test.rb

require 'minitest/autorun'

class MyTest < MiniTest::Unit::TestCase
  class SomeClass
    def calls_takes_args
      takes_args "hello world"
    end

    private
    def takes_args arg
      puts "i was called"
    end
  end

  def setup
    #pretend this instance came from somewhere else
    @instance = SomeClass.new
  end

  def test_immediate_check_args
    tc = self

    @instance.extend Module.new {
      define_method(:takes_args) { |arg|
        tc.assert_equal 'hello world', arg
        super(arg)
      }
    }

    @instance.calls_takes_args
  end

  def test_check_args_later
    args = []

    @instance.extend Module.new {
      define_method(:takes_args) { |arg|
        args << arg
        super(arg)
      }
    }

    @instance.calls_takes_args

    assert_equal ['hello world'], args
  end
end