Python 3 Deep Dive Part 4 Oop High Quality

Descriptors are the mechanism behind property(), classmethod(), and super(). They allow you to define reusable attribute logic.

A descriptor is a class that implements any of __get__, __set__, or __delete__.

When to NOT use metaclasses:
Almost always. Class decorators and __init_subclass__ (Python 3.6+) solve 99% of metaclass use cases more simply. python 3 deep dive part 4 oop high quality

__init_subclass__ example:

class Base:
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        print(f"Subclass cls.__name__ created")
class Child(Base):
pass
Python has no real access control. We rely on naming conventions: When to NOT use metaclasses:  
Almost always

Name mangling example:
class Foo:
    def __init__(self):
        self.__secret = 42
    def get_secret(self):
        return self.__secret
f = Foo()
print(f._Foo__secret)  # 42 – still accessible, but harder to accidentally access

High-quality wisdom: Do not overuse __ (double underscore). It breaks subclassing. Use _ for internal attributes and trust other developers. Python is a "consenting adults" language.

Implement these to integrate with language features:
| Feature               | Dunder Method(s)                     |
|-----------------------|--------------------------------------|
| Object representation | __repr__, __str__                |
| Container protocol    | __len__, __getitem__, __setitem__ |
| Callable objects      | __call__                           |
| Context manager       | __enter__, __exit__              |
| Arithmetic ops        | __add__, __sub__, __mul__, etc.|
| Hashing / equality    | __hash__, __eq__                 | Name mangling example : class Foo: def __init__(self):
Example – a sequence‑like class:
class ShoppingCart:
    def __init__(self):
        self._items = []
    def __len__(self):
        return len(self._items)
    def __getitem__(self, index):
        return self._items[index]
    def __setitem__(self, index, value):
        self._items[index] = value