From: Paul Brannan <pbrannan@...>
Date: 2003-01-11T01:29:31+09:00
Subject: iterating in a test

Short question: if I have a test that iterates over a list of items, how
can I get assertion failure messages and exceptions to report which
iteraation I am on?

Longer question:

Suppose I have a test like this:

  require 'test/unit'

  def f(x)
    return x**2
  end

  class MyTest < Test::Unit::TestCase
    def initialize(*args)
      super(*args)
      p *args
    end

    def test_x_squared
      assert_equal 1, f(1)
      assert_equal 4, f(2)
      assert_equal 9, f(3)
    end
  end

I can rewrite test_x_squared like this:

    def test_x_squared
      for i in 1..3 do
        assert_equal i*i, f(i)
      end
    end

but if the test fails (I rewrite f(x) to return x**4), I get:

  "test_x_squared"
  Loaded suite test
  Started...
  ..
  Failure occurred in test_x_squared(MyTest) [test.rb:21]: Expected <4> but was <16>

which isn't very helpful.  If f(x) raises an exception, the message is
also not very helpful:

  "test_x_squared"
  Loaded suite test
  Started...
  ..
  Error occurred in test_x_squared(MyTest): RuntimeError: f(x) failed
          test.rb:4:in `f'
          test.rb:22:in `test_x_squared'
          test.rb:21:in `each'
          test.rb:21:in `test_x_squared'
          test.rb:20

The solution I came up with is something like this:

    def test_x_squared
      for i in 1..3 do
        appendage = "while testing for i=#{i}"
        begin
          assert_equal i*i, f(i), appendage
        ensure
          $!.message.replace($!.message + ' ' + appendage) if $!
        end
      end
    end

but it seems like there should be a cleaner or more generic solution
than this.  What is it?

Thanks,

Paul