python: len() function and __len__() method

len is a function to get the length of a collection. It works by calling an object's __len__ method.

class AClass(object):
    def __init__(self):
        self._list = [1,2,3]

    def __len__(self):
        return len(self._list)

a=AClass()
print(a.__len__()) # 3
print(len(a))      # 3