Ducktools: Class Builder

ducktools-classbuilder is the Python package that will bring you the joy of writing... functions... that will bring back the joy of writing classes.

Maybe.

While attrs and dataclasses are class boilerplate generators, ducktools.classbuilder is intended to provide the tools to help make a customized version of the same concept.

Install from PyPI with: python -m pip install ducktools-classbuilder

Included Implementations

There are 2 different implementations provided with the module each of which offers a subclass based and decorator based option.

Tip

For more information on using these tools to create your own implementations using the builder see the tutorial for a full tutorial and extension_examples for other customizations.

Core

These tools are available from the main ducktools.classbuilder module.

• @slotclass
  • A decorator based implementation that uses a special dict subclass assigned to __slots__ to describe the fields for method generation.
• AnnotationClass
  • A subclass based implementation that works with __slots__, type annotations or Field(...) attributes to describe the fields for method generation.
  • If __slots__ isn't used to declare fields, it will be generated by a metaclass.

Each of these forms of class generation will result in the same methods being attached to the class after the field information has been obtained.

from ducktools.classbuilder import Field, SlotFields, slotclass

@slotclass
class SlottedDC:
    __slots__ = SlotFields(
        the_answer=42,
        the_question=Field(
            default="What do you get if you multiply six by nine?",
            doc="Life, the Universe, and Everything",
        ),
    )
    
ex = SlottedDC()
print(ex)

Prefab

This is available from the ducktools.classbuilder.prefab submodule.

This includes more customization including __prefab_pre_init__ and __prefab_post_init__ functions for subclass customization.

A @prefab decorator and Prefab base class are provided. Similar to AnnotationClass, Prefab will generate __slots__ by default. However decorated classes with @prefab that do not declare fields using __slots__ will not be slotted and there is no slots argument to apply this.

Here is an example of applying a conversion in __post_init__:

from pathlib import Path
from ducktools.classbuilder.prefab import Prefab

class AppDetails(Prefab, frozen=True):
    app_name: str
    app_path: Path

    def __prefab_post_init__(self, app_path: str | Path):
        # frozen in `Prefab` is implemented as a 'set-once' __setattr__ function.
        # So we do not need to use `object.__setattr__` here
        self.app_path = Path(app_path)

steam = AppDetails(
    "Steam",
    r"C:\Program Files (x86)\Steam\steam.exe"
)

print(steam)

What is the issue with generating __slots__ with a decorator

If you want to use __slots__ in order to save memory you have to declare them when the class is originally created as you can't add them later.

When you use @dataclass(slots=True)¹ with dataclasses, the function has to make a new class and attempt to copy over everything from the original.

This is because decorators operate on classes after they have been created while slots need to be declared beforehand. While you can change the value of __slots__ after a class has been created, this will have no effect on the internal structure of the class.

By using a metaclass or by declaring fields using __slots__ however, the fields can be set before the class is constructed, so the class will work correctly without needing to be rebuilt.

For example these two classes would be roughly equivalent, except that @dataclass has had to recreate the class from scratch while AnnotationClass has created __slots__ and added the methods on to the original class. This means that any references stored to the original class before @dataclass has rebuilt the class will not be pointing towards the correct class.

Here's a demonstration of the issue using a registry for serialization functions.

This example requires Python 3.10 or later as earlier versions of dataclasses did not support the slots argument.

import json
from dataclasses import dataclass
from ducktools.classbuilder import AnnotationClass, Field


class _RegisterDescriptor:
    def __init__(self, func, registry):
        self.func = func
        self.registry = registry

    def __set_name__(self, owner, name):
        self.registry.register(owner, self.func)
        setattr(owner, name, self.func)


class SerializeRegister:
    def __init__(self):
        self.serializers = {}

    def register(self, cls, func):
        self.serializers[cls] = func

    def register_method(self, method):
        return _RegisterDescriptor(method, self)

    def default(self, o):
        try:
            return self.serializers[type(o)](o)
        except KeyError:
            raise TypeError(f"Object of type {type(o).__name__} is not JSON serializable")


register = SerializeRegister()


@dataclass(slots=True)
class DataCoords:
    x: float = 0.0
    y: float = 0.0

    @register.register_method
    def to_json(self):
        return {"x": self.x, "y": self.y}


# slots=True is the default for AnnotationClass
class BuilderCoords(AnnotationClass, slots=True):
    x: float = 0.0
    y: float = Field(default=0.0, doc="y coordinate")

    @register.register_method
    def to_json(self):
        return {"x": self.x, "y": self.y}


# In both cases __slots__ have been defined
print(f"{DataCoords.__slots__ = }")
print(f"{BuilderCoords.__slots__ = }\n")

data_ex = DataCoords()
builder_ex = BuilderCoords()

objs = [data_ex, builder_ex]

print(data_ex)
print(builder_ex)
print()

# Demonstrate you can not set values not defined in slots
for obj in objs:
    try:
        obj.z = 1.0
    except AttributeError as e:
        print(e)
print()

print("Attempt to serialize:")
for obj in objs:
    try:
        print(f"{type(obj).__name__}: {json.dumps(obj, default=register.default)}")
    except TypeError as e:
        print(f"{type(obj).__name__}: {e!r}")

Output (Python 3.12):

DataCoords.__slots__ = ('x', 'y')
BuilderCoords.__slots__ = {'x': None, 'y': 'y coordinate'}

DataCoords(x=0.0, y=0.0)
BuilderCoords(x=0.0, y=0.0)

'DataCoords' object has no attribute 'z'
'BuilderCoords' object has no attribute 'z'

Attempt to serialize:
DataCoords: TypeError('Object of type DataCoords is not JSON serializable')
BuilderCoords: {"x": 0.0, "y": 0.0}

What features does this have?

Included as an example implementation, the slotclass generator supports default_factory for creating mutable defaults like lists, dicts etc. It also supports default values that are not builtins (try this on Cluegen).

It will copy values provided as the type to Field into the __annotations__ dictionary of the class. Values provided to doc will be placed in the final __slots__ field so they are present on the class if help(...) is called.

AnnotationClass offers the same features with additional methods of gathering fields.

If you want something with more features you can look at the prefab submodule which provides more specific features that differ further from the behaviour of dataclasses.

Will you add <feature> to classbuilder.prefab?

No. Not unless it's something I need or find interesting.

The original version of prefab_classes was intended to have every feature anybody could possibly require, but this is no longer the case with this rebuilt version.

I will fix bugs (assuming they're not actually intended behaviour).

However the whole goal of this module is if you want to have a class generator with a specific feature, you can create or add it yourself.

Credit

Heavily inspired by David Beazley's Cluegen

Footnotes

1. or @attrs.define. ↩