python: a little bit about dunder functions

What I learned from looking at Fluent Python yesterday:

• Functions defined with surrounding double-underlines (“dunder”) are called differently
•  e.g. len(my_object) instead of my_object.len()
• In general, we shouldn’t create new dunder  functions
• One reason is that newer versions of Python might use the same name
• __repre__ is called by __str__ if no __str__ defined, so it’s more important to define __repre__()  
• If no __bool__ defined, then python uses __len__() and returns False if 0, True otherwise