I asked this question to core developer Raymond Hettinger in 2013 and the key to his answer was a quote from the Zen of Python: “ practicality beats purity ”. In “ How special methods are used ” on page 8 I described how len(x) runs very fast when x is an instance of a built-in type. No method is called for the built-in objects of CPython: the length is simply read from a field in a C struct. Getting the number of items in a collection is a common operation and must work efficiently for such basic and diverse types as str, list, memoryview etc. In other words, len is not called as a method because it gets special treatment as part of the Python Data Model, just like abs. But thanks to the special method __len__ you can also make len work with your own custom objects. This is fair compromise between the need for efficient built-in objects and the consistency of the language. Also from the Zen of Python: “ Special cases aren ’ t special enough to break the rules.”
------《 Fluent Python 》