Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder2.enfer-du-nord.net!cs.uu.nl!news.stack.nl!newsfeed.xs4all.nl!newsfeed6.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
MIME-Version: 1.0
In-Reply-To: <CAFYqXL8PPOi2dy7M0ae+zCVB2yD6hPDdr7q8MhL0waadqoWvuw@mail.gmail.com>
References: <4F1AC1D4.2080402@gmail.com> <mailman.4909.1327157944.27778.python-list@python.org> <pL-dnUjq2eH3t4bSnZ2dnUVZ5vudnZ2d@giganews.com> <CAFYqXL8PPOi2dy7M0ae+zCVB2yD6hPDdr7q8MhL0waadqoWvuw@mail.gmail.com>
From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Mon, 23 Jan 2012 10:39:41 -0700
Subject: Re: while True or while 1
To: =?ISO-8859-1?Q?Giampaolo_Rodol=E0?= <g.rodola@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Cc: python-list@python.org
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.4968.1327340413.27778.python-list@python.org>
Lines: 58
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:19270

On Mon, Jan 23, 2012 at 9:41 AM, Giampaolo Rodol=E0 <g.rodola@gmail.com> wr=
ote:
>
> Il 21 gennaio 2012 22:13, Erik Max Francis <max@alcyone.com> ha scritto:
> > The real reason people still use the `while 1` construct, I would imagi=
ne,
> > is just inertia or habit, rather than a conscious, defensive decision. =
=A0If
> > it's the latter, it's a case of being _way_ too defensive.
>
> It's also because while 1 is faster:


That's because, as has already been pointed out in the thread, the
compiler is able to store the 1 literal with the code, whereas "True"
requires a name lookup.=A0 If you try the same timing test in Python 3,
you will find that there is no longer any difference.

> Think about receiving a 10 GB file by using a socket. You'd tipically
> have something like this:
>
> while 1:
> =A0 =A0chunk =3D sock.recv(1024):
> =A0 =A0if not chunk:
> =A0 =A0 =A0 =A0 =A0break
> =A0 =A0 ...
>
>
> Now, that's a case where I (personally) want to explicitly use "while
> 1" instead of "while True".


I disagree.=A0 10 GB in 1 KB chunks is going to be 10,000,000 loops,
which as you just demonstrated above, carries an overhead of about 0.4
seconds if we use True instead of 1.=A0 That's entirely trivial compared
to the time needed to actually transfer the file.

Moreover, unless you're doing a significant amount of processing on
the file as you read it, a socket-reading loop like that is likely to
be IO-bound -- which means that after the "while 1" check, the process
is usually going to go to sleep anyway as it waits for more data.=A0 In
that case, the extra 40 nanoseconds for the name lookup makes no
difference at all, as the process wasn't going to do anything useful
with those cycles anyway.

Actually, I have a hard time envisioning a case where the difference
between "while 1" and "while True" in Python 2.x is worth caring
about.=A0 At the point where it might actually start to be noticeable
(say around 100 million iterations), there are surely higher-priority
optimizations to be made, including rewriting parts of the program in
C.

"We should forget about small efficiencies, say about 97% of the time:
premature optimization is the root of all evil"
-- Donald Knuth

Cheers,
Ian