一文带你搞懂Python中的描述符(Descriptor)

class ReadOnly:
    def __init__(self, initval=None, name='var'):
        self.val = initval
        self.name = name
    def __get__(self, obj, objtype):
        return self.val
    def __set__(self, obj, val):
        raise AttributeError(f"{self.name} is read-only")
class MyClass:
    attr = ReadOnly(10, 'attr')

class Typed:
    def __init__(self, name, required_type):
        self.name = name
        self.required_type = required_type
    def __get__(self, instance, owner):
        return instance.__dict__[self.name]
    def __set__(self, instance, value):
        if not isinstance(value, self.required_type):
            raise TypeError(f"Expected {self.required_type}")
        instance.__dict__[self.name] = value
class MyClass:
    attr = Typed('attr', int)

my_obj = MyClass()
my_obj.attr = 10
print(my_obj.attr)  # Output: 10
try:
    my_obj.attr = 'hello'
except TypeError as e:
    print(e)  # Output: Expected <class 'int'>

class LazyProperty:
    def __init__(self, function):
        self.function = function
        self.name = function.__name__
    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        value = self.function(obj)
        setattr(obj, self.name, value)
        return value
class MyClass:
    @LazyProperty
    def expensive_computation(self):
        print("Computing...")
        return sum(range(1000000))
my_instance = MyClass()
print(my_instance.expensive_computation)  # Output: Computing... 499999500000
print(my_instance.expensive_computation)  # Output: 499999500000