caught in the import web again

Michael Torrie torriem at gmail.com
Tue Nov 18 08:00:42 CET 2014


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.




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