python - 如何扩展图像类？

Question

我想在 PIL 中扩展“图像”类。

#module Image
def open(file): ...
class Image:
    def method1:...
    def method2:...

#module myOriginal
from Image import Image
class ExtendedImage(Image):
    def method3:...

#module test
import myOriginal
im = myOriginal.open("picture.jpg")

结果:Error.myOriginal 没有属性“open”。

如何在不重写 open() 方法的情况下扩展 Image 类？

Answer 1

According to Fredrik Lundh ，PIL作者:

the Image class isn't designed to be subclassed by application code. if you want custom behaviour, use a delegating wrapper.

myOriginal.py:

委托(delegate)单个方法:

class ExtendedImage(object):
    def __init__(self,img):
        self._img=img
    def method1(self):
        return self._img.method1()    #<-- ExtendedImage delegates to self._img
    def method3(self):
        ...

或者将(几乎)所有内容委托(delegate)给 self._img，您可以使用 __getattr__:

class ExtendedImage(object):
    def __init__(self,img):
        self._img=img
    def __getattr__(self,key):
        if key == '_img':
            #  http://nedbatchelder.com/blog/201010/surprising_getattr_recursion.html
            raise AttributeError()
        return getattr(self._img,key)
    def method3(self):
        print('Hiya!')

测试.py:

import Image
import myOriginal
im = myOriginal.ExtendedImage(Image.open("picture.jpg"))
im.method3()