初探Python元编程之理解并使用元编程改变代码的代码示例

def simple_decorator(function):
    def wrapper():
        print("Before function execution")
        function()
        print("After function execution")
    return wrapper
@simple_decorator
def hello():
    print("Hello, world!")
hello()

class Meta(type):
    def __init__(cls, name, bases, attrs):
        attrs['say_hello'] = lambda self: f"Hello, I'm {name}"
        super().__init__(name, bases, attrs)
class MyClass(metaclass=Meta):
    pass
obj = MyClass()
print(obj.say_hello())  # 输出: Hello, I'm MyClass

class MyClass:
    def hello(self):
        return "Hello, world!"
def new_hello(self):
    return "Hello, Python!"
MyClass.hello = new_hello
obj = MyClass()
print(obj.hello())  # 输出: Hello, Python!

def cache_decorator(function):
    cache = {}
    def wrapper(n):
        if n not in cache:
            cache[n] = function(n)
        return cache[n]
    return wrapper
@cache_decorator
def fibonacci(n):
    if n < 2:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)
print(fibonacci(10))  # 输出: 55

class SingletonMeta(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]
class Singleton(metaclass=SingletonMeta):
    pass
obj1 = Singleton()
obj2 = Singleton()
print(obj1 is obj2)  # 输出: True

class MyClass:
    pass
obj = MyClass()
# 动态添加属性
obj.new_attr = "Hello, world!"
print(obj.new_attr)  # 输出: Hello, world!
# 动态添加方法
from types import MethodType
def new_method(self):
    return "This is a new method."
obj.new_method = MethodType(new_method, obj)
print(obj.new_method())  # 输出: This is a new method.