diff -r 5f3ac68f34f2 Doc/faq/design.rst
--- a/Doc/faq/design.rst	Mon Dec 26 18:52:46 2016 +0900
+++ b/Doc/faq/design.rst	Mon Dec 26 20:59:50 2016 +0900
@@ -210,24 +210,49 @@ objects using the ``for`` statement.  Fo
 Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?
 ----------------------------------------------------------------------------------------------------------------
 
-The major reason is history. Functions were used for those operations that were
-generic for a group of types and which were intended to work even for objects
-that didn't have methods at all (e.g. tuples).  It is also convenient to have a
-function that can readily be applied to an amorphous collection of objects when
-you use the functional features of Python (``map()``, ``zip()`` et al).
+Guido says:
 
-In fact, implementing ``len()``, ``max()``, ``min()`` as a built-in function is
-actually less code than implementing them as methods for each type.  One can
-quibble about individual cases but it's a part of Python, and it's too late to
-make such fundamental changes now. The functions have to remain to avoid massive
-code breakage.
+   There are two bits of "Python rationale" that I'd like to explain first.
+   
+   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.
+   
+   Saying the same thing in another way, I see 'len' as a built-in
+   *operation*. I'd hate to lose that. I can't say for sure whether you
+   meant that or not, but 'def len(self): ...' certainly sounds like you
+   want to demote it to an ordinary method. I'm strongly -1 on that.
+   
+   The second bit of Python rationale I promised to explain is the reason
+   why I chose special methods to look __special__ and not merely
+   special. I was anticipating lots of operations that classes might want
+   to override, some standard (e.g. __add__ or __getitem__), some not so
+   standard (e.g. pickle's __reduce__ for a long time had no support in C
+   code at all). I didn't want these special operations to use ordinary
+   method names, because then pre-existing classes, or classes written by
+   users without an encyclopedic memory for all the special methods,
+   would be liable to accidentally define operations they didn't mean to
+   implement, with possibly disastrous consequences. Ivan Krstić
+   explained this more concise in his message, which arrived after I'd
+   written all this up.
 
-.. XXX talk about protocols?
-
-.. note::
-
-   For string operations, Python has moved from external functions (the
-   ``string`` module) to methods.  However, ``len()`` is still a function.
+https://mail.python.org/pipermail/python-3000/2006-November/004643.html
 
 
 Why is join() a string method instead of a list or tuple method?