bpo-27671: Update FAQ about why len is function (GH-8432)

(cherry picked from commit c48e26dcad)

Co-authored-by: INADA Naoki <methane@users.noreply.github.com>

Doc/faq/design.rst

@@ -215,24 +215,25 @@ objects using the ``for`` statement.  For example, :term:`file objects
 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).
+As Guido said:
 
-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.
+    (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.
 
-.. XXX talk about protocols?
+    (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.
 
-.. 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?