# 允许的命名
name = "张三"
age = 25
_user_id = 123
count2 = 10
my_variable = "值"

# 不允许的命名
2count = 10      # 不能以数字开头
my-variable = 1  # 不能包含连字符
class = "test"   # 不能使用关键字

# 单个变量赋值
x = 10
name = "Python"

# 同时给多个变量赋值
a = b = c = 100
x, y, z = 1, 2, 3

# 变量类型可以改变
var = 10        # 现在是整数
var = "hello"   # 现在是字符串
var = [1,2,3]   # 现在是列表

# 整数
age = 25

# 浮点数
price = 19.99

# 字符串
name = "Alice"

# 布尔值
is_valid = True

# 空值
result = None

# 列表
numbers = [1, 2, 3, 4]

# 元组
coordinates = (10, 20)

# 字典
person = {"name": "Bob", "age": 30}

# 集合
unique_numbers = {1, 2, 3}

def my_function():
    local_var = "我在函数内部"  # 局部变量
    print(local_var)

my_function()
# print(local_var)  # 错误：局部变量在外部不可访问

global_var = "我是全局变量"    # 全局变量

def test_function():
    print(global_var)         # 可以访问全局变量

test_function()

count = 0

def increment():
    global count              # 声明使用全局变量
    count += 1

increment()
print(count)  # 输出: 1

a = b = c = 0  # 三个变量都赋值为0

# 元组解包
x, y = (10, 20)

# 列表解包
name, age = ["Tom", 25]

# 交换变量值
a, b = 1, 2
a, b = b, a  # 交换a和b的值

# 收集多余的值
first, *middle, last = [1, 2, 3, 4, 5]
# first=1, middle=[2,3,4], last=5

import sys

a = [1, 2, 3]
print(sys.getrefcount(a))  # 查看引用计数

a = [1, 2, 3]
b = a
c = [1, 2, 3]

print(a is b)  # True，指向同一个对象
print(a is c)  # False，内容相同但不是同一个对象

# 好的命名
student_count = 50
user_name = "john_doe"
is_logged_in = True

# 避免的命名
sc = 50        # 不明确
un = "john"    # 缩写不清晰
flag = True    # 含义不明确

# 常量通常使用全大写
MAX_CONNECTIONS = 100
PI = 3.14159
DATABASE_URL = "postgresql://localhost:5432/mydb"

# print(undefined_var)  # NameError: name 'undefined_var' is not defined

count = 10
# print(coutn)  # 拼写错误导致NameError

def bad_function():
    # 这里试图修改全局变量但没有声明global
    # count += 1  # UnboundLocalError
    pass