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

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

Answer 2

There are several differences:

1. Only the second approach will give you the familiar len(obj) syntax for foo. The first will require obj.len().
2. If the length of self.data can change post-construction, only the second version will reflect the new length.