一文详解Python的变量作用域

total = 0
def process_numbers():
    while True:
        num = input("请输入数字（输入q退出）：")
        if num == 'q':
            break
        total = total + int(num)
        print(f"当前总和：{total}")
process_numbers()
print(f"最终总和：{total}")

请输入数字（输入q退出）：5
UnboundLocalError: local variable 'total' referenced before assignment

count = 10
def show():
    print(count)
show()  # 输出10，正常运行

def greet():
    message = "你好"
    print(message)
greet()  # 输出：你好
print(message)  # 报错！NameError: name 'message' is not defined

name = "张三"
def say():
    age = 18
    print(name)  # 能读到外面的name
    print(age)   # 能读到自己的age
say()
print(age)  # 报错！外面读不到函数里的age

# 全局变量
app_name = "我的应用"
version = "1.0"
def show_info():
    print(app_name)  # 函数里可以读全局变量
    print(version)
show_info()  # 正常运行
print(app_name)  # 外面也能用

counter = 0
def increment():
    counter = counter + 1  # 报错！
increment()

counter = 0
def increment():
    global counter  # 声明：这个counter是全局的
    counter = counter + 1
increment()
print(counter)  # 1

def outer():
    x = 10  # 外层函数的变量
    def inner():
        print(x)  # 内层函数可以读外层变量
    inner()
outer()  # 输出10

def outer():
    x = 10
    def inner():
        x = 20  # 这是在inner里创建了一个新的局部变量x
        print("inner里的x:", x)
    inner()
    print("outer里的x:", x)
outer()
# 输出：
# inner里的x: 20
# outer里的x: 10

def outer():
    x = 10
    def inner():
        nonlocal x  # 声明：这个x来自外层函数
        x = 20
    inner()
    print(x)  # 20
outer()

print(len("hello"))  # len是内置函数
print(sum([1,2,3]))  # sum是内置函数
print(str(100))      # str是内置类型

len = 10  # 你定义了一个叫len的变量
print(len("hello"))  # 报错！因为len现在是10，不是函数了

x = "global x"  # 全局
def outer():
    x = "outer x"  # 外层函数
    def inner():
        x = "inner x"  # 局部
        print(x)
    inner()
outer()  # 输出 "inner x"

def outer():
    x = "outer x"
    def inner():
        print(x)  # 没有局部x，就去外层找
    inner()
outer()  # 输出 "outer x"

x = "global x"
def outer():
    def inner():
        print(x)  # 局部没有，外层没有，就去全局找
    inner()
outer()  # 输出 "global x"

print = "hello"
print("world")  # 报错，因为print现在是字符串，不能当函数调用

if True:
    x = 10
print(x)  # 10，x还在！
for i in range(5):
    pass
print(i)  # 4，i还在！

name = "张三"
def change():
    name = "李四"  # 这不是修改全局，是创建局部
    print("函数内:", name)
change()  # 函数内: 李四
print("全局:", name)  # 全局: 张三 —— 没变！

x = 10
squares = [x**2 for x in range(5)]
print(x)  # Python 3里还是10，Python 2里会变成4

funcs = []
for i in range(3):
    funcs.append(lambda: i)
for f in funcs:
    print(f())  # 2 2 2，不是0 1 2

funcs = []
for i in range(3):
    funcs.append(lambda i=i: i)  # 把当前i的值固定下来
for f in funcs:
    print(f())  # 0 1 2

config = "production"
def get_env():
    return config  # 读全局，不需要global
print(get_env())  # production

mode = "read"
def switch_to_write():
    global mode
    mode = "write"
switch_to_write()
print(mode)  # write

def make_counter():
    count = 0
    def increment():
        nonlocal count
        count += 1
        return count
    return increment
counter = make_counter()
print(counter())  # 1
print(counter())  # 2
print(counter())  # 3

total = 0
def process_numbers():
    # 这里少了一行
    while True:
        num = input("请输入数字：")
        if num == 'q':
            break
        total = total + int(num)  # total被赋值，被视为局部变量

def process_numbers():
    global total
    # 剩下的代码不变