Re: [Python-Dev] PEP 559 - built-in noop()

On Sat, Sep 9, 2017 at 10:54 PM, Victor Stinner wrote:

I always wanted this feature (no kidding).

Would it be possible to add support for the context manager?

with noop(): ...

Maybe noop can be an instance of:

class Noop: def __enter__(self, *args, **kw): return self def __exit__(self, *args): pass def __call__(self, *args, **kw): return self

​This worries me. Clearly, assuming a None-coercing noop, we must have: noop(foo) is None noop[foo] is None noop * ​foo is None foo * noop is None noop + foo is None foo + noop is None noop - noop is None ... noop / 0 is None ... (noop == None) is None which can all sort of be implicitly extrapolated to be in the PEP. But how are you planning to implement: (noop is None) is None (obj in noop) is None (noop in obj) is None or (None or noop) is None (None and noop) is None and finally: foo(noop) is None ? Sooner or later, someone will need all these features, and the PEP does not seem to address the issue in any way. -- Koos

Victor

Le 9 sept. 2017 11:48 AM, "Barry Warsaw" a écrit :

...

I couldn’t resist one more PEP from the Core sprint. I won’t reveal where or how this one came to me.

-Barry

PEP: 559 Title: Built-in noop() Author: Barry Warsaw Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 2017-09-08 Python-Version: 3.7 Post-History: 2017-09-09

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.

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``.

Coghlin'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.

Copyright =========

This document has been placed in the public domain.

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