Deskriptoren: __get__, __set__, __delete__

Was ist ein Deskriptor?

class Descriptor:
    def __get__(self, obj, objtype=None):
        print(f"__get__ called, obj={obj}")
        return 42

class MyClass:
    attr = Descriptor()

print(MyClass().attr)   # __get__ called ... 42

Daten- vs. Nicht-Daten-Deskriptoren

class DataDesc:
    def __get__(self, obj, t): return "data"
    def __set__(self, obj, val): pass

class NonDataDesc:
    def __get__(self, obj, t): return "nondata"

class C:
    d = DataDesc()
    n = NonDataDesc()

c = C()
c.__dict__["n"] = "instance"  # shadows the non-data desc
c.__dict__["d"] = "instance"  # does NOT shadow data desc
print(c.n)   # instance
print(c.d)   # data

__get__-Signatur

class Verbose:
    def __get__(self, obj, objtype=None):
        if obj is None:
            return self   # accessed on class
        return f"value for {obj!r}"

class Widget:
    label = Verbose()

print(Widget.label)        # <Verbose object>
print(Widget().label)      # value for <Widget object>

__set__ und Validierung

class PositiveInt:
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, obj, t):
        return obj.__dict__.get(self.name)

    def __set__(self, obj, val):
        if not isinstance(val, int) or val <= 0:
            raise ValueError(f"{self.name} must be a positive int")
        obj.__dict__[self.name] = val

class Rect:
    width  = PositiveInt()
    height = PositiveInt()

r = Rect()
r.width = 10
# r.width = -1  # ValueError

__set_name__

class Typed:
    def __set_name__(self, owner, name):
        self.public  = name
        self.private = "_" + name

    def __get__(self, obj, t):
        return None if obj is None else getattr(obj, self.private, None)

    def __set__(self, obj, val):
        setattr(obj, self.private, val)

class User:
    name = Typed()

u = User()
u.name = "Alice"
print(u.name)     # Alice

__delete__ zum Entfernen von Attributen

class Protected:
    def __set_name__(self, owner, name): self.name = name
    def __get__(self, obj, t):
        return obj.__dict__.get(self.name)
    def __set__(self, obj, v): obj.__dict__[self.name] = v
    def __delete__(self, obj):
        raise AttributeError(f"Cannot delete {self.name}")

class Config:
    host = Protected()

c = Config(); c.host = "localhost"
# del c.host   # AttributeError

Funktionen sind Nicht-Daten-Deskriptoren

def greet(self):
    return f"Hello from {self}"

class C: pass
C.greet = greet

c = C()
print(c.greet())    # Hello from <C object>

classmethod und staticmethod sind Deskriptoren

class MyClass:
    @classmethod
    def from_string(cls, s):
        return cls()

# classmethod.__get__ returns a bound method with cls
# staticmethod.__get__ returns the plain function

Deskriptor für verzögerte Attribute

class LazyAttr:
    def __init__(self, func): self.func = func
    def __set_name__(self, owner, name): self.name = name
    def __get__(self, obj, t):
        if obj is None: return self
        val = self.func(obj)
        obj.__dict__[self.name] = val   # shadows descriptor
        return val

class Report:
    @LazyAttr
    def summary(self):
        print("computing...")
        return "done"

r = Report()
print(r.summary)   # computing... done
print(r.summary)   # done (cached)

property ist ein Deskriptor

class Circle:
    def __init__(self, r): self._r = r

    @property
    def radius(self): return self._r

    @radius.setter
    def radius(self, v):
        if v < 0: raise ValueError
        self._r = v

# property is: property.__get__ = fget, property.__set__ = fset

Reihenfolge der Deskriptorauflösung

# Pseudocode for obj.attr:
# 1. Check type(obj).__mro__ for a data descriptor
# 2. Check obj.__dict__
# 3. Check type(obj).__mro__ for non-data descriptor or class var
# 4. Raise AttributeError

Kurzer Test

Zusammenfassung