pick a subclass based on a parameter

Question

I have a module (db.py) which loads data from different database types (sqlite,mysql etc..) the module contains a class db_loader and subclasses (sqlite_loader,mysql_loader) which inherit from it.

The type of database being used is in a separate params file,

How does the user get the right object back?

i.e how do I do:

loader = db.loader()

Do I use a method called loader in the db.py module or is there a more elegant way whereby a class can pick its own subclass based on a parameter? Is there a standard way to do this kind of thing?

Answer 1

Sounds like you want the Factory Pattern. You define a factory method (either in your module, or perhaps in a common parent class for all the objects it can produce) that you pass the parameter to, and it will return an instance of the correct class. In python the problem is a bit simpler than perhaps some of the details on the wikipedia article as your types are dynamic.

class Animal(object):

    @staticmethod
    def get_animal_which_makes_noise(noise):
        if noise == 'meow':
            return Cat()
        elif noise == 'woof':
            return Dog()

class Cat(Animal):
    ...

class Dog(Animal):
    ...