python/day02/dict_basics.py

"""dict_basics.py - Python 字典基础示例

包含常见的字典操作示例：创建、访问、添加、修改、删除、遍历、嵌套和推导式等。
运行后会打印每个示例的结果，并在代码中注释预期输出。
"""


def main():
    # 1. 创建字典
    student = {"name": "Tom", "age": 18, "city": "Beijing"}
    print("student ->", student)

    # 2. 访问值
    print("student['name'] ->", student["name"])  # Tom
    print("student['age'] ->", student["age"])    # 18

    # 3. 安全获取值
    print("student.get('score') ->", student.get("score"))        # None
    print("student.get('score', 0) ->", student.get("score", 0))  # 0

    # 4. 添加和修改
    student["score"] = 95
    student["age"] = 19
    print("after add/modify ->", student)

    # 5. 删除键值对
    del student["city"]
    print("after del student['city'] ->", student)

    # pop() 删除指定键并返回值
    age = student.pop("age")
    print("age popped ->", age)
    print("after pop('age') ->", student)

    # 6. 判断 key 是否存在
    print("'name' in student ->", "name" in student)       # True
    print("'score' in student ->", "score" in student)     # True
    print("'city' in student ->", "city" in student)       # False

    # 7. 遍历字典
    for key in student:
        print("loop key ->", key)

    for key, value in student.items():
        print("loop item ->", key, value)

    for value in student.values():
        print("loop value ->", value)

    # 8. 常见方法
    print("student.keys() ->", student.keys())
    print("student.values() ->", student.values())
    print("student.items() ->", student.items())
    print("len(student) ->", len(student))

    # 9. 字典嵌套
    users = {
        "alice": {"age": 20, "city": "Shanghai"},
        "bob": {"age": 22, "city": "Hangzhou"}
    }
    print("users['alice']['city'] ->", users["alice"]["city"])  # Shanghai

    # 10. 字典推导式
    squares = {x: x * x for x in range(1, 5)}
    print("squares ->", squares)  # {1: 1, 2: 4, 3: 9, 4: 16}

    # 11. 统计频次
    counts = {"apple": 3, "banana": 2}
    counts["apple"] += 1
    print("counts ->", counts)

    # 12. 说明：
    # - 键唯一
    # - 通过 key 快速查找 value
    # - 适合表达“映射关系”
    # - Python 3.7+ 保留插入顺序


if __name__ == '__main__':
    main()