如何重新加载__getitem__，以便在键不存在时可以创建它 [英] How can reload __getitem__ so that, when key doesn't exist, can create it

问题描述

是否可以创建尚不存在的键值，重新加载方法__ getitem __?

Is it possible to create a key value that does not yet exist, reload the method __ getitem__?

我尝试:

class Dict_(dict):
  def __getitem__(self, key):
     if key in self:
         self[key] = {}
         return self[key]
     else:
         return self[key]

我了解为什么会有堆栈溢出，但我不明白为什么没有溢出

I understand why there is a stack overflow, but I do not understand why there is no overflow

class Dict_(dict):
     def __getitem__(self, key):
         if key not in self:
             self[key] = {}
             return self.get(key)
         else:
             return self.get(key)

请参见说明，并发现使用get时也使用__ getitem__-D [k]

See description and see that when using get also uses __ getitem__ - D[k]

 D.get(k[,d]) -> D[k] if k in D, else d.  d defaults to None.

请告诉我，如何通过 __ getitem __ 做我想做的事情?

Please tell me, how to do what I want through __getitem__?

推荐答案

需要调用原创 __getitem__;使用 self [key] 调用顶级 __ getitem __ 并创建无限递归.

You need to call the original __getitem__; using self[key] calls the top-level __getitem__ and creates an infinite recursion.

执行此操作:

class Dict_(dict):
  def __getitem__(self, key):
     if key not in self:
         self[key] = {}
     return super(Dict_, self).__getitem__(key)

请注意，内置的Python dict 类型已经支持您的用例.如果您定义 __ missing __()方法，它将被称为自动:

Note that the built-in Python dict type already has support for your use-case; if you define a __missing__() method it'll be called automatically:

class Dict_(dict):
  def __missing__(self, key):
     self[key] = {}
     return self[key]

这是 collections.defaultdict() 确实，提供了一个 __ missing __ 方法，该方法调用了提供的工厂方法:

This is what collections.defaultdict() does, provide a __missing__ method that calls a provided factory method:

somedict = collections.defaultdict(dict)

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