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Re: [XML-SIG] RE: Equality tests on DOM nodes

From: Fred L. Drake <fdrake@cnri.reston.va.us>
Date: Tue, 15 Dec 1998 09:37:52 -0500 (EST)
Message-ID: <13942.29753.814023.621356@weyr.cnri.reston.va.us>
To: John Cowan <cowan@locke.ccil.org>
Cc: DOM List <www-dom@w3.org>, xml-sig@python.org

John Cowan writes:
 > The trouble with that scheme is that it makes equality hard to
 > reason about.  Intuitively, we expect equality to be transitive,
 > (if a = b and b = c then a = c), reflexive (a = a), and symmetrical
 > (if a = b then b = a).  Making equality depend on mutable properties
 > defeats this: a might = b at one time, but a later check for 
 > b = a might fail.

  That is correct.  This is very important for the programmer to know
about, and is a real consideration when designing a class for which
equality or ordering are important issues.  This is one reason why
many Python programmers use a minimalist approach for immutable data:
it's clear that a particular value will not change underneath you.
  However, I don't think comparison of mutable objects is necessarily
a signigicant problem.  I think most programmers expect equality of
objects to be meaning only when the comparison is made; any longevity
of the result depends on the specific guarantees made by that object.

 > >         a.reverse()
 > I presume this is a *destructive* reverse (leaves a reversed)?

  Yes, that's exactly how the list .reverse() method operates..
  I think we're sufficiently off-topic; we can move this to personal
email or some other forum if you wish to continue.  The topic is
interesting.  This might be good for comp.lang.python.


Fred L. Drake, Jr.	     <fdrake@acm.org>
Corporation for National Research Initiatives
1895 Preston White Dr.	    Reston, VA  20191
Received on Tuesday, 15 December 1998 10:46:21 UTC

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