Why use setup method for test suites in Ruby on Rails? - ruby-on-rails

Question

I am working through railstutorial.org and in our first test suite we setup an instance variable in a setup method.
require 'test_helper'
class StaticPagesControllerTest < ActionDispatch::IntegrationTest
def setup
#base_title = "Ruby on Rails Tutorial Sample App"
end
test "should get home" do
get static_pages_home_url
assert_response :success
assert_select "title", "Home | #{#base_title}"
end
test "should get help" do
get static_pages_help_url
assert_response :success
assert_select "title", "Help | #{#base_title}"
end
test "should get about" do
get static_pages_about_url
assert_response :success
assert_select "title", "About | #{#base_title}"
end
end
Why do we do this instead of just defining it straight up as an instance variable?

Answer 1

You are defining it as an instance variable. When the test is run the class (StaticPagesControllerTest) is instantiated and the setup function called on that instance, not on the class. If you tried to do the following instead it would define a class level instance variable not accessible from instances:
class StaticPagesControllerTest < ActionDispatch::IntegrationTest
#base_title = "Ruby on Rails Tutorial Sample App"
test "should get home" do
get static_pages_home_url
assert_response :success
assert_select "title", "Home | #{#base_title}" # XXX not defined on instance
end
...
See this page for one explanation of the difference.
You could use a class variable (##base_title) but that might cause surprising consequences later, e.g. if you wanted to subclass the test.
Here's a quick demonstration:
class First
#a = 'hoge'
##b = 'hoge'
def initialize
#c = 'hoge'
end
def give_me_a
#a
end
def give_me_b
##b
end
def give_me_c
#c
end
end
f = First.new
# => #<First:0x007fb81607fcd0>
f.give_me_a
# => nil
f.give_me_b
# => "hoge"
f.give_me_c
# => "hoge"