test.rb

require 'test/unit'
require 'rubygems'
require 'mocha'

class Foo
  def self.say_hello_to(name)
    Bar.hello(name)
  end
end

class Bar
end

class TestFooWithStubbingInSetup < Test::Unit::TestCase
  def setup
    @test = states("test").starts_as("stubbing")
    @hello = mock("said hello")
    Bar.stubs(:hello).returns(@hello).when(@test.is("stubbing"))
  end

  def test_something
    Bar.expects(:hello).with("zab").when(@test.is("expecting"))
    @test.become("expecting")
    Foo.say_hello_to("baz")
  end

  def test_something_else
    assert_equal @hello, Foo.say_hello_to("bob")
  end

  # Lots of other tests so it's convenient to stub Bar.hello in the setup...
end

class TestFooWithoutStubbingInSetup < Test::Unit::TestCase
  def setup
    @hello = mock("said hello")
  end

  def test_something
    Bar.expects(:hello).with("zab")
    Foo.say_hello_to("baz")
  end

  def test_something_else
    Bar.stubs(:hello).returns(@hello)
    assert_equal @hello, Foo.say_hello_to("bob")
  end
end

Test output is now as follows :-

  1) Failure:
test_something(TestFooWithStubbingInSetup)
    [test.rb:7:in `say_hello_to'
     test.rb:24:in `test_something']:
unexpected invocation: Bar.hello('baz')
unsatisfied expectations:
- expected exactly once, not yet invoked: Bar.hello('zab'); when test is 'expecting'
satisfied expectations:
- allowed any number of times, not yet invoked: Bar.hello(any_parameters); when test is 'stubbing'
states:
- test is 'expecting'

  2) Failure:
test_something(TestFooWithoutStubbingInSetup)
    [test.rb:7:in `say_hello_to'
     test.rb:41:in `test_something']:
unexpected invocation: Bar.hello('baz')
unsatisfied expectations:
- expected exactly once, not yet invoked: Bar.hello('zab')