工廠方法模式（Factory Method Pattern）：

定義了一個建立物件的介面，但由子類決定要例項化類的哪一個；即通過子類來建立物件。

原則：

要依賴抽象，不要依賴具體類。

案例：

先解釋什麼是工廠：
如果你開一家Pizza店（PizzaStore抽象類）賣各種風味的Pizza（Pizza子類），那麼你需要根據客戶要求準備相應的Pizza（建立Pizza物件），然後烘烤、切片、包裝；
最簡單的做法就是在PizzaStore中根據客戶要求（型別判斷）建立相應的Pizza物件，然後呼叫Pizza自身（由Pizza抽象類實現）的烘烤、切片和包裝方法；
但這樣的程式碼缺乏彈性，因為你讓一個抽象類去依賴具體的物件；我們可以建立一個工廠來生產Pizza，根據傳入的不同型別值返回不同Pizza物件，即從PizzaStore中將建立物件的程式碼挪到工廠中。但這只是一個程式設計技巧，並不算模式。

在工廠方法模式中，我們在PizzaStore中定義一個抽象介面（create_pizza）作為抽象的工廠，而order_pizza是它的客戶；將Pizza物件的建立放到PizzaStore子類去解決。

現有Cheese和Clam兩款Pizza，以及NY和Chicago兩家分店，每家店的同款Pizza的口味不同——為迎合當地口味做了改進，主要差別來自不同的原材料，因此我們實現四個Pizza型別（NYStyleCheesePizza、NYStyleClamPizza、ChicagoStyleCheesePizza和ChicagoStyleClamPizza），每種使用不同的原材料組合，根據客戶所在城市和選擇款式我們建立不同的物件；根據工廠方法，我們將物件建立的程式碼放到PizzaStore子類去實現。

程式碼：

#!/usr/bin/python


class Pizza:
    name = ""
    dough = ""
    sauce = ""
    toppings = []

    def prepare(self):
        print "Preparing %s" % self.name
        print "    dough: %s" % self.dough
        print "    sauce: %s" % self.sauce
        print "    add toppings:"
        for n in self.toppings:
            print "        %s" % n

    def bake(self):
        print "Bake for 25 minutes at 350."

    def cut(self):
        print "Cutting into diagonal slices."

    def box(self):
        print "Put into official box."

    def get_name(self):
        return self.name


class PizzaStore:
    def order_pizza(self, pizza_type):
        self.pizza = self.create_pizza(pizza_type)
        self.pizza.prepare()
        self.pizza.bake()
        self.pizza.cut()
        self.pizza.box()
        return self.pizza

    def create_pizza(self, pizza_type):
        pass


class NYStyleCheesePizza(Pizza):
    def __init__(self):
        self.name = "NY Style Cheese Pizza"
        self.dough = "NY Dough"
        self.sauce = "NY Sauce"
        self.toppings.append("NY toopping A")
        self.toppings.append("NY toopping B")


class ChicagoStyleCheesePizza(Pizza):
    def __init__(self):
        self.name = "Chicago Style Cheese Pizza"
        self.dough = "Chicago Dough"
        self.sauce = "Chicago Sauce"
        sefl.toppings.append("Chicago toopping A")

    def cut(self):
        print "Cutting into square slices."


class NYStyleClamPizza(Pizza):
    def __init__(self):
        self.name = "NY Style Clam Pizza"
        self.dough = "NY Dough"
        self.sauce = "NY Sauce"
        self.toppings.append("NY toopping A")
        self.toppings.append("NY toopping B")


class ChicagoStyleClamPizza(Pizza):
    def __init__(self):
        self.name = "Chicago Style Clam Pizza"
        self.dough = "Chicago Dough"
        self.sauce = "Chicago Sauce"
        self.toppings.append("Chicago toopping A")

    def cut(self):
        print "Cutting into square slices."


class NYPizzaStore(PizzaStore):
    def create_pizza(self, pizza_type):
        if pizza_type == "cheese":
            return NYStyleCheesePizza()
        elif pizza_type == "clam":
            return NYStyleClamPizza()
        else:
            return None


class ChicagoPizzaStore(PizzaStore):
    def create_pizza(self, pizza_type):
        if pizza_type == "cheese":
            return ChicagoStyleCheesePizza()
        elif pizza_type == "clam":
            return ChicagoStyleClamPizza()
        else:
            return None

if __name__ == "__main__":
    ny_store = NYPizzaStore()
    chicago_store = ChicagoPizzaStore()

    pizza = ny_store.order_pizza("cheese")
    print "Mike ordered a %s." % pizza.get_name()
    print 

    pizza = chicago_store.order_pizza("clam")
    print "John ordered a %s." % pizza.get_name()
    print

 

輸出：

Preparing NY Style Cheese Pizza
    dough: NY Dough
    sauce: NY Sauce
    add toppings:
        NY toopping A
        NY toopping B
Bake for 25 minutes at 350.
Cutting into diagonal slices.
Put into official box.
Mike ordered a NY Style Cheese Pizza.

Preparing Chicago Style Clam Pizza
    dough: Chicago Dough
    sauce: Chicago Sauce
    add toppings:
        NY toopping A
        NY toopping B
        Chicago toopping A
Bake for 25 minutes at 350.
Cutting into square slices.
Put into official box.
John ordered a Chicago Style Clam Pizza.

參考：