如何制作Python工厂类

Question

我希望能够基于枚举类创建对象，并使用字典。像这样：

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()

但是我得到了错误：

NameError: name 'IngredientFactory' is not defined

由于某些原因，无法创建字典。我在哪里错了？

Answer 1

看了Bruce Eckel 的书后，我想到了这一点：

#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()

这让用户从枚举中选择一个类类型，并阻止任何其他类型。这也意味着用户不太可能写出带有拼写错误的“坏字符串”。他们只使用枚举。测试的输出是这样的：

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

Answer 2

Python不是Java，不需要将所有内容都放在类中。在这里，您的IngredientFactory类没有状态，只有静态方法，因此它实际上只是一个单例名称空间，在python中，使用模块作为单例名称空间和普通函数可以正常完成此操作。另外，由于Python类已经可以调用，因此将实例化包装在函数中没有任何意义。简单，直接的pythonic实现是：

# 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()

和客户端代码：

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

# or much more simply:

egg = ingredients.Egg()

FWIW IngredientType枚举在这里并没有带来太多好处，甚至使事情变得更加复杂-您可以使用简单的字符串：

# 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()

和客户端代码：

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

或者，如果您确实要使用an Enum，则可以至少choices按照MadPhysicist的建议，通过将类本身用作枚举的值来摆脱字典：

# 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()

和客户端代码

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

但我真的也看不到任何好处

编辑：您提到：

我试图实现工厂模式，以隐藏对象的创建。然后，工厂的用户无需了解具体类型就可以处理“成分”

用户仍然必须指定他想要哪种食材（ingredient_type参数），因此我不确定我是否了解这里的好处。您实际的用例是什么？（编造/精简示例的问题在于它们无法说明全部内容）。