Why sorted() and reversed() are built-in functions and not methods of sequences?
python, python-3.3
Solution
The short answer is that these decisions were made for human reasons rather than technical reasons.
The reasoning for `len()` versus `obj.length()` is explained by Guido van Rossum (Python's Benevolent Dictator For Life):
First of all, I chose len(x) over x.len() for HCI reasons (def `__len__()` came much later). There are two intertwined reasons actually, both HCI:
(a) For some operations, prefix notation just reads better than postfix — prefix (and infix!) operations have a long tradition in mathematics which likes notations where the visuals help the mathematician thinking about a problem. Compare the easy with which we rewrite a formula like x*(a+b) into x*a + x*b to the clumsiness of doing the same thing using a raw OO notation.
(b) When I read code that says len(x) I know that it is asking for the length of something. This tells me two things: the result is an integer, and the argument is some kind of container. To the contrary, when I read x.len(), I have to already know that x is some kind of container implementing an interface or inheriting from a class that has a standard len(). Witness the confusion we occasionally have when a class that is not implementing a mapping has a get() or keys() method, or something that isn’t a file has a write() method.
He also explains why `sorted()` and `reversed()` aren't methods:
Also note that many functions are defined in terms of informal interfaces; for example, reversed works on anything that supports random access to items and has a known length. In practice, implementing things like max, sum, map, any, in and others, as built-in functions and operators is actually less code than implementing them as methods for each and every type that needs to support them.
Problem
I'm digging into Python 3.3 now and I wonder why some functions (for example, `sorted()`, `reversed()`) for managing collections/iterable are built-in but some are implemented as methods of collection objects? I can append item to list using method `append()`, but to get a length of it I should using built-in function `len()`. It seems inconsistent to me, but I think I just missing some point in new language.