Python闭包原理与nonlocal关键字实战指南

def outer_function(x):
    """外部函数"""
    def inner_function(y):
        """内部函数 - 闭包"""
        return x + y  # 引用了外部函数的变量x
    return inner_function  # 返回内部函数
# 创建闭包
closure = outer_function(10)
# 调用闭包
print(closure(5))   # 输出: 15
print(closure(20))  # 输出: 30

def make_multiplier(n):
    def multiplier(x):
        return x * n
    return multiplier
double = make_multiplier(2)
triple = make_multiplier(3)
# 查看闭包信息
print(double.__closure__)  # (<cell at 0x...: int object at 0x...>,)
print(double.__closure__[0].cell_contents)  # 2
print(triple.__closure__[0].cell_contents)  # 3

# 普通函数
def regular_function():
    return 42
# 闭包
def make_closure():
    value = 42
    def closure():
        return value
    return closure
closure_func = make_closure()
# 比较
print(regular_function.__closure__)  # None
print(closure_func.__closure__)      # (<cell at ...>,)

def counter():
    count = 0
    def increment():
        # count += 1  # ❌ 报错：UnboundLocalError
        nonlocal count  # ✅ 声明使用外部函数的count
        count += 1
        return count
    return increment
counter_a = counter()
print(counter_a())  # 1
print(counter_a())  # 2
print(counter_a())  # 3
counter_b = counter()
print(counter_b())  # 1 (独立的计数器)

config = {"debug": False}  # 全局变量
def outer():
    value = 10  # 外部函数变量
    def inner():
        global config      # 引用全局变量
        nonlocal value     # 引用外部函数变量
        config["debug"] = True
        value += 1
        return value
    return inner
func = outer()
print(func())  # 11
print(config)  # {'debug': True}

def create_account(initial_balance):
    """创建一个银行账户（使用闭包实现数据隐藏）"""
    balance = initial_balance
    def account(action, amount=0):
        nonlocal balance
        if action == "deposit":
            balance += amount
            return f"存入 {amount}，当前余额: {balance}"
        elif action == "withdraw":
            if amount > balance:
                return "余额不足"
            balance -= amount
            return f"取出 {amount}，当前余额: {balance}"
        elif action == "balance":
            return f"当前余额: {balance}"
        else:
            return "未知操作"
    return account
# 创建账户
my_account = create_account(1000)
print(my_account("balance"))     # 当前余额: 1000
print(my_account("deposit", 500)) # 存入 500，当前余额: 1500
print(my_account("withdraw", 200)) # 取出 200，当前余额: 1300
# balance变量无法直接访问，实现了数据隐藏

def make_power(exponent):
    """创建幂函数工厂"""
    def power(base):
        return base ** exponent
    return power
# 创建不同的幂函数
square = make_power(2)   # 平方函数
cube = make_power(3)     # 立方函数
quartic = make_power(4)  # 四次方
print(square(5))    # 25
print(cube(3))      # 27
print(quartic(2))   # 16

def count_calls(func):
    """统计函数调用次数的装饰器"""
    count = 0
    def wrapper(*args, **kwargs):
        nonlocal count
        count += 1
        result = func(*args, **kwargs)
        print(f"{func.__name__} 被调用了 {count} 次")
        return result
    return wrapper
# 使用闭包装饰器
@count_calls
def greet(name):
    return f"Hello, {name}!"
greet("Alice")  # greet 被调用了 1 次
greet("Bob")    # greet 被调用了 2 次
greet("Carol")  # greet 被调用了 3 次

def memoize(func):
    """简单的记忆化装饰器"""
    cache = {}
    def wrapper(*args):
        if args not in cache:
            cache[args] = func(*args)
        return cache[args]
    return wrapper
@memoize
def fibonacci(n):
    """斐波那契数列（带缓存）"""
    if n < 2:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)
# 快速计算大数
print(fibonacci(50))  # 12586269025，速度极快

def create_multipliers():
    """这个函数有bug！"""
    multipliers = []
    for i in range(5):
        def multiplier(x):
            return x * i  # i是延迟绑定的！
        multipliers.append(multiplier)
    return multipliers
# 错误的结果
m = create_multipliers()
print([m(2) for m in m])  # [8, 8, 8, 8, 8] 而不是 [0, 2, 4, 6, 8]
# 正确的写法
def create_multipliers_fixed():
    """修复后的版本"""
    multipliers = []
    for i in range(5):
        def make_multiplier(n):  # 使用默认参数捕获当前值
            def multiplier(x):
                return x * n
            return multiplier
        multipliers.append(make_multiplier(i))
    return multipliers
m = create_multipliers_fixed()
print([m(2) for m in m])  # [0, 2, 4, 6, 8] ✅