dataclassbase: Implement custom dataclasses

What it is and how to get it

dataclassbase is a small toolkit for providing your own dataclasses. For dataclasses, attributes are declared via annotations, and constructors are created automatically making development fast.

However, often data in dataclasses shares common properties (e.g., data shape, etc.) or we might want to have more control over the fields in dataclasses. This is where dataclassbase comes to help: You can define your own dataclass-like behavior using typing.dataclass_transform or metaclasses, and control the behavior of your dataclass in a fine-grained manner.

You can install it into your environment from PyPI via

pip install dataclassbase

General note

While the project is generally usable, it is not yet production-ready. It is, however, being actively developed since I plan to use it as a base for further work.

License

MIT

Overview

The dataclasses module provides very basic dataclasses with little customization. The objective of this package is to overcome this limitation by providing customizable dataclasses.

A possible definition for the built-in dataclasses is

from dataclasses import dataclass

@dataclass
class SomeClass:
    some_field: str
    some_defaulting_field: int = 1

In this framework, a similar approach is possible:

from dataclassbase import dataclass

@dataclass
class SomeClass:
    some_field: str
    some_defaulting_field: int = 1

The most obvious difference above is that we import from dataclassbase rather than dataclasses. However, looking at the signature of dataclassbase.dataclass, we note that there are two notable parameters: field_provider and object_handler. field_provider allows customization of the field generation and processing (if the user provides a dataclassbase.Field as a default value), object_handler implements the logic for the initialization (__init__) and attribute assignment (__setattr__) of objects of the generated dataclass. We therefore can hook into or override the behavior of the dataclass.

If you omit the field_provider or object_handler, the respective default implementations provided in the FieldProvider and ObjectHandler classes, which reproduce the default dataclass behavior, are used instead. If you provide a callable matching the signature of dataclassbase.Field to field_provider, a FieldProvider is generated. Therefore, you can directly specify the type of field to use, as long as it matches the respective signature. You can also replace the default-value if it indicates a value you want to process.

Note that while it suffices to call dataclassbase.dataclass on a class to initialize dataclass-like behavior, it does not truly behave like a dataclass to static type checkers since it lacks the typing.dataclass_transform decorator. My reason for that choice is that I would have to fix the field_specifiers parameter. (Instances of the types provided in field_specifiers are allowed as default values for fields since they are assumed to be fields or to be converted into some. We do that conversion in the provided field_provider.) In doing so, I would constrain proper static type checking on the fields. Therefore, I would rather let you indicate the types yourself. To achieve true dataclass-like behavior on a type checker, you can call typing.dataclass_transform yourself or rely on a metaclass derived from DataclassMetaBase[TField]:

Rather than using dataclassbase.dataclass as above, you can define your own dataclass by deriving a metaclass from class DataclassMetaBase[TField] and attaching a typing.dataclass_transform to it. DataclassMetaBase invokes make_dataclass when creating the new type (dataclass is the decorator-variant of make_dataclass). It provides the methods _field_provider and _object_handler which act as factories for the field provider and object handler for the types using the metaclass.

from dataclassbase import DataclassMetaBase, Field, FieldProvider
from typing import dataclass_transform


class CustomField(Field):
    # you can override the methods provided by `Field` to control the behavior of the field in derived classes,
    # superclasses, to check the assignments, etc. You can also change the behavior of `__init__` as long as its 
    # signature matches the one from the base
    pass


@dataclass_transform
class CustomDataclassMeta(DataclassMetaBase[Field]):
    @classmethod
    def _field_provider(cls) -> FieldProvider[Field]:
        # we want it to return our custom field. The factory works since the signature of `CustomField.__init__` matches
        # the one of `Field.__init__`
        return FieldProvider(CustomField)
    
class VariantA(metaclass=CustomDataclassMeta):
    field: int
    defaulting: int = 1

dataclassbase.Fields themselves can check whether the value assigned to them is valid. Furthermore, they can check whether fields in derived classes are valid and, in the opposite direction, whether they are valid overrides of fields of superclasses. As an example, you can look at the provided dataclassbase.TypeConstrainedField which provides simple type checking via isinstance.

The default implementation is DataclassMeta which just uses the default values and allows any dataclasses.Field as a default value for its fields. While it behaves in the same way as the built-in dataclasses-dataclasses, it does however not give us any benefits over it.

from dataclassbase import DataclassMeta

class VariantB(metaclass=DataclassMeta):
    field: int
    defaulting: int = 1    

Why dataclassbase and not ...?

Pydantic

Pydantic's objective is to provide fast data validation and serialization [1]. The package is much more abstract since it is not constrained to a single use-case such as data validation, and thus does (hopefully) not give too strong restrictions on what you can/cannot do.