Advanced Descriptors

Descriptor protocol, get, set, delete, and descriptors.

Overview

Master descriptor protocol.

Data Descriptors

class Validated:
    def __init__(self, name, validator):
        self.name = name
        self.validator = validator
    
    def __set_name__(self, owner, name):
        self.name = name
    
    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        return getattr(obj, f'_{self.name}', None)
    
    def __set__(self, obj, value):
        if not self.validator(value):
            raise ValueError(f"Invalid value for {self.name}")
        setattr(obj, f'_{self.name}', value)

class Positive:
    def __call__(self, value):
        return isinstance(value, (int, float)) and value > 0

class User:
    age = Validated("age", Positive())

user = User()
user.age = 25
print(user.age)  # 25
# user.age = -5  # ValueError

Non-Data Descriptors

class Method:
    def __init__(self, func):
        self.func = func
    
    def __get__(self, obj, objtype=None):
        if obj is None:
            return self.func
        return lambda *args, **kwargs: self.func(obj, *args, **kwargs)

class MyClass:
    @Method
    def greet(self, name):
        return f"Hello, {name}!"

obj = MyClass()
print(obj.greet("Alice"))  # Hello, Alice!

Practice

Implement a caching descriptor for expensive computations.