Python 使用 len() 和 def __len__(self): 来构建一个类
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Using len() and def __len__(self): to build a class
提问by Gianni Spear
Just curious,
只是好奇，
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?
使用len()或def __len__()构建类之间有什么区别（优点和缺点）吗？哪个是最好的 Python 风格？
class foo(object):
def __init__(self,obs=[])
self.data = obs
self.max = max(obs)
self.min = min(obs)
self.len = len(obs)
or
或者
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)
采纳答案by Martijn Pieters
There is a hugedifference.
有很大的不同。
The __len__()method is a hook method. The len()functionwill use the __len__method if present to query your object for it's length.
该__len__()方法是一个钩子方法。该len()函数将使用该__len__方法（如果存在）来查询您的对象的长度。
The normalAPI people expect to use is the len()method, using a .lenattribute instead would deviate from that norm.
人们期望使用的正常API 是len()方法，而使用.len属性会偏离该规范。
If the length of self.datais not expected to change, you can always cache the length in an attribute and have .__len__()return that attribute.
如果self.data预期的长度不会改变，您始终可以将长度缓存在属性中并.__len__()返回该属性。
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
回答by NPE
There are several differences:
有几个区别：
- Only the second approach will give you the familiar
len(obj)syntax forfoo. The first will requireobj.len(). - If the length of
self.datacan change post-construction, only the second version will reflect the new length.
- 只有第二种方法才能为您提供熟悉
len(obj)的foo. 第一个将需要obj.len(). - 如果
self.data构建后的长度可以更改，则只有第二个版本会反映新的长度。
