proxy pattern in Python

Question

This is my try at the proxy pattern.

What do you Pythoneers think of my attempt?

class Image:
    def __init__( self, filename ):
        self._filename = filename
    def load_image_from_disk( self ):
        print("loading " + self._filename )
    def display_image( self ):
        print("display " + self._filename)

class Proxy:
    def __init__( self, subject ):
        self._subject = subject
        self._proxystate = None

class ProxyImage( Proxy ):
    def display_image( self ):
        if self._proxystate == None:
            self._subject.load_image_from_disk()
            self._proxystate = 1
        print("display " + self._subject._filename )

proxy_image1 = ProxyImage ( Image("HiRes_10Mb_Photo1") )
proxy_image2 = ProxyImage ( Image("HiRes_10Mb_Photo2") )


proxy_image1.display_image() # loading necessary
proxy_image1.display_image() # loading unnecessary
proxy_image2.display_image() # loading necessary
proxy_image2.display_image() # loading unnecessary
proxy_image1.display_image() # loading unnecessary

Output:

loading HiRes_10Mb_Photo1
display HiRes_10Mb_Photo1
display HiRes_10Mb_Photo1
loading HiRes_10Mb_Photo2
display HiRes_10Mb_Photo2
display HiRes_10Mb_Photo2
display HiRes_10Mb_Photo1

Answer 1

It seems like overkill to implement a class to represent the proxy pattern. Design patterns are patterns, not classes. What do you gain from your implementation compared to a simple approach like the one shown below?

class Image(object):
    def __init__(self, filename):
        self._filename = filename
        self._loaded = False

    def load(self):
        print("loading {}".format(self._filename))
        self._loaded = True

    def display(self):
        if not self._loaded:
            self.load()
        print("displaying {}".format(self._filename))

Some other notes on your code:

1. It doesn't follow PEP8. In particular, "Avoid extraneous whitespace [...] immediately inside parentheses."

2. No docstrings.

3. It makes more sense to use True and False for a binary condition than to use None and 1.

4. It's not necessary to append the class name to all the methods. In a class called Image, the methods should just be called load and display, not load_image and display_image.

5. You should use new-style classes (inheriting from object) to make your code portable between Python 2 and 3.