Lære __get__, __set__, __delete__ | What Descriptors Are

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A descriptor is any object that defines at least one of three special methods: __get__, __set__, or __delete__. When an instance of such an object is assigned as a class attribute, Python's attribute lookup machinery calls these methods automatically instead of returning the object directly.

The Three Methods

__get__(self, obj, objtype=None) is called when the attribute is read. obj is the instance accessing the attribute (None when accessed from the class). objtype is the class:


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# A minimal read-only descriptor
class Descriptor:
    def __get__(self, obj, objtype=None):
        if obj is None:
            return self  # Accessed from the class – return the descriptor itself
        return f"Value accessed on {obj.__class__.__name__}"

class Report:
    status = Descriptor()  # Descriptor assigned as a class attribute

report = Report()
print(report.status)   # "Value accessed on Report"
print(Report.status)   # The Descriptor object itself

__set__(self, obj, value) is called when the attribute is written:


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# A descriptor that validates and stores a value
class PositiveNumber:
    def __set_name__(self, owner, name):
        self._name = name  # Storing the attribute name for error messages

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        return obj.__dict__.get(self._name)

    def __set__(self, obj, value):
        if not isinstance(value, (int, float)) or value <= 0:
            raise ValueError(f"{self._name} must be a positive number, got {value!r}")
        obj.__dict__[self._name] = value

class Product:
    price = PositiveNumber()

    def __init__(self, name, price):
        self.name = name
        self.price = price  # Triggers __set__

laptop = Product("ThinkPad", 1200.0)
print(laptop.price)   # 1200.0

laptop.price = -50    # Raises ValueError

__delete__(self, obj) is called when the attribute is deleted with del:


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# Adding deletion support to the descriptor
class PositiveNumber:
    def __set_name__(self, owner, name):
        self._name = name

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        return obj.__dict__.get(self._name)

    def __set__(self, obj, value):
        if not isinstance(value, (int, float)) or value <= 0:
            raise ValueError(f"{self._name} must be a positive number, got {value!r}")
        obj.__dict__[self._name] = value

    def __delete__(self, obj):
        obj.__dict__.pop(self._name, None)
        print(f"{self._name} removed from {obj!r}")

class Product:
    price = PositiveNumber()

    def __init__(self, name, price):
        self.name = name
        self.price = price

laptop = Product("ThinkPad", 1200.0)
del laptop.price  # Triggers __delete__
print(laptop.__dict__)  # {'name': 'ThinkPad'}

`__set_name__`

__set_name__(self, owner, name) is called automatically when the descriptor is assigned to a class attribute. It gives the descriptor access to its own attribute name without hardcoding it:


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# __set_name__ receives the class and attribute name at class creation time
class TrackedAttribute:
    def __set_name__(self, owner, name):
        print(f"Descriptor assigned to {owner.__name__}.{name}")
        self._name = name

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        return obj.__dict__.get(self._name)

    def __set__(self, obj, value):
        obj.__dict__[self._name] = value

class Invoice:
    amount = TrackedAttribute()   # Prints: Descriptor assigned to Invoice.amount
    currency = TrackedAttribute() # Prints: Descriptor assigned to Invoice.currency

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