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 i

Answer 1

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.