How to implement a factory class?

Question

I want to be able to create objects based on an enumeration class, and use a dictionary. Something like this:

class IngredientType(Enum):
    SPAM        = auto() # Some spam
    BAKE_BEANS  = auto() # Baked beans
    EGG         = auto() # Fried egg

class Ingredient(object):
    pass    
class Spam(Ingredient):
    pass
class BakedBeans(Ingredient):
    pass
class Egg(Ingredient):
    pass


class IngredientFactory(object):
    """Factory makes ingredients"""

    choice = {
        IngredientType.SPAM: IngredientFactory.MakeSpam,
        IngredientType.BAKED_BEANS: IngredientFactory.MakeBakedBeans,
        IngredientType.EGG: IngredientFactory.MakeEgg
    }

    @staticmethod
    def make(type):
        method = choice[type]
        return method()

    @staticmethod
    def makeSpam():
        return Spam()

    @staticmethod
    def makeBakedBeans():
        return BakedBeans()

    @staticmethod
    def makeEgg():
        return Egg()

But I get the error:

NameError: name 'IngredientFactory' is not defined

For some reason the dictionary can't be created. Where am I going wrong here?

Answer 1

Python is not Java and doesn't require everything to be in a class. Here your IngredientFactory class has no states and only staticmethods, so it's actually just a singleton namespace, which in python is canonically done using the module as singleton namespace and plain functions. Also since Python classes are already callable, wrapping the instanciation in a function doesn't make sense. The simple, straightforwrad pythonic implementation would be:

# ingredients.py

class IngredientType(Enum):
    SPAM        = auto() # Some spam
    BAKE_BEANS  = auto() # Baked beans
    EGG         = auto() # Fried egg

class Ingredient(object):
    pass    

class Spam(Ingredient):
    pass

class Beans(Ingredient):
    pass

class Egg(Ingredient):
    pass


_choice = {
        IngredientType.SPAM: Spam,
        IngredientType.BAKED_BEANS: Beans,
        IngredientType.EGG: Egg
    }

def make(ingredient_type):
    cls = _choice[ingredient_type]
    return cls()

And the client code:

import ingredients
egg = ingredients.make(ingredients.IngredientType.EGG)

# or much more simply:

egg = ingredients.Egg()

FWIW the IngredientType enum doesn't bring much here, and even makes things more complicated that they have to be - you could just use plain strings:

# ingredients.py

class Ingredient(object):
    pass    

class Spam(Ingredient):
    pass

class Beans(Ingredient):
    pass

class Egg(Ingredient):
    pass


_choice = {
        "spam": Spam,
        "beans": Beans,
        "egg": Egg
    }

def make(ingredient_type):
    cls = _choice[ingredient_type]
    return cls()

And the client code:

import ingredients
egg = ingredients.make("egg")

Or if you really want to use an Enum, you can at least get rid of the choices dict by using the classes themselves as values for the enum as suggested by MadPhysicist:

# ingredients.py

class Ingredient(object):
    pass    

class Spam(Ingredient):
    pass

class Beans(Ingredient):
    pass

class Egg(Ingredient):
    pass

class IngredientType(Enum):
    SPAM = Spam
    BEANS = Beans
    EGG = Egg

    @staticmethod
    def make(ingredient_type):
        return ingredient_type.value()

and the client code

 from ingredients import IngredientType
 egg = IngredientType.make(IngredientType.EGG)

But I really don't see any benefit here either

EDIT: you mention:

I am trying to implement the factory pattern, with the intent of hiding the creation of objects away. The user of the factory then just handles 'Ingredients' without knowledge of the concrete type

The user still have to specify what kind of ingredients he wants (the ingredient_type argument) so I'm not sure I understand the benefit here. What's your real use case actually ? (the problem with made up / dumbed down examples is that they don't tell the whole story).

Answer 2

After looking at Bruce Eckel's book I came up with this:

#Based on Bruce Eckel's book Python 3 example
# A simple static factory method.
from __future__ import generators
import random
from enum import Enum, auto

class ShapeType(Enum):
    CIRCLE  = auto() # Some circles
    SQUARE  = auto() # some squares

class Shape(object):
    pass

class Circle(Shape):
    def draw(self): print("Circle.draw")
    def erase(self): print("Circle.erase")

class Square(Shape):
    def draw(self): print("Square.draw")
    def erase(self): print("Square.erase")

class ShapeFactory(object):

    @staticmethod
    def create(type):
        #return eval(type + "()") # simple alternative
        if type in ShapeFactory.choice:
            return ShapeFactory.choice[type]()

        assert 0, "Bad shape creation: " + type    

    choice = { ShapeType.CIRCLE:  Circle,
               ShapeType.SQUARE:  Square                
             }

# Test factory
# Generate shape name strings:
def shapeNameGen(n):

    types = list(ShapeType)

    for i in range(n):
        yield random.choice(types)

shapes = \
  [ ShapeFactory.create(i) for i in shapeNameGen(7)]

for shape in shapes:
    shape.draw()
    shape.erase()

This gets the user to select a class type from the enumeration, and blocks any other type. It also means user's are less likely to write 'bad strings' with spelling mistakes. They just use the enums. The output from the test is then, something like this:

Circle.draw
Circle.erase
Circle.draw
Circle.erase
Square.draw
Square.erase
Square.draw
Square.erase
Circle.draw
Circle.erase
Circle.draw
Circle.erase
Square.draw
Square.erase