Python factory method - creating objects from dictionaries

README.md

Python factory method

This is a simple approach for a factory that makes objects out of dictionaries. The only two ingredients are

• a metaclass Factory to manage the class registration and export a make function
• a base class Creatable that implements the creation funtion by calling the initializer

The user only has to derive classes from the base class Creatable and then has access to a dictionary-based creation of instances of such classes.

Please note

In the file where Factory.make() is called, it is necessary to import the Creatable concrete class otherwise the metaclass initialisation will not run.

factory.py

#!/usr/bin/env python3
"""
Simple factory implementation where we are using a metaclass to register
creation functions of classes derived from a base class Creatable.
"""

class Factory(type):
    """
    Metaclass to hold the registration dictionary of the constructor functions
    """
    register = {}
    def __init__(cls, name, bases, d):
        cls.register[cls.__name__] = cls.create

    @classmethod
    def make(mcs, concrete, kwargs):
        return mcs.register[concrete](kwargs)


class Creatable(object, metaclass=Factory):
    """
    Base class for factory creatable objects, implementing create()
    """
    @classmethod
    def create(cls, kwargs):
        return cls(**kwargs)


if __name__ == '__main__':
    
    # Here we are in user land where we create new classes derived from
    # Creatable
    class A(Creatable):
        def __init__(self, number):
            self.number = number
            print('created A: number =', self.number)


    class B(Creatable):
        def __init__(self, word):
            self.word = word
            print('created B: word =', self.word)

    # Now we can factory-create objects of our classes from lists of
    # dictionaries
    data = [{'A': {'number': 4}}, {'B': {'word': 'hello world!'}}]

    for d in data:
        for obj, ctor in d.items():
            Factory.make(obj, ctor)
        
    # The script will print the following lines:
    # created A: number = 4
    # created B: word = hello world!