rspec - what is the syntax of before :each and before :all vs let! and let

Question

When can I use let or let! and how can I then refer to variables inside the test?

How does the usage of let and let! compare to using before :each and before :all?

Answer 1

The 'classic' way of using before(:each) and before(:all) is

describe "tests" do
  before :each do
    @var1='a'
    @var2='b'
  end
end

You then refer to the instance variables by preceeding them with an @ sign inside the tests, i.e.

describe "tests" do
  before :each do
    @var1='a'
    @var2='b'
  end
  it  "does stuff" do
    a=some_class.some_method(@var1)
    expect(a).to eq @var2
  end
end

If you want a variable to have the same value in each test you can use before :all.
If you want a variable to have the same value generated for each test use before :each.
Generally use before :each as improper use of before :all can too easily lead to intermittent test failures.
Think of before :all as defining constants.

The let/let! format looks like this:

describe "tests" do
  let(:var1)  {'a'}
  let!(:var2) {srand(25)}
  it  "does stuff" do
    a=some_class.some_method(var1)
    expect(a).to eq var2
  end
end

Notice several things:

1. var1 and var2 now look like local variables, i.e. var1 instead of @var1 when used in the test and setup. In actual fact they are test class methods. The important thing to note for usage is that you do NOT use the @ sign when subsequently using them unlike the before :each and before :all where you did.
2. In the first case of the let's example, let is used with var1. This allow the memoization across examples to work. In the second case let! is used with var2. This is needed because you want the variable (method) to be called/set explicitly before each use. srand() is a great example of when you would need this as it seeds the random number generator but only for the next use of rand() so if using it in multiple test examples resetting it is needed each time.