "Alice",
        "age": 25
    },
    "user2": {
        "name": "Bob",
        "age": 30
    }
}

print(users["user1"]["name"])  # Alice
print(users["user2"]["age"])   # 30

setdefault()

如果 key 存在，回傳其值；如果不存在，設定預設值並回傳：

person = {"name": "Alice"}

# key 存在
result = person.setdefault("name", "Unknown")
print(result)  # Alice
print(person)  # {'name': 'Alice'}

# key 不存在
result = person.setdefault("age", 25)
print(result)  # 25
print(person)  # {'name': 'Alice', 'age': 25}