如何为用户定义的对象定义类似字典的值赋值？

Question

以下代码使用Tkinter模块创建一个简单的gui.

import Tkinter

root = Tkinter.Tk()

myContainer1 = Tkinter.Frame(root)
myContainer1.pack()

button1 = Tkinter.Button(myContainer1) 
button1["text"]= "Hello, World!"  
button1["background"] = "green"     
button1.pack()                 

root.mainloop()

现在看下面的代码:

button1["text"]= "Hello, World!"

此语法与为字典键分配值相同.如何为用户定义的对象定义此行为的可能性？你能用一个例子来表示:

class Example(object):
    def __init__(self, length = 1, width= 2):
        self.length = length
        self.width = width

---

编辑1(对kindall和jsbueno的响应):我正在尝试通过执行以下操作来调用42分配的值e["alpha"]:

print e["alpha"]

我的代码添加了:

def __getitem__(self, key, value):
    if key in self.validkeys:
        getattr(self, key, value)
    else:
        raise KeyError

但我仍然得到'示例'不是可订阅的TypeError.

---

编辑2

def __getitem__(self, key):
    if key in self.validkeys:
        return getattr(self, key)
    else:
        raise KeyError

Answer 1

定义__setitem__(); 看到这里.也有相应的__getitem__().

至于一个例子:

class Example(object):
    # define which names can be assigned using [...] syntax
    validkeys = set("alpha beta gamma".split())

    # assign valid names as attributes on our object
    def __setitem__(self, key, value):
        if key in self.validkeys:
            setattr(self, key, value)
        else:
            raise KeyError("Key must be one of %s", ", ".join(self.validkeys))

e = Example()
e["alpha"] = 42
print(e.alpha)   # prints 42

查看有关模拟字典和其他内置类型的完整文档:http: //docs.python.org/reference/datamodel.html