From 0b376eb0d63e0c51ed55c620b40edae6ded4ea48 Mon Sep 17 00:00:00 2001 From: "Miss Islington (bot)" <31488909+miss-islington@users.noreply.github.com> Date: Mon, 30 Jul 2018 22:56:27 -0700 Subject: [PATCH] bpo-27671: Update FAQ about why len is function (GH-8432) (cherry picked from commit c48e26dcadbff8620bb5881d3bd148fc8894d0ef) Co-authored-by: INADA Naoki --- Doc/faq/design.rst | 37 +++++++++++++++++++------------------ 1 file changed, 19 insertions(+), 18 deletions(-) diff --git a/Doc/faq/design.rst b/Doc/faq/design.rst index bb20f04c55..234dc9c12b 100644 --- a/Doc/faq/design.rst +++ b/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). - -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. - -.. 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. +As Guido said: + + (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. + + -- https://mail.python.org/pipermail/python-3000/2006-November/004643.html Why is join() a string method instead of a list or tuple method? -- 2.40.0