__len__ — Advanced Playground

# __len__ with descriptors
class ValidatedAttribute:
    def __init__(self, validator):
        self.validator = validator
        self.name = None

    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}: {value}")
        setattr(obj, f"_{self.name}", value)

class User:
    name = ValidatedAttribute(lambda v: isinstance(v, str) and len(v) > 0)
    age = ValidatedAttribute(lambda v: isinstance(v, int) and 0 <= v <= 150)

u = User()
u.name = "Alice"
u.age = 30
print(f"{u.name}, age {u.age}")

try:
    u.age = -1
except ValueError as e:
    print(f"Validation error: {e}")