classes

What it is and what it does

Classes provides a typeclass system for Python—a way to bind behavior to types without creating class hierarchies or interfaces. Instead of writing many subclasses (like IntField, BoolField, etc.), you define a typeclass as a function and register implementations for specific types using decorators. This approach reduces boilerplate, improves type safety through mypy integration, and makes it easier to extend behavior for new types without modifying existing code.

The package is designed for declarative, functional-style business logic where different data types need different handling. A common use case is serialization (converting objects to JSON based on their type), but it applies anywhere you'd otherwise use a type-based dispatch pattern. It requires mypy plugin configuration to achieve full type safety; without it, the type checker will report violations.

Use it for

• Implement serialization/deserialization logic that varies by type (e.g., custom JSON converters for int, bool, datetime).
• Build validation rules that apply different checks to different data types without inheritance chains.
• Create extensible handler registries where third-party code can add support for custom types.
• Write functional-style data transformation pipelines where operations dispatch based on input type.
• Reduce boilerplate in libraries that need type-specific behavior (like ORMs or REST frameworks).

Worth the install?

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Install

classes on PyPI

Before you install

Low friction install with a single runtime dependency (typing_extensions). Maintenance is dormant—last release was 2022-04-11, over 1586 days ago, though the repository remains active with a recent commit on 2024-12-17. Suitable for stable, mature use cases but not for projects requiring active upstream support.

Requires mypy plugin configuration in setup.cfg or mypy.ini under [mypy] with 'plugins = classes.contrib.mypy.classes_plugin' for full type safety; without it, type violations will be reported.

License in practice

BSD-2-Clause is permissive and imposes minimal restrictions. You can use, modify, and distribute this package freely in commercial and private projects provided you include the license notice.

Quickstart

pip install classes

from classes import typeclass

@typeclass
def to_json(instance) -> str:
    """Convert things to json."""

@to_json.instance(int)
def _to_json_int(instance: int) -> str:
    return str(instance)

result = to_json(value)

Package facts

Evidence: classes-0.4.1-py3-none-any.whl

Tags

More Python Modules packages

See also fasttransform · overloading · multipledispatch · visitor · simplegeneric · methoddispatch · plum-dispatch · singledispatch · returns · mypy-zope