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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: Encapsulation unpythonic?
Date: Tue, 03 Sep 2013 22:41:13 -0400
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Xref: csiph.com comp.lang.python:53591

On Tue, 03 Sep 2013 00:29:42 +0100, "Rhodri James"
<rhodri@wildebst.demon.co.uk> declaimed the following:

>On Sat, 31 Aug 2013 12:46:52 +0100, Steven D'Aprano  
><steve+comp.lang.python@pearwood.info> wrote:
>
>> # THIS DOES NOT HAPPEN IN PYTHON
>> # or any other language, as far as I am aware
>> x = 23
>> y = x  # y now has the value 23
>> x = 42  # change the value of the object  ### NOT SO! ###
>> print y
>> => prints 42
>
>Not directly, but FORTRAN did (does?) allow you to achieve this  
>oh-so-undesirable result indirectly.
>
>FORTRAN passes arguments to functions and procedures by reference.  That  
>includes passing constants, which effectively get put into hidden  
>variables and passed across.  Changes to the "constant" parameters in the  
>function can happen just as with any pass-by-reference, which would be  
>fine if you promptly threw the constant away.  Unfortunately smart  
>compilers used to (still do?) keep pools of these common constant  
>"variables" around rather than creating new ones all the time, so changes  
>to "constants" persisted.  Many's the poor natural scientist who was  
>perplexed to find that 0 suddenly had the value 1!

	Granted, this tended to be with older implementations. Most more modern
(F77 and later) tend to allocate such literals in a read-only block of
memory, so attempts to change them now cause a significant program failure.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/