classical.descriptors module

Descriptors/properties for classes

class classical.descriptors.ArgumentedSubclass(*args, **kwargs)[source]

Bases: classical.descriptors.FactoryProperty

A descriptor that returns an argumented_subclass() of the owner class when accessed.

It allows a class to have attributes that are its own subclasses with additional arguments passed to __init__:

class Tree:
    Peach = ArgumentedSubclass(fruit='peach')
    Pine = ArgumentedSubclass(fruit='cone')
    # both will return subclasses of Tree when accessed

    def __init__(self, fruit):
        self.fruit = fruit

issubclass(Tree.Pine, Tree)  # True
Tree.Pine().fruit  # 'cone'

These properties can be used recursively in combination with each other:

class Polygon:
    Blue = ArgumentedSubclass(color='blue')
    Pentagon = ArgumentedSubclass(sides=5)

    def __init__(self, color=None, sides=3):
        self.color = color
        self.sides = sides

blue_pentagon = Polygon.Pentagon.Blue()
# blue_pentagon.color == 'blue'
# blue_pentagon.sides == 5

class classical.descriptors.AttributedSubclass(**attributes)[source]

Bases: classical.descriptors.FactoryProperty

A descriptor that returns an attributed_subclass() of the owner class when accessed.

It allows a class to have attributes that are its own subclasses with additional or redefined class attributes:

class Paint:
    solvent = None

    Oil = AttributedSubclass(solvent='turpentine')
    Watercolor = AttributedSubclass(solvent='water')

issubclass(Paint.Oil, Paint)  # True
Paint.Oil.solvent  # 'turpentine'

These properties can be used recursively and in combination with one another.

class classical.descriptors.AutoProperty(*args, **kwargs)[source]

Bases: classical.descriptors.FactoryProperty

A descriptor that returns an instance of the owner class when accessed.

The instance is created with the custom arguments that are passed to the property’s constructor.

Acts somewhat like an Enum

class Thing:
    book = AutoProperty(color='brown', size=5)
    pencil = AutoProperty(color='green', size=1)
    # both will return instances of Thing when accessed

    def __init__(self, color, size):
        self.color = color
        self.size = size

isinstance(Thing.book, Thing)  # True
Thing.book.color  # 'brown'
Thing.book is Thing.book  # True (the same instance is returned every time)

These properties can be used in a subclass to produce instances of the subclass:

class ClassyThing(Thing):
    pass

isinstance(ClassyThing.book, ClassyThing)  # True

Use AutoProperty(...).terminal to produce instances of the original owner class inside a subclass:

class Thing:
    terminal_thing = AutoProperty().terminal

class ClassyThing(Thing):
    pass

isinstance(ClassyThing.terminal_thing, ClassyThing)  # False
ClassyThing.terminal_thing.__class__  # Thing

class classical.descriptors.DummySubclass[source]

Bases: classical.descriptors.FactoryProperty

A descriptor that returns a copy of the owner class when accessed (but with a new name equal to the attribute’s name).

class classical.descriptors.FactoryProperty(factory, *args, **kwargs)[source]

Bases: object

A descriptor that returns an object related to the owner class and created by factory when accessed.

Parameters

factory – the object factory. Should have the following signature: factory(cls, name, *args, **kwargs). For example, argumented_subclass() can be used here
args – positional arguments for factory
kwargs – keyword arguments for factory

FactoryProperty is the base class for:

ArgumentedSubclass

AttributedSubclass

AutoProperty

DummySubclass

property terminal: classical.descriptors.FactoryProperty

Return a “terminal” version of the property: if the owner class is subclassed, the object factory will be applied to the original owner of the property instead of the subclass

Consider:

class Thing:
    my_thing = AutoProperty()
    terminal_thing = AutoProperty().terminal

class ClassyThing(Thing):
    pass

isinstance(ClassyThing.my_thing, ClassyThing)  # True
isinstance(ClassyThing.terminal_thing, ClassyThing)  # False

ClassyThing.my_thing.__class__  # ClassyThing
ClassyThing.terminal_thing.__class__  # Thing