Extendable

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.

Let's define a first python class.

from extendable import ExtendableMeta

class Person(metaclass=ExtendableMeta):

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

    def __repr__(self) -> str:
        return self.name

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):
        super().__init__(name)
        self._firstname = None

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

    @firstname.setter
    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()
context.extendable_registry.set(_registry)
_registry.init_registry()

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.