使用len()和def __len __(self):建立一个类 [英] Using len() and def __len__(self): to build a class

问题描述

很好奇，

在构建类时，使用len()或def __len__()有什么区别(优点和缺点)?最好的Python风格是什么?

Is there any difference (advantages and disadvantages) between using len() or def __len__() when I build a class? And which is the best Python style?

   class foo(object):
      def __init__(self,obs=[])
         self.data = obs
         self.max = max(obs)
         self.min = min(obs)
         self.len = len(obs)

或

   class foo(object):
      def __init__(self,obs=[])
         self.data = obs
         self.max = max(obs)
         self.min = min(obs)
      def __len__(self):
         return len(self.data)

推荐答案

有很大的区别.

__len__()方法是挂钩方法. len() 函数将使用__len__方法(如果存在)来查询对象的长度.

The __len__() method is a hook method. The len() function will use the __len__ method if present to query your object for it's length.

人们期望使用的 normal API是len()方法，而使用.len属性会偏离该规范.

The normal API people expect to use is the len() method, using a .len attribute instead would deviate from that norm.

如果预计self.data的长度不会改变，则始终可以将长度缓存在属性中，并让.__len__()返回该属性.

If the length of self.data is not expected to change, you can always cache the length in an attribute and have .__len__() return that attribute.

class foo(object):
    def __init__(self, obs=None):
        if obs is None:  # provide a default if no list was passed in.
            obs = []
        self.data = obs
        self.max = max(obs)
        self.min = min(obs)
        self._data_len = len(obs)

    def __len__(self):
        return self._data_len

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