Using len() and def __len__(self): to build a class

Question

Just curious,

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) 

or

   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) 

Answer 1

There is a huge difference.

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.

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

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