Python 3 Object-Oriented Programming - Third Edition

The len() function

The simplest example is the len() function, which counts the number of items in some kind of container object, such as a dictionary or list. You've seen it before, demonstrated as follows::

>>> len([1,2,3,4])
4  

You may wonder why these objects don't have a length property instead of having to call a function on them. Technically, they do. Most objects that len() will apply to have a method called __len__() that returns the same value. So len(myobj) seems to call myobj.__len__().

Why should we use the len() function instead of the __len__ method? Obviously, __len__ is a special double-underscore method, suggesting that we shouldn't call it directly. There must be an explanation for this. The Python developers don't make ...