My notes when reading Luciano Ramalho's Fluent Python.
By implementing special methods (A.K.A. magic methods, like __len__, __getitem__, __repr__ etc.. Often pronounced dunder xx), your objects can utilize built-in functions and syntax like len(), [], for ... in ... and thus be considered Pythonic.
Understanding the Pythonic len(xx) over xx.len(): Think of these functions as unary operators.