Module dataclass_baseclass

DataClass - inheritable contagious base class.

Instead of (endless?) @dataclass decorating.

Usage

class A(DataClass): # it's a dataclass
class B(A): # it's a dataclass too

as opposed to:

@dataclass
class A(): ... # it's a dataclass
class B(A): ... # it's *not* a dataclass, needs decorating
@dataclass
class B(A): ... # now it's a dataclass

Also:

class B(DataClass, A): ... # all attributes from A become fields

as opposed to:

class A(): ...
@dataclass
class B(A): ... # no attributes from A are fields

Instantiation

class C(DataClass):
 a: str
 b: str
defaults: Data = {"a": "A", "b": "B"}
c = C(defaults, a="a", b="b")
or just:
c = C(a="a", b="b")

It kind of supports freezing/unfreezing on the fly, but it is best to keep all classes in the chain either frozen or not.

frozen attribute is dominant, ie as soon as you have one frozen parent class in the mix, class becomes frozen. If you want it unfrozen you need to specify dataclass_params.frozen as False:

class Unfrozen(Frozen, dataclass_params={"frozen": False}

Gotchas and features

Turning field into property in subclass

This works:

class C(DataClass):
 s: str
class CC(C):
 @property
 def s(self) -> str:
 return "S"

Create a frozen (readonly) copy with _frozen_copy()

Loaders

Tested with following dataclass loaders:

• apischema
• typedload
• dataclasses-json

dataclasses-json

Works with DataClassJsonMixin and from_dict() (actually _decode_dataclass()). Unfortunately we turn dataclass_json_config into an attribute.

I could not get it to work with @dataclass_json decorator, probably did not try hard enough.

Documentation and examples

Documentation

Tests should give a good idea of how to use it.

Test report

Name Stmts Miss Cover
--------------------------------------------
dataclass_baseclass.py 204 0 100%
--------------------------------------------
TOTAL 204 0 100%

Notes / FAQ

And Pydantic?

Pydantic is OK if you want to enter that world, stay there and comply. Some limitations with inheritance:

Straight multiple inheritance

class A(BaseModel): ...
class B(BaseModel): ...
class C(A, B): ...

The official stance on this (at least what I could figure out at the time of writing) is:

It will probably work, but not guaranteed, not officially supported

It could be argued, of course, that multiple inheritance is an anti-pattern and it is good that it is not supported. I have no strong opinion on that. But:

Protocols (or mix-ins, or whatever)

class A(BaseModel): ...
class P(Protocol): ...
class C(A, P): ...

That is a no-go.

With DataClass, we aim to enable all that.

Why not from scratch, why wrapping dataclasses?

Considering the effort that was put into dataclasses my conclusion is that dataclasses is the recommended way to standardise directly accessible class/instance properties in the standard library.

Has this been tested in real life?

I am using it in my personal (hobby?) projects. But nothing of a decent size in business environment.

A rant

Metaclasses. A quote from the official docs:

The potential uses for metaclasses are boundless. Some ideas that have been explored include enum, logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.

One could be easily forgiven to think that creating custom metaclasses is a valid thing to do, at least not discouraged. Some official examples of how to roll out your own metaclass, how to subclass type? I could not find it. type (meta)class is implementyed in C, and it is not light reading. Quite frustrating if one is after "what methods are available for overriding and what are their footprints".

One is condemned to trawling the internet, which comes up with the venerable "Let's make a singleton" example in 99.98% of the cases. That gives you a clue that you need to override __new__(), which has the same footprint as type(). Then you look in some corners of the internet, or much better ask ChatGPT, which gives you a hint that you could also play with __call__() method.

To sum it up, a laborious process. Why not documenting some examples and make life a tad easier...