gistfile1.rb

# Use constant directly
class V1
  def caculate
    Math::PI
  end
end

# Use a method
class V2
  def pi
    Math::PI
  end

  def caculate
    pi
  end
end

# Use dependency injection
class V3
  def initialize(pi=Math::PI)
    @pi = pi
  end

  def caculate
    @pi
  end
end


describe V1 do
	
  it "should return PI" do
    my_custom_pi = 3.14 
    stub_const("Math::PI", my_custom_pi) # RSpec 2.11 new feature. Very magic!
    V1.new.caculate.should == my_custom_pi
  end

end

describe V2 do
	
  it "should return PI" do
    my_custom_pi = 3.14
    v2 = V2.new
    v2.stub(:pi).and_return(my_custom_pi) # stub the method
    v2.caculate.should == my_custom_pi
  end

end

describe V3 do

  it "should return PI" do
    my_custom_pi = 3.14
    v3 = V3.new(my_custom_pi) # inject the dependency. Need not any magic stub.
    v3.caculate.should == my_custom_pi
  end

end