Python and factories

Question

I cant seem to grasp the proper concepts of a factory.

Can anyone help me code a simple test? I read some texts over the internet and cant code it the same way. Actually i cant understand the process. Copying code is easy, but i need to learn why this wont work.

class Factory:

    def __init__(self):
        self.msg = "teste"

    def fabricateAnotherObject(self,obj,**kwargs):
        return apply(obj,**kwargs)

class testClass:
    def __init__(self,nome,salario,endereco):
        self.nome = nome
        self.salario = salario
        self.endereco = endereco

    def __str__(self):
        return "Nome: " + str(self.nome) + "\nEndereco: " + str(self.endereco) + "\nSalario: " + str(self.salario) 

a = Factory()
emp = a.fabricateAnotherObject(testClass,"George",2000,"Three Four Five Avenue")
print str(emp)

Answer 1

Your code is counter-productive (sorry, I must say it).

The sense of a factory is, that you don't have to know the class of your constructed object at the position where you create it.

The reason is, that object creation creates a gap in object oriented abstraction. You must be concrete in creating the object. But sometimes you just want to create an object with some behaviour but somebody else should decide (centrally) what concrete class it is.

For example you must create one kind of object in 100 places. But later you might find out, that you must change the class -- you would have to change all those places.

The factory will eliminate this need by defining one place that you must change.

The simplest factory would be:

def fabricateAnotherObject(self, **kwargs):
    return testClass(**kwargs)

Of course, this might be of little help in some situations. So some factories might also load the class names from db or some other configuration. But the most simple solution is a hard-coded construction of the object -- only this method must be changed in our example, when you choose to always call this method.

A somewhat more dynamic solution (without need for a db):

class Factory(object):
   def __init__(self, theClass):
       self.theClass = theClass
   def create(self, **kwargs):
       self.theClass(**kwargs)

myFactory = Factory(testClass)

The myFactory instance can be used in different locations for creating the correct instances. The problem is, how to initialize myFactory -- in some special module??