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A lib to define class extendable at runtime.

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Extendable is a module that aims to provide a way to define extensible python classes. This module was born out of the need to find a way to allow the definition of modules whose behaviour can be extended by other modules by extending the initial definition of classes at runtime.

Quick start

Let's define a first python class.

from extendable import ExtendableMeta

class Person(metaclass=ExtendableMeta):

    def __init__(self, name: str): = name

    def __repr__(self) -> str:

Someone using the module where the class is defined would need to extend the person definition with a firstname field.

from extendable import ExtendableMeta

class PersonExt(Person, extends=Person):
    def __init__(self, name: str):
        self._firstname = None

    def firstname(self) -> str:
        return self._firstname

    def firstname(self, value:str) -> None:
        self._firstname = value

    def __repr__(self) -> str:
        res = super().__repr__()
        return f"{res}, {self.firstname or ''}"

At this time we have defined that PersonExt extends the initial definition of Person. To finalyse the process, we need to instruct the runtime that all our class declarations are done by building the final class definitions and making it available into the current execution context.

from extendable import context, registry

_registry = registry.ExtendableClassesRegistry()

Once it's done the Person and PersonExt classes can be used interchangeably into your code since both represent the same class...

p = Person("Mignon")
p.firstname = "Laurent"
print (p)
#> Mignon, Laurent

:warning: This way of extending a predefined behaviour must be used carefully and in accordance with the Liskov substitution principle It doesn't replace others design patterns that can be used to make your code pluggable.


To run tests, use tox. You will get a test coverage report in htmlcov/index.html. An easy way to install tox is pipx install tox.

This project uses pre-commit to enforce linting (among which black for code formating, isort for sorting imports, and mypy for type checking).

To make sure linters run locally on each of your commits, install pre-commit (pipx install pre-commit is recommended), and run pre-commit install in your local clone of the extendable repository.

To release:

  • run ``bumpversion patch|minor|major --list
  • Check the new_version value returned by the previous command
  • run towncrier build.
  • Inspect and commit the updated HISTORY.rst.
  • git tag {new_version} ; git push --tags.


All kind of contributions are welcome.

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