None类型 #

# None是Python的空值
x = None

print(type(x))  # <class 'NoneType'>
print(x)        # None

# None是唯一的NoneType实例
print(None is None)  # True

# None是单例对象
a = None
b = None
print(a is b)  # True（同一个对象）

# None的布尔值是False
print(bool(None))  # False

# None不等于0、空字符串、空列表等
print(None == 0)       # False
print(None == "")      # False
print(None == [])      # False
print(None == False)   # False

# 函数参数默认值
def greet(name=None):
    if name is None:
        print("你好，陌生人")
    else:
        print(f"你好，{name}")

greet()         # 你好，陌生人
greet("Tom")    # 你好，Tom

# 没有return语句的函数返回None
def say_hello():
    print("Hello")

result = say_hello()
print(result)  # None

# return不带值也返回None
def do_nothing():
    return

result = do_nothing()
print(result)  # None

# 初始化时使用None
result = None

# 后续赋值
result = calculate_result()
if result is not None:
    print(f"结果是：{result}")

def find_user(user_id):
    # 如果找到返回用户信息，否则返回None
    users = {1: "Tom", 2: "Jerry"}
    return users.get(user_id)  # get方法找不到时返回None

user = find_user(3)
if user is not None:
    print(f"找到用户：{user}")
else:
    print("用户不存在")

person = {"name": "Tom"}

# 使用get方法，键不存在返回None
age = person.get("age")  # None
print(age)

# 可以指定默认值
age = person.get("age", 0)  # 0
print(age)

# setdefault方法
person.setdefault("age", 25)
print(person)  # {'name': 'Tom', 'age': 25}

x = None

# 推荐：使用is
if x is None:
    print("x是None")

if x is not None:
    print("x不是None")

class MyNone:
    """一个返回False的类"""
    def __eq__(self, other):
        return True  # 与任何值比较都返回True
    
    def __bool__(self):
        return False

my_none = MyNone()

# == 可能会被重载
print(my_none == None)  # True（被欺骗了）
print(my_none is None)  # False（正确的判断）

# None是单例，is是检查身份
x = None
print(x == None)  # True
print(x is None)  # True（推荐）

x = None

# None在布尔上下文中为False
if not x:
    print("x是假值")  # 会执行

# 但这不能区分None和其他假值
y = []
if not y:
    print("y是假值")  # 会执行，但y不是None

# 正确判断None
if x is None:
    print("x确实是None")

# None表示"没有值"，0是一个具体的数字
score = None  # 还没有分数
score = 0     # 分数是0

# 统计时需要区分
scores = [85, 90, None, 78, None]

# 只计算有效分数
valid_scores = [s for s in scores if s is not None]
average = sum(valid_scores) / len(valid_scores)
print(f"平均分：{average}")

# None表示"没有值"，""表示空字符串
name = None   # 名字未知
name = ""     # 名字是空字符串

# 处理表单输入
def process_name(name):
    if name is None:
        return "名字未提供"
    elif name == "":
        return "名字为空"
    else:
        return f"名字是：{name}"

# None表示"没有数据"，[]表示空数据
data = None   # 数据还没获取
data = []     # 数据获取了，但为空

def process_data(data):
    if data is None:
        print("数据未加载")
        return
    if len(data) == 0:
        print("数据为空")
        return
    print(f"处理{len(data)}条数据")

# None表示"无值"，False是一个布尔值
flag = None   # 状态未知
flag = False  # 状态是假

# 三态逻辑
def check_permission(user):
    if user is None:
        return None  # 用户不存在
    return user.has_permission  # True或False

result = check_permission(None)
if result is None:
    print("请先登录")
elif result:
    print("有权限")
else:
    print("无权限")

# 使用None作为默认参数（可变默认值的替代）
def add_item(items=None, item=None):
    if items is None:
        items = []
    if item is not None:
        items.append(item)
    return items

print(add_item())           # []
print(add_item(item='a'))   # ['a']
print(add_item([1, 2], 3))  # [1, 2, 3]

# 为什么不用[]作为默认值？
# 错误示例：
def bad_append(item, items=[]):
    items.append(item)
    return items

print(bad_append(1))  # [1]
print(bad_append(2))  # [1, 2] - 意外的行为！

def create_file(filename, content=None, encoding='utf-8'):
    """创建文件，content可选"""
    if content is None:
        content = ""
    
    with open(filename, 'w', encoding=encoding) as f:
        f.write(content)

create_file("empty.txt")  # 创建空文件
create_file("hello.txt", "Hello World")

def find_first_negative(numbers):
    """查找第一个负数，找不到返回None"""
    for num in numbers:
        if num < 0:
            return num
    return None

result = find_first_negative([1, 2, 3])
if result is not None:
    print(f"找到负数：{result}")
else:
    print("没有找到负数")

def process_file(filename):
    """处理文件，失败返回None"""
    try:
        with open(filename, 'r') as f:
            content = f.read()
    except FileNotFoundError:
        return None
    
    # 处理内容
    return content.upper()

result = process_file("data.txt")
if result is None:
    print("文件处理失败")

from typing import Optional

def find_user(user_id: int) -> Optional[str]:
    """返回用户名或None"""
    users = {1: "Tom", 2: "Jerry"}
    return users.get(user_id)

# Optional[str] 等价于 str | None（Python 3.10+）

# Python 3.10+ 简写
def get_value(key: str) -> str | None:
    """获取值，可能返回None"""
    data = {"name": "Tom"}
    return data.get(key)

x = None

# 会报错
# result = x + 1  # TypeError

# 应该先检查
if x is not None:
    result = x + 1

# 可能出现None的错误
result = get_data().process().save()  # 如果get_data()返回None会报错

# 安全链式调用
data = get_data()
if data is not None:
    processed = data.process()
    if processed is not None:
        processed.save()

# 或使用短路求值
result = get_data() and get_data().process() and get_data().process().save()

data = {"name": "Tom", "age": None}

# get方法返回None有两种情况：
# 1. 键不存在
# 2. 键存在但值是None

print(data.get("name"))  # "Tom"
print(data.get("age"))   # None
print(data.get("email")) # None

# 区分方法：使用in检查
if "email" in data:
    email = data["email"]  # 存在但可能是None
else:
    print("email键不存在")

# all()：全为真才返回True，None被视为False
values = [1, True, "hello", None]
print(all(values))  # False

# any()：有一个为真就返回True
values = [None, False, 0, "", "hello"]
print(any(values))  # True

# 空列表
print(all([]))  # True（空序列为真）
print(any([]))  # False

# 过滤掉None值
items = [1, None, 2, None, 3]
filtered = [x for x in items if x is not None]
print(filtered)  # [1, 2, 3]

# 使用filter
filtered = list(filter(lambda x: x is not None, items))
print(filtered)  # [1, 2, 3]

# None使用最佳实践

# 1. 检查None用is
if x is None:
    pass

# 2. 可变默认参数用None
def func(items=None):
    if items is None:
        items = []

# 3. 类型注解用Optional
def find(id: int) -> Optional[str]:
    pass

# 4. 区分None和其他假值
if value is not None:
    # 有值（可能是0或空字符串）
    pass