Python 中 __new__的实现示例

class Foo:
    def __new__(cls, *args, **kwargs):
        print(f"[__new__] 分配内存，cls={cls}")
        instance = super().__new__(cls)  # 创建实例
        return instance                  # 必须 return！

    def __init__(self, value):
        print(f"[__init__] 初始化，self={self}")
        self.value = value

obj = Foo(42)
# [__new__] 分配内存，cls=<class 'Foo'>
# [__init__] 初始化，self=<__main__.Foo object>

object.__new__(cls[, *args, **kwargs])

# type.__call__ 的内部行为（伪代码）
def __call__(cls, *args, **kwargs):
    obj = cls.__new__(cls, *args, **kwargs)
    if isinstance(obj, cls):        # 只有返回 cls 的实例才调 __init__
        obj.__init__(*args, **kwargs)
    return obj

class Singleton:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance  # 始终返回同一个实例

a = Singleton()
b = Singleton()
print(a is b)  # True

class UpperStr(str):
    def __new__(cls, value):
        return super().__new__(cls, value.upper())
        # ⚠️ 不能在 __init__ 里改，str 是不可变的

s = UpperStr("hello")
print(s)        # HELLO
print(type(s))  # <class '__main__.UpperStr'>

class Shape:
    def __new__(cls, kind, *args):
        subclasses = {"circle": Circle, "rect": Rect}
        target_cls = subclasses.get(kind, cls)
        return super().__new__(target_cls)

class Circle(Shape): pass
class Rect(Shape):   pass

s = Shape("circle")
print(type(s))  # <class '__main__.Circle'>

class Meta(type):
    def __new__(mcs, name, bases, namespace):
        # 在类定义时，自动把所有方法名转大写
        new_ns = {k.upper() if not k.startswith('_') else k: v
                  for k, v in namespace.items()}
        return super().__new__(mcs, name, bases, new_ns)

class Foo(metaclass=Meta):
    def hello(self): return "hi"

Foo().HELLO()  # "hi"  — 方法被重命名了

class Bad:
    def __new__(cls):
        super().__new__(cls)  # 没有 return！→ 返回 None
                              # __init__ 不会被调用，obj 是 None

# Python 会同时把参数传给 __new__ 和 __init__
# 若签名不匹配会报 TypeError

class Ok:
    def __new__(cls, x, y):      # 接受 x, y
        return super().__new__(cls)
    def __init__(self, x, y):    # 同样接受 x, y ✓
        self.x, self.y = x, y