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Re: Importing variables non-deterministic?

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From Dave Angel <davea@davea.name>
Subject Re: Importing variables non-deterministic?
Date Mon, 19 Aug 2013 07:45:00 +0000 (UTC)
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Antoon Pardon wrote:

> Op 17-08-13 17:01, Steven D'Aprano schreef:
>> 
>> And here you re-import the name "y" from struct_global. That rebinds the 
>> current module's "y" with whatever value struct_global.y has *now*, 
>> rather than a second (or a minute, or an hour) earlier when the first 
>> import took place. Obviously at some point between the first import and 
>> the second import, struct_global.y must have been reassigned from -1 to 
>> 62.
>> 
>> This goes to show why global variables are considered harmful, and why 
>> clean, modern program design tries to reduce the use of them as much as 
>> possible. Global variables are too easily modified by, well, *anything*. 
>> The sort of behaviour you are seeing is sometimes called "action at a 
>> distance" -- something, anything, anywhere in your program, possibly 
>> buried deep, deep down inside some function you might never suspect, is 
>> changing the global variable.
>
> I think you are overstating your case. Classes and functions are
> variables too and in general nobody seems to have a problem with them
> being global.
>

It's global *variables* that are to be avoided.  constants like clsases
and functions are fine.  On the other hand, class attributes can be
variable, and thus are to be avoided when reasonable.

There *are* places where global variables make sense, such as for
caching, or counting.  But those are typically hidden, so they are
global in lifetime, but not in scope.

-- 
DaveA

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Thread

Importing variables non-deterministic? tmellman@googlemail.com - 2013-08-17 07:25 -0700
  Re: Importing variables non-deterministic? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-17 15:01 +0000
    Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 09:14 +0200
    Re: Importing variables non-deterministic? Dave Angel <davea@davea.name> - 2013-08-19 07:45 +0000
    Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 10:16 +0200
      Re: Importing variables non-deterministic? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-19 16:57 +0000
        Re: Importing variables non-deterministic? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-19 17:16 +0000
          Re: Importing variables non-deterministic? Chris Angelico <rosuav@gmail.com> - 2013-08-19 18:25 +0100
        Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 19:40 +0200
    Re: Importing variables non-deterministic? Chris Angelico <rosuav@gmail.com> - 2013-08-19 09:32 +0100
      Re: Importing variables non-deterministic? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-19 17:05 +0000
        Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 22:34 +0200
          Re: Importing variables non-deterministic? Steven D'Aprano <steve@pearwood.info> - 2013-08-20 05:48 +0000
            Re: Importing variables non-deterministic? wxjmfauth@gmail.com - 2013-08-19 23:40 -0700
            Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-20 08:55 +0200
              Re: Importing variables non-deterministic? wxjmfauth@gmail.com - 2013-08-20 00:31 -0700
                Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-20 09:55 +0200
                Re: Importing variables non-deterministic? wxjmfauth@gmail.com - 2013-08-20 02:15 -0700
    Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 11:03 +0200
    Re: Importing variables non-deterministic? Chris Angelico <rosuav@gmail.com> - 2013-08-19 10:18 +0100
    Re: Importing variables non-deterministic? Peter Otten <__peter__@web.de> - 2013-08-19 11:49 +0200
    Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 13:54 +0200
    Re: Importing variables non-deterministic? Dave Angel <davea@davea.name> - 2013-08-19 12:33 +0000
    Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 16:55 +0200
    Re: Importing variables non-deterministic? Chris Angelico <rosuav@gmail.com> - 2013-08-19 16:04 +0100
    Re: Importing variables non-deterministic? Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-08-19 19:25 +0200
    Re: Importing variables non-deterministic? Ben Finney <ben+python@benfinney.id.au> - 2013-08-20 11:14 +1000

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