Metaclass lan descriptor minangka fitur Python canggih kanggo meta-programming — ngontrol cara kelas lan akses atribut berperilaku. Padha kuwat "magic" ing framework (ORM, Pydantic), nanging sampeyan jarang nulis langsung ing kode aplikasi.
:
(Dog)
(Dog())
Kayata kelas nggawe instansi, metaclass nggawe kelas. Kanthi nyesuaikan metaclass, sampeyan bisa nganggit lan ngubah pembuatan kelas:
class Meta(type):
def __new__(mcs, name, bases, namespace):
# runs when a class USING this metaclass is DEFINED
namespace["created_by"] = "Meta" # inject an attribute into every such class
return super().__new__(mcs, name, bases, namespace)
class MyClass(metaclass=Meta):
pass
MyClass.created_by # "Meta" — injected by the metaclass at class-creation time
Iki ngidini framework kanthi otomatis ndhaftar kelas, validasi definisi, utawa nambah perilaku menyang saben subkelas — contone carane ORM ngubah atribut kelas dadi kolom database.
Sebuah descriptor yaiku objek sing ngetrapake __get__/__set__/__delete__, ngontrol apa sing kedadeyan nalika atribut dibacal utawa ditulis:
class Positive:
def __set_name__(self, owner, name):
self.name = f"_{name}"
def __get__(self, obj, objtype=None):
return getattr(obj, self.name)
def __set__(self, obj, value):
if value < 0:
raise ValueError("must be positive") # validate on assignment
setattr(obj, self.name, value)
class Account:
balance = Positive() # a descriptor as a class attribute
a = Account()
a.balance = 100 # calls Positive.__set__ → validates
a.balance = -5 # ❌ ValueError
Descriptor nganggit akses atribut — nggawea validasi, atribut terkomputasi, lan lazy loading. Bener, @property, @classmethod, lan @staticmethod kabeh diterapake minangka descriptor.
Metaclasses → Django/SQLAlchemy ORM models, ABCs, Pydantic, framework registration
Descriptors → @property, ORM fields, validation libraries, lazy attributes
Metaclass lan descriptor yaiku machinery meta-programming ing balik akeh library Python paling kuwat — ORM sing ngubah definisi kelas dadi skema database, framework validasi kaya Pydantic, lan malah built-in kaya @property (descriptor).
Sampeyan jarang nulis langsung (lan ora kudu ngiro-iro anggo saka alat sing luwih prasaja), nanging ngerti padha ngroso carane framework iki ngati "magic" saka kostumisasi tingkat kelas lan akses atribut sing dikontrol.
Pengetahuan iki berharga kanggo desain library canggih, debug perilaku framework, lan ngerti banget object model Python — iku apa sing misahake intermediate saka expert-level pemahaman basa iki.