Python Design Patterns II(Chapter 9 of Python 3 Object Oriented Programming)

>>> import weakref

>>> class CarModel:

    _models = weakref.WeakValueDictionary()

    def __new__(cls, model_name, *args, **kwargs):

        model = cls._models.get(model_name)

        if not model:

            model = super().__new__(cls)

            cls._models[model_name] = model

        return model

    def __init__(self, model_name, air=False, tilt=False,

             cruise_control=False, power_locks=False,

             alloy_wheels=False, usb_charger=False):

        if not hasattr(self, "initted"):

            self.model_name = model_name

            self.air = air

            self.tilt = tilt

            self.cruise_control = cruise_control

            self.power_locks = power_locks

            self.alloy_wheels = alloy_wheels

            self.usb_charger = usb_charger

            self.initted = True

    def check_serial(self, serial_number):

        print("Sorry, we are unable to check "

              "the serial number {0} on the {1} "

              "at this time".format(serial_number, self.model_name))



        

>>> class Car:

    def __init__(self, model, color, serial):

        self.model = model

        self.color = color

        self.serial = serial

    def check_serial(self):

        return self.model.check_serial(self.serial)



    

>>> dx = CarModel("FIT DX")

>>> lx = CarModel("FIT LX", air=True, cruise_control=True, power_locks=True, tilt=True)

>>> car1 = Car(dx, "blue", "12345")

>>> car2 = Car(dx, "black", "12346")

>>> car3 = Car(lx, "red", "12347")

>>> id(lx)

20634736

>>> del lx

>>> del car3

>>> import gc

>>> gc.collect()

0

>>> lx = CarModel("FIT LX", air=True, cruise_control=True, power_locks=True, tilt=True)

>>> id(lx)

20634224

 

 

 1 >>> class Window:
 2     def exit(self):
 3         sys.exit(0)
 4 
 5 >>> class Document:
 6     def __init__(self, filename):
 7         self.filename = filename
 8         self.contents = "This file cannot be modified"
 9     def save(self):
10         with open(self.filename, 'w') as file:
11             file.write(self.contents)
12 
13             
14 >>> class ToolbarButton:
15     def __init__(self, name, iconname):
16         self.name = name
17         self.iconname = iconname
18     def click(self):
19         self.command.execute()
20 
21         
22 >>> class MenuItem:
23     def __init__(self, menu_name, menuitem_name):
24         self.menu = menu_name
25         self.item = menuitem_name
26     def click(self):
27         self.command.execute()
28 
29         
30 >>> class KeyboardShortcut:
31     def __init__(self, key, modifier):
32         self.key = key
33         self.modifier = modifier
34     def keypress(self):
35         self.command.execute()
36 
37         
38 >>> class SaveCommand:
39     def __init__(self, document):
40         self.document = document
41     def execute(self):
42         self.document.save()
43 
44         
45 >>> class ExitCommand:
46     def __init__(self, window):
47         self.window = window
48     def execute(self):
49         self.window.exit()
50 
51 >>> window = Window()
52 >>> document = Document("c:\\1.txt")
53 >>> save = SaveCommand(document)
54 >>> exit = ExitCommand(window)
55 >>> save_button = ToolbarButton('save', 'save.png')
56 >>> save_button.command = save
57 >>> save_keystroke = KeyboardShortcut('s', 'ctrl')
58 >>> save_keystroke.command = save
59 >>> exit_menu = MenuItem("File", "Exit")
60 >>> exit_menu.command = exit