From: James Wenton <kkaitan@...>
Date: 2010-05-23T23:00:26+09:00
Subject: How do I write an RSpec test to unit-test this interesting 	metaprogramming code?

I'm a little stumped by this problem. Here's some simple code that,
for each argument specified, will add specific get/set methods named
after that argument. If you write `attr_option :foo, :bar`, then you
will see `#foo/foo=` and `#bar/bar=` instance methods on `Config`:

    module Configurator
      class Config
        def initialize()
          @options = {}
        end

        def self.attr_option(*args)
          args.each do |a|
            if not self.method_defined?(a)
              define_method "#{a}" do
                @options[:"#{a}"] ||= {}
              end

              define_method "#{a}=" do |v|
                @options[:"#{a}"] = v
              end
            else
              throw Exception.new("already have attr_option for #{a}")
            end
          end
        end
      end
    end

So far, so good. I want to write some RSpec tests to verify this code
is actually doing what it's supposed to. But there's a problem! If I
invoke `attr_option :foo` in one of the test methods, that method is
now forever defined in Config. So a subsequent test will fail when it
shouldn't, because `foo` is already defined:

      it "should support a specified option" do
        c = Configurator::Config
        c.attr_option :foo
        # ...
      end

      it "should support multiple options" do
        c = Configurator::Config
        c.attr_option :foo, :bar, :baz   # Error! :foo already defined
                                         # by a previous test.
        # ...
      end

Is there a way I can give each test an anonymous "clone" of the
`Config` class which is independent of the others?