dict和collections.defaultdict有什么区别？

问题描述

我正在检查Peter Norvig的，了解如何编写简单的拼写检查器。开始时，他使用这段代码将字词插入字典。

  def train（features）：
 model = collections.defaultdict（lambda：1）
 for f in features：
 model [f] + = 1 
 return model 
  

Python dict和这里使用的Python dict有什么区别？另外，什么是 lambda ？我检查了API文档，它表示defaultdict实际上是从dict派生而是一个决定哪一个使用？

解决方案

区别在于一个 defaultdict 如果该键尚未设置，将会默认一个值。如果您没有使用 defaultdict ，则必须检查该密钥是否存在，如果没有，请将其设置为您想要的。 p>

lambda为默认值定义工厂。该函数在需要默认值时被调用。您可以假设有一个更复杂的默认功能。

 模块集合中的类defaultdict帮助：
 
 class defaultdict（__ builtin __。dict）
 | defaultdict（default_factory） - > dict的默认工厂
 | 
 |调用默认工厂无参数生成
 |一个键值不存在时的新值，仅在__getitem__中。 
 | defaultdict与相同项目的dict相等。 
 | 
  

（来自 help（type（collections.defaultdict（））） code>）

{}。setdefault 本质上类似，但是接受一个值，而不是一个工厂功能。它用于设置值，如果它不存在...这有点不同，但是。

I was checking out Peter Norvig's code on how to write simple spell checkers. At the beginning, he uses this code to insert words into a dictionary.

def train(features):
    model = collections.defaultdict(lambda: 1)
    for f in features:
        model[f] += 1
    return model

What is the difference between a Python dict and the one that was used here? In addition, what is the lambda for? I checked the API documentation here and it says that defaultdict is actually derived from dict but how does one decide which one to use?

解决方案

The difference is that a defaultdict will "default" a value if that key has not been set yet. If you didn't use a defaultdict you'd have to check to see if that key exists, and if it doesn't, set it to what you want.

The lambda is defining a factory for the default value. That function gets called whenever it needs a default value. You could hypothetically have a more complicated default function.

Help on class defaultdict in module collections:

class defaultdict(__builtin__.dict)
 |  defaultdict(default_factory) --> dict with default factory
 |  
 |  The default factory is called without arguments to produce
 |  a new value when a key is not present, in __getitem__ only.
 |  A defaultdict compares equal to a dict with the same items.
 |  

(from help(type(collections.defaultdict())))

{}.setdefault is similar in nature, but takes in a value instead of a factory function. It's used to set the value if it doesn't already exist... which is a bit different, though.

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