How to Write Better Code with Mutation Testing

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How to Write Better Code with Mutation Testing John Backus - @backus - [email protected]

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Talk Outline

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Talk Outline Introduction

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Talk Outline Introduction Improving coverage

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Talk Outline Introduction Improving coverage Learning about Ruby and the code you rely on

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Talk Outline Improving coverage X-Raying Legacy Code Learning about Ruby and the code you rely on

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Talk Outline Incredible Dead Code Detection Learning about Ruby and the code you rely on X-Raying Legacy Code

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Talk Outline Thorough Test Coverage X-Raying Legacy Code Incredible Dead Code Detection

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Talk Outline Code Simplification Incredible Dead Code Detection Thorough Test Coverage

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Talk Outline Practicality Thorough Test Coverage Code Simplification

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Talk Outline Mutation Testing on the Job Code Simplification Practicality

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Line Coverage Lines of Code Run by Tests Total Lines of Code ÷

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Mutation Coverage How much of your code can I change without breaking your tests?

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Mutation Testing by Hand

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1 class Gluttons 2 def initialize(twitter_client) 3 @twitter = twitter_client 4 end 5 6 def recent 7 query = @twitter.search('"I really enjoy #pizza"') 8 9 query.first(2).map { |tweet| "@#{tweet.author}" } 10 end 11 end 1 RSpec.describe Gluttons do 2 it 'lists the two most recent gluttonous tweeters' do 3 tweets = [double(author: 'John'), double(author: 'Jane')] 4 gluttons = Gluttons.new(double(search: tweets)) 5 6 expect(gluttons.recent).to eql(%w[@John @Jane]) 7 end 8 end

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1 class Gluttons 2 def recent 3 - query = @twitter.search('"I really enjoy #pizza"') 4 + query = @twitter.search('"I really enjoy #hotdogs"') 1 example, 0 failures

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1 class Gluttons 2 def recent 3 - query = @twitter.search('"I really enjoy #pizza"') 4 + query = @twitter.search('') 1 example, 0 failures

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1 class Gluttons 2 def recent 3 - query = @twitter.search('"I really enjoy #pizza"') 4 + query = @twitter.search 1 example, 0 failures

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1 class Gluttons 2 def recent 3 - query.first(2).map { |tweet| "@#{tweet.author}" } 4 + query.first(1).map { |tweet| "@#{tweet.author}" } Failure expected: ["@John", "@Jane"] got: ["@John"]

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1 - query.first(2).map { |tweet| "@#{tweet.author}" } 2 + query.first(3).map { |tweet| "@#{tweet.author}" } 3 end 1 example, 0 failures

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Manual Mutation Testing Tedious

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Manual Mutation Testing Hard to outsmart yourself

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$ gem install mutest $ gem install mutant or

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1 def recent 2 - query = @twitter.search('"I really enjoy #pizza"') 3 + query = @twitter.search

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1 def recent 2 - query = @twitter.search('"I really enjoy #pizza"') 3 + query = @twitter.search(nil)

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1 - query.first(2).map { |tweet| "@#{tweet.author}" } 2 + query.last(2).map { |tweet| "@#{tweet.author}" } 3 end

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1 - query.first(2).map { |tweet| "@#{tweet.author}" } 2 + query.map { |tweet| "@#{tweet.author}" } 3 end

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1 it 'lists the two most recent gluttonous tweeters' do 2 tweets = [ 3 double(author: 'John'), 4 double(author: 'Jane'), 5 double(author: 'Devon') 6 ] 7 8 client = double('Client') 9 gluttons = Gluttons.new(client) 10 11 allow(client) 12 .to receive(:search) 13 .with('"I really enjoy #pizza"') 14 .and_return(tweets) 15 16 expect(gluttons.recent).to eql(%w[@John @Jane]) 17 end

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Mutation Testing with mutest Automated

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Probably more clever than you Mutation Testing with mutest

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Example #1: Internal API

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1 it 'returns a user when given a valid id' do 2 expect(get(:show, id: 1)).to eq(id: 1, name: 'John') 3 end 4 5 it 'renders JSON error when given an invalid id' do 6 expect(get(:show, id: 0)) 7 .to eq(error: "Could not find User with 'id'=0") 8 end 1 class UsersController < ApplicationController 2 def show 3 render json: User.find(params[:id].to_i) 4 rescue User::RecordNotFound => error 5 render json: { error: error.to_s } 6 end 7 end

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1 def show 2 - render json: User.find(params[:id].to_i) 3 + render json: User.find(Integer(params[:id])) 4 rescue User::RecordNotFound => error 5 render json: { error: error.to_s } 6 end

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1 def show 2 - render json: User.find(params[:id].to_i) 3 + render json: User.find(params.fetch(:id).to_i) 4 rescue User::RecordNotFound => error 5 render json: { error: error.to_s } 6 end

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1 def show 2 - render json: User.find(params[:id].to_i) 3 + render json: User.find(Integer(params.fetch(:id))) 4 rescue User::RecordNotFound => error 5 render json: { error: error.to_s } 6 end

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1 class UsersController < ApplicationController 2 def created_after 3 after = Date.parse(params[:after]) 4 render json: User.recent(after) 5 end 6 end

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1 def created_after 2 - after = Date.parse(params[:after]) 3 + after = Date.iso8601(params[:after]) 4 render json: User.recent(after) 5 end

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1 def created_after 2 - after = Date.parse(params[:after]) 3 + after = Date.iso8601(params[:after]) 4 render json: User.recent(after) 5 end

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1 def created_after 2 - after = Date.parse(params[:after]) 3 + after = Date.iso8601(params[:after]) 4 render json: User.recent(after) 5 end “2017-05-01" "H29.05.01" "Tue May 01 00:00:00 2017" "Tue, 01 May 2017 00:00:00 +0000" "Tue, 01 May 2017 00:00:00 GMT" "May" "I may be complete garbage" “2017-05-01" Date.parse Date.iso8601

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Example #2: Hardening Regular Expressions

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1 usernames.select do |username| 2 username =~ /^(John|Alain).+$/ 3 end

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1 usernames.select do |username| 2 - username =~ /^(John|Alain).+$/ 3 + username =~ /^(?:John|Alain).+$/ 4 end

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1 usernames.select do |username| 2 - username =~ /^(John|Alain).+$/ 3 + username.match?(/^(John|Alain).+$/) 4 end

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1 usernames.select do |username| 2 - username =~ /^(John|Alain).+$/ 3 + username.match?(/\A(?:John|Alain).+\z/) 4 end

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Example #3: Learning about your HTTP Client

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1 def stars_for(repo) 2 url = "https://api.github.com/repos/#{repo}" 3 data = HTTParty.get(url).to_h 4 5 data['stargazers_count'] 6 end

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1 def stars_for(repo) 2 url = "https://api.github.com/repos/#{repo}" 3 - data = HTTParty.get(url).to_h 4 + data = HTTParty.get(url) 5 6 data['stargazers_count'] 7 end

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X-Raying Legacy Code

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1 class UsersController < ApplicationController 2 def created_after 3 after = Date.parse(params[:after]) 4 render json: User.recent(after) 5 end 6 end

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1 def created_after 2 - after = Date.parse(params[:after]) 3 + after = Date.iso8601(params[:after]) 4 render json: User.recent(after) 5 end

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1 def created_after 2 - after = Date.parse(params[:after]) 3 + after = Date.iso8601(params[:after]) 4 render json: User.recent(after) 5 end What was the author’s intent?

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1 def created_after 2 - after = Date.parse(params[:after]) 3 + after = Date.iso8601(params[:after]) 4 render json: User.recent(after) 5 end How is this code used today?

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1 def created_after 2 - after = Date.parse(params[:after]) 3 + after = Date.iso8601(params[:after]) 4 render json: User.recent(after) 5 end Should I update the code or the tests?

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Thorough Test Coverage

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1 def can_buy_alcohol?(age) 2 age >= 21 3 end

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1 def can_buy_alcohol?(age) 2 - age >= 21 3 + age >= 22 4 end 1 def can_buy_alcohol?(age) 2 - age >= 21 3 + age >= 20 4 end

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1 def can_edit_post?(editor, post) 2 case editor.role 3 when :guest, :muted then false 4 when :user 5 editor == post.author && !post.locked? 6 when :moderator then !post.author.admin? 7 when :admin then true 8 end 9 end when :guest 1 :muted 2

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guest muted user moderator admin guest muted editor different user moderator admin Editor’s Role Post Author’s Role

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1 def mailing_list(users) 2 users.map do |user| 3 next unless user.email && !user.unsubscribed? 4 5 user.email 6 end.compact 7 end

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1 it 'filters out users without emails' do 2 users = [good_user, user_without_email] 3 expect(mailing_list(users)).to eql([good_user.email]) 4 end 5 6 7 it 'filters out users unsubscribed users' do 8 users = [good_user, unsubscribed_user] 9 expect(mailing_list(users)).to eql([good_user.email]) 10 end

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1 def mailing_list(users) 2 users.map do |user| 3 - next unless user.email && !user.unsubscribed? 4 + break unless user.email && !user.unsubscribed? 5 6 user.email 7 end.compact 8 end

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1 it 'filters out users without emails' do 2 - users = [good_user, user_without_email] 3 + users = [user_without_email, good_user] 4 expect(mailing_list(users)).to eql(good_user.email) 5 end 6 7 8 it 'filters out users unsubscribed users' do 9 - users = [good_user, unsubscribed_user] 10 + users = [unsubscribed_user, good_user] 11 expect(mailing_list(users)).to eql(good_user.email) 12 end

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Incredible Dead Code Detection

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1 class User < ActiveRecord::Base 2 def self.find_by_name(name) 3 find_by(name: name) 4 end 5 end

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1 class User < ActiveRecord::Base 2 def find_by_name(name) 3 - find_by(name: name) 4 + super 5 end 6 end

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1 class PostsController < ApplicationController 2 private 3 4 def authorized?(user = current_user) 5 # ... 6 end 7 end

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1 class PostsController < ApplicationController 2 def edit 3 return unauthorized unless authorized?(current_user) 4 end 5 6 def admin_edit 7 return unauthorized unless authorized?(current_user) 8 end 9 10 private 11 12 def authorized?(user = current_user) 13 # ... 14 end 15 end

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1 class PostsController < ApplicationController 2 - def authorized?(user = current_user) 3 + def authorized?(user) 4 # ... 5 end 6 end

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1 class PostsController < ApplicationController 2 def edit 3 return unauthorized unless authorized? 4 end 5 6 def admin_edit 7 return unauthorized unless authorized? 8 end 9 10 private 11 12 def authorized?(user = current_user) 13 # ... 14 end 15 end

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1 class PostsController < ApplicationController 2 def authorized?(user = current_user) 3 + user = current_user 4 # ... 5 end 6 end

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1 module MyApp 2 class User 3 def posted? 4 ::MyApp::Post.exists?(user_id: id) 5 end 6 end 7 end

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1 module MyApp 2 class User 3 def posted? 4 - ::MyApp::Post.exists?(user_id: id) 5 + Post.exists?(user_id: id) 6 end 7 end 8 end

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1 class PostsController < ApplicationController 2 def show 3 render json: Post.find(params[:id]), status: :ok 4 end 5 end

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1 class PostsController < ApplicationController 2 def show 3 - render json: Post.find(params[:id]), status: :ok 4 + render json: Post.find(params[:id]) 5 end 6 end

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Code Simplification

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1 def show_many 2 User.find(*params[:user_ids]) 3 end

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1 def show_many 2 - User.find(*params[:user_ids]) 3 + User.find(params[:user_ids]) 4 end

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1 class UserDecorator 2 attr_reader :user 3 4 def initialize(user) 5 @user = user 6 end 7 8 def greeting 9 "Welcome, #{@user.name}!" 10 end 11 end

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1 class UserDecorator 2 attr_reader :user 3 4 def initialize(user) 5 @user = user 6 end 7 8 def greeting 9 - "Welcome, #{@user.name}!" 10 + "Welcome, #{user.name}!" 11 end 12 end

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1 def greeting 2 "Welcome, #{@usre.name}!" 3 end

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1 def greeting 2 "Welcome, #{@usre.name}!" 3 end NoMethodError: undefined method `name' for nil:NilClass

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1 def greeting 2 "Welcome, #{usre.name}!" 3 end

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1 def greeting 2 "Welcome, #{usre.name}!" 3 end NameError: undefined local variable or method `usre`

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1 def expand_home(path) 2 path.gsub(%r{\A~/}, ENV['HOME']) 3 end

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1 def expand_home(path) 2 - path.gsub(%r{\A~/}, ENV['HOME']) 3 + path.sub(%r{\A~/}, ENV['HOME']) 4 end

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1 def prune_inactive_images 2 images = Image.where("last_viewed_at > ?", 2.years.ago) 3 4 images.map do |image| 5 puts "Deleting image #{image.name}" 6 end 7 8 images.destroy_all 9 10 images.count 11 end

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1 def prune_inactive_images 2 images = Image.where("last_viewed_at > ?", 2.years.ago) 3 4 - images.map do |image| 5 + images.each do |image| 6 puts "Deleting image #{image.name}" 7 end 8 9 images.destroy_all 10 11 images.count 12 end

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1 require 'logger' 2 3 logger = Logger.new($stdout) 4 5 logger.formatter = 6 Proc.new do |severity, datetime, name, msg| 7 "[#{severity}] #{datetime} #{name} -- #{msg}\n" 8 end

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1 require 'logger' 2 3 logger = Logger.new($stdout) 4 5 logger.formatter = 6 - Proc.new do |severity, datetime, name, msg| 7 + lambda do |severity, datetime, name, msg| 8 "[#{severity}] #{datetime} #{name} -- #{msg}\n" 9 end

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formatter = proc { |a, b| [a, b].inspect }

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formatter = proc { |a, b| [a, b].inspect }

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formatter = proc { |a, b| [a, b].inspect } formatter.call() # => "[nil, nil]" formatter.call(1) # => "[1, nil]" formatter.call(1, 2) # => "[1, 2]" formatter.call(1, 2, 3) # => "[1, 2]"

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formatter = proc { |a, b| [a, b].inspect } formatter.call() # => "[nil, nil]" formatter.call(1) # => "[1, nil]" formatter.call(1, 2) # => "[1, 2]" formatter.call(1, 2, 3) # => "[1, 2]" formatter.call([]) # => "[nil, nil]" formatter.call([1]) # => "[1, nil]" formatter.call([1, 2]) # => "[1, 2]" formatter.call([1, 2, 3]) # => "[1, 2]"

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formatter = lambda { |a, b| [a, b].inspect } formatter.call() # => ArgumentError formatter.call(1) # => ArgumentError formatter.call(1, 2) # => "[1, 2]" formatter.call(1, 2, 3) # => ArgumentError formatter.call([]) # => ArgumentError formatter.call([1]) # => ArgumentError formatter.call([1, 2]) # => ArgumentError formatter.call([1, 2, 3]) # => ArgumentError

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Practicality

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/^(John|Alain).+$/ /\A(John|Alain).+$/ /^(John|Alain).+\z/ /^(Alain).+$/ /^(John).+$/ /^(?:John|Alain).+$/

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$ mutest --use rspec --since master

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$ mutest \ --use rspec \ --since master \ ‘YourApp::User#validate_email’

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1 module YourApp 2 class User < ActiveRecord::Base 3 # Dozens of includes, scopes, class methods 4 5 def validate_email 6 # Simple method you're fixing 7 end 8 end 9 end

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1 RSpec.describe YourApp::User do 2 # 100s of tests and setup unrelated to your task 3 4 describe '#validate_email' do 5 # Half dozen tests you are focusing on 6 end 7 end