Testing inherited classes

citromatik has asked for the wisdom of the Perl Monks concerning the following question:

Hi all

Recently, one of my CPAN modules have been criticized for not having enough tests. This module is a class that only serves methods to other classes by inheritance and lacks a constructor.

The problem is that different independent modules can inherit this one:

Base  # No constructor
Base::X # Inherits Base, has constructor
Base::Y # Inherits Base, has constructor
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What is the best way to test Base without assuming that Base::X or Base::Y are installed? (In fact, they can not be installed before Base, because package Base is a pre-requisite of Base::X and Base::Y). Currently I test Base as part of the Base::X and Base::Y testing

Thanks in advance

citromatik

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Re: Testing inherited classes by chromatic (Archbishop) on Nov 09, 2010 at 16:51 UTC
If this class exists so that other classes can inherit from it, write a class which inherits from it and test that the subclass behaves as you would expect it to behave if it inherited properly from this class.	[reply]
Re: Testing inherited classes by choroba (Cardinal) on Nov 09, 2010 at 14:52 UTC
Wouldn't it be better for Base to have a constructor that would die with "Cannot instantiate an abstract class"? Update: You can also create Base::Test in your test script. Thus you can test what Base is really designed to do - to provide abstract parent.	[reply]
Re: Testing inherited classes by sundialsvc4 (Abbot) on Nov 09, 2010 at 16:07 UTC
Or, since constructors can have any name, what if you defined `sub testing_constructor`? This method would only be invoked by the module-testing code. To my way of thinking, the tests of the base-class (having thus been made possible by this alternate, special-purpose constructor) would of course be limited to functionality that was strictly in the base ... and that could, meaningfully, be “tested” in such abstract isolation. (Resist the temptation to have “tests to have tests.” Tests must actually make sense.) You might consider coding the abstract base-class so that it had such things as ... a `new` method that blew up, and stub-methods that also blew up. Just so that the abstract class wouldn’t do anything harmful if accidentally invoked in a ~~senior moment~~ brief episode of amnesia...
Re: Testing inherited classes by kcott (Archbishop) on Nov 10, 2010 at 07:08 UTC
With your current setup, you are maintaining the same tests in multiple subclasses. If you add a Base::NewSubclass in the future, you're going to have to repeat all of your Base tests in that module too and add to the list of modules that need to be maintained in parallel. Anyone adding their own Base::CustomSubclass is also going to face the same issues. You've already received good general advice above. If you identify the Base class in question, you may receive better specific advice. -- Ken	[reply]

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