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[2.7] bpo-27671: Update FAQ about why len is function (GH-8432) #8579

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miss-islington merged 1 commit into from
Jul 31, 2018
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[2.7] bpo-27671: Update FAQ about why len is function (GH-8432) #8579

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37 changes: 19 additions & 18 deletions Doc/faq/design.rst
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Expand Up @@ -226,24 +226,25 @@ Python file objects support the iterator protocol, so you can now write simply::
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?
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