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

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@ -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))? Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?
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The major reason is history. Functions were used for those operations that were As Guido said:
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).
In fact, implementing ``len()``, ``max()``, ``min()`` as a built-in function is (a) For some operations, prefix notation just reads better than
actually less code than implementing them as methods for each type. One can postfix -- prefix (and infix!) operations have a long tradition in
quibble about individual cases but it's a part of Python, and it's too late to mathematics which likes notations where the visuals help the
make such fundamental changes now. The functions have to remain to avoid massive mathematician thinking about a problem. Compare the easy with which we
code breakage. 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:: -- https://mail.python.org/pipermail/python-3000/2006-November/004643.html
For string operations, Python has moved from external functions (the
``string`` module) to methods. However, ``len()`` is still a function.
Why is join() a string method instead of a list or tuple method? Why is join() a string method instead of a list or tuple method?