PEP: 559
Title: Built-in noop()
Author: Barry Warsaw <barry@python.org>
Status: Rejected
Type: Standards Track
Created: 08-Sep-2017
Python-Version: 3.7
Post-History: 09-Sep-2017
Resolution: https://mail.python.org/pipermail/python-dev/2017-September/149438.html


Abstract
========

This PEP proposes adding a new built-in function called ``noop()`` which does
nothing but return ``None``.


Rationale
=========

It is trivial to implement a no-op function in Python.  It's so easy in fact
that many people do it many times over and over again.  It would be useful in
many cases to have a common built-in function that does nothing.

One use case would be for :pep:`553`, where you could set the breakpoint
environment variable to the following in order to effectively disable it::

    $ setenv PYTHONBREAKPOINT=noop


Implementation
==============

The Python equivalent of the ``noop()`` function is exactly::

    def noop(*args, **kws):
        return None

The C built-in implementation is available as a pull request [1]_.


Rejected alternatives
=====================

``noop()`` returns something
----------------------------

YAGNI.

This is rejected because it complicates the semantics.  For example, if you
always return both ``*args`` and ``**kws``, what do you return when none of
those are given?  Returning a tuple of ``((), {})`` is kind of ugly, but
provides consistency.  But you might also want to just return ``None`` since
that's also conceptually what the function was passed.

Or, what if you pass in exactly one positional argument, e.g. ``noop(7)``.  Do
you return ``7`` or ``((7,), {})``?  And so on.

The author claims that you won't ever need the return value of ``noop()`` so
it will always return ``None``.

Coghlan's Dialogs (edited for formatting):

    My counterargument to this would be ``map(noop, iterable)``,
    ``sorted(iterable, key=noop)``, etc. (``filter``, ``max``, and
    ``min`` all accept callables that accept a single argument, as do
    many of the itertools operations).

    Making ``noop()`` a useful default function in those cases just
    needs the definition to be::

       def noop(*args, **kwds):
           return args[0] if args else None

    The counterargument to the counterargument is that using ``None``
    as the default in all these cases is going to be faster, since it
    lets the algorithm skip the callback entirely, rather than calling
    it and having it do nothing useful.


References
==========

.. [1] https://github.com/python/cpython/pull/3480


Copyright
=========

This document has been placed in the public domain.
