let vs def

1_let.rb

desc "A user's comment" do
  let(:user) { User.create! name: "John" }
  let(:comment) { user.comments.create! }

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

2_def.rb

desc "A user's comment" do
  def user
    @user ||= User.create! name: "John"
  end

  def comment
    @comment ||= user.comments.create!
  end

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

3_def_condensed.rb

desc "A user's comment" do
  def user; @user ||= User.create! name: "John"; end
  def comment; @comment ||= user.comments.create!; end

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

You could also do this:

desc "A user's comment" do
  attr_reader :user, :comment

  setup do
    @user    = User.create! name: "John"
    @comment = user.comments.create!
  end

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

The thing that bugs me about the lazy initialize is that it makes it difficult to reason about the data that is required to execute a particular test. Since it's possible that not all tests will call all lazy initializers. I like to be able to look at a test class and say "oh, that's the data I need".

Oooh I didn't even know about setup. I like how that looks.

Typo in attr_reader. It should be

attr_reader :user, :comment

I never really understood the value added by let and it's lazy evaluation. If I am going to use variables in my test then I tend to declare them outright in a before block. The attr_reader but other coders seem annoyed by that (for some reason). I like the concept of using attr_read tho. Let seems to confuse a lot of newbies who don't understand how the symbol :tasty_steak magically becomes the variable tasty_steak. Even after you grasp that concept, it is unclear as to exactly when a let is evaluated durring execution.

Readability is important in test bc it is the first place I go to understand a project. Let is useful for that. Option 2 breaks the code up too much for me and the semi-colons get in my way when scanning option 3. I like Aaron's technique of putting assignment all into that setup block.

@KeysetTS It doesn't become a variable at all, it defines a method with that name at runtime that returns the value returned by the block given, which is then cached.

let(:user) { User.new }

has the exact same end result as

def user
  @user ||= User.new
end

As for when it is evaluated, the object isn't instantiated until you call user unless you use the let! form, which is evaluated immediately upon entering every test within the describe or context block in which that it is defined. Sure, it's a little advanced and it's a bit of an acquired taste, but the conciseness is always nice when you're writing dozens of them. :-)