One class per test file (was --fqname option to test/unit/au

(Moved to ruby-talk from ruby-core since we’re on a more general topic)

Could you expand on why?

I would say granularity, maintenance and mental clarity.

By granularity, I mean the tests as a whole that are defined in your
TestCase subclass should be able to stand on their own, i.e. the tests
in one TestCase subclass should not depend on the results from another
TestCase subclass.

Likewise, the individual tests should be able to stand on their own.
This rule can be broken in exceptional circumstances, e.g. socket
programming tests, though even then the general philosophy is to uses
mocks and/or stubs instead of relying on one test to fulfill a prereq
for another test. I realize that’s not always possible or easy, though.

By maintenance I mean that it’s usually more difficult to maintain a
single test file when that file contains multiple subclasses of
TestCase. Say, for example, that you have a Foo class and Bar class.
Also say that you’ve created a TC_Bar and a TC_Foo class (both
subclasses of TestCase), and they both have a method called
“test_length”, and put both of them and their tests in the same file.

This creates two problems. First, using -n will be problematic. What
if I want to run the test_length test? From which class will it run? Or
will it run both? I don’t actually know without looking, and I don’t
want to know. Second, what if you decide to change the Bar API down the
road? Now you’ll have to modify a file that contains test code
completely unrelated to your changes.

One final point on maintenance. When I see a list of files like
tc_foo.rb, tc_bar.rb, etc, I can quickly figure out what each file is
testing i.e. a class or module called ‘foo’ and ‘bar’, respectively. If
someone later has to come along and maintain code that you’ve long
abandonded, it’s going to be more difficult for them to figure out where
you’re testing classes or modules, if at all. They’ll have to manually
inspect the code. I know that if I had to do that I would become very
irritated very quickly.

Lastly there’s the issue of mental clarity. When I open up the test
file I only want to mentally deal with ONE class or module. I don’t
want to worry about a bunch of code that’s unrelated to the code that I
want to test. Furthermore, if I’m taking over maintenance of someone
else’s code, I might start to wonder why the other subclasses of
TestCase are in the same file. Are they related? Do the tests from one
class impact another? Or did the original author just slap everything
into a single file?

(On a side note, in my own independant test suite for Ruby, I’ve broken
the test cases down by method to keep it even more mentally

It never occurred to me that there
might be a
reason not to. I’ve been dividing up my test cases based on chunks of
functionality, not on implementation details.

If you have classes with similar functionality being tested separately,
then I would say you probably have a design issue (you should probably
be using a mixin or subclass) or your tests are just not setup very
well. Note that I say “probably” because I’m not entirely certain
without seeing your test cases.

Well, anyway, there you have it.



PS - I wonder if anyone proposed a “Philosphy of Unit Testing” at
RubyConf this year.

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