pick a subclass based on a parameter

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):
    ...

I'd store the name of the subclass in the params file, and have a factory method that would instantiate the class given its name:

class loader(object):
  @staticmethod
  def get_loader(name):
    return globals()[name]()

class sqlite_loader(loader): pass

class mysql_loader(loader): pass

print type(loader.get_loader('sqlite_loader'))
print type(loader.get_loader('mysql_loader'))

Store the classes in a dict, instantiate the correct one based on your param:

db_loaders = dict(sqlite=sqlite_loader, mysql=mysql_loader)
loader = db_loaders.get(db_type, default_loader)()

where db_type is the paramter you are switching on, and sqlite_loader and mysql_loader are the "loader" classes.