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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: Comparison Style
Date: Thu, 25 Apr 2013 22:37:10 -0400
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On Thu, 25 Apr 2013 15:57:49 +1000, Chris Angelico <rosuav@gmail.com>
declaimed the following in gmane.comp.python.general:

> On Thu, Apr 25, 2013 at 3:49 PM, llanitedave <llanitedave@veawb.coop> wrote:
> > Given that
> >
> > s = some static value
> > i = a value incremented during a loop
> >
> > I'm used to comparing them as
> >
> > if i == s:
> >     # some code
> >
> > But for some unknown reason I did a switch
> >
> > if s == i:
> >     # same code
> >
> > It didn't seem to make any difference at first glance, so I just got to wondering --
> 
> It won't make any difference in any sort of sane code. If there's any
> situation in which == is not reflexive, something seriously nasty is
> going on.
> 
> > Is there a standard for comparison order?  Is there any kind of performance difference?  Is there even a tradition for one or the other?  Are there any gotchas?
> 
> It's conventional to compare variables to constants, not constants to
> variables (even in C where there's the possibility of mucking up the
> operator, most people still compare variables to constants). I'd
> normally use "i == s" there, treating s as a constant for the purpose
> of the loop. Unless you're deliberately being poetical, language such
> as "Three is the number thou shalt count" is distinctly abnormal, so
> saying "if (5 == i)" is equally awkward. It's nothing major; mostly
> it's like the algebraic convention of putting the more-known elements
> earlier in a term (eg 2ðix - 2 is known, 3.14159.... is mostly known,
> i is imaginary but at least it's constant, and x is unknown).
>

	The main reason for /literal/ first is to trap C/C++ assignment as
expression.

	if (x = 5)

ends up assigning x the value 5 and THEN, since 5 is "true" executing
the "then" part.

	if (5 = x)

OTOH is a compiler error.

	But since Python doesn't allow assignment/binding as an expression,
either would be flagged an error.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/