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caught in the import web again

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  caught in the import web again "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2014-11-15 22:52 +0000
    Re: caught in the import web again Ben Finney <ben+python@benfinney.id.au> - 2014-11-16 10:43 +1100
    Re: caught in the import web again Denis McMahon <denismfmcmahon@gmail.com> - 2014-11-16 01:11 +0000
    Re: caught in the import web again dieter <dieter@handshake.de> - 2014-11-16 08:14 +0100
      Re: caught in the import web again "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2014-11-16 08:53 +0000
        Re: caught in the import web again Chris Angelico <rosuav@gmail.com> - 2014-11-16 20:20 +1100
        Re: caught in the import web again dieter <dieter@handshake.de> - 2014-11-17 08:08 +0100
          Re: caught in the import web again "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2014-11-17 20:07 +0000
            Re: caught in the import web again Dave Angel <davea@davea.name> - 2014-11-17 17:17 -0500
            Re: caught in the import web again Ian Kelly <ian.g.kelly@gmail.com> - 2014-11-17 15:52 -0700
            Re: caught in the import web again Dave Angel <davea@davea.name> - 2014-11-17 20:20 -0500
            Re: caught in the import web again Ian Kelly <ian.g.kelly@gmail.com> - 2014-11-18 08:28 -0700
            Re: caught in the import web again Dave Angel <davea@davea.name> - 2014-11-18 14:09 -0500
            Re: caught in the import web again Chris Angelico <rosuav@gmail.com> - 2014-11-19 07:18 +1100
            Re: caught in the import web again Dave Angel <davea@davea.name> - 2014-11-18 17:58 -0500
        Re: caught in the import web again Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-11-18 09:45 +1100
          Re: caught in the import web again Rick Johnson <rantingrickjohnson@gmail.com> - 2014-11-17 18:17 -0800
            Re: caught in the import web again Chris Angelico <rosuav@gmail.com> - 2014-11-18 13:25 +1100
            Re: caught in the import web again Steven D'Aprano <steve@pearwood.info> - 2014-11-18 07:13 +0000
          Re: caught in the import web again Michael Torrie <torriem@gmail.com> - 2014-11-18 00:00 -0700
            Re: caught in the import web again "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2014-11-18 09:13 +0000

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#81000

On Tue, 18 Nov 2014 00:00:42 -0700, Michael Torrie wrote:

> On 11/17/2014 03:45 PM, Steven D'Aprano wrote:
> 
>> Circular dependencies are not just a problem in Python, they are a
>> problem throughout most of software design.
> 
> Personally I find that duck typing eliminates a lot of the circular
> dependency problems.  Class A doesn't necessarily have to know about
> Class B to work with instances of Class B.  All that has to be known is
> what methods are going to be available on those instances.  The
> interface pattern can help here if you wanted a bit more safety (like
> what Zope provides).  Either way it breaks up the dependency cycle
> nicely.
> 
> If Class A needs to instantiate instances of Class B (which depends on
> Class A), then it could call a factory method to do so, perhaps set up
> by a third party after both modules have been imported.
> 
> There are lots of methods of avoiding the issue entirely.
> 
> In the C or C++ world, I find that signalling frameworks can also be
> used to break a dependency cycle.  For example, if Class A needs to call
> a method on an instance of Class B (which in turn depends on Class A),
> that could be done instead as a signal, which is later connected to
> Class B's method after everything is successfully instantiated.  Also
> makes testing a bit easier, because you can simply hook up the signals
> to a test harness.



A bunch of useful thoughts.  Thanks, folks!

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