rspec 3.0: under the covers

RSpec 3.0: Under the CoversAchieving “Zero Monkey-Patching Mode”

Brian Gesiak

March 13th, 2014

Research Student, The University of Tokyo

@modocache

• Monkey patching • How does RSpec work?

• The rspec executable • Loading spec files • Example groups: describe and context

• RSpec 2.11: describe no longer added to every Object

• Running examples (it blocks) • Expectations

• RSpec 2.11: expect-based syntax removes need for adding should to every Object

Monkey PatchingHow to Do It and Why You Shouldn’t

class Array def sum # Also defined in `activesupport`! inject { |sum, x| sum + x } end end !expect([1, 2, 3].sum).to eq 6

Monkey patching can lead to cryptic errors

Monkey Patching Root ObjectsGo Big or Go Home

class Object def should # ... end end

module Kernel def describe # ... end end

# Global method describe

# All objects respond # to method Object.new.should

RSpec 3.0

Historically, RSpec has extensively used monkey patching to create its readable syntax, adding methods…to every object. !In the last few 2.x releases, we’ve worked towards reducing the amount of monkey patching done by RSpec.

Zero Monkey-Patching Mode

Myron Marston, RSpec Core Member @myronmarston

RSpec 3.0Achieving “Zero Monkey-Patching Mode”

$ rspec meetup_spec.rb

The rspec Executablerspec-core/exe/rspec

#!/usr/bin/env ruby !require 'rspec/core' RSpec::Core::Runner.invoke

The rspec Executablerspec-core/exe/rspec

#!/usr/bin/env ruby !require 'rspec/core' RSpec::Core::Runner.invoke

Loading Spec Files

class CommandLine def run(err, out) # ... @configuration.load_spec_files # ... begin @configuration.hooks.run(:before, :suite) @world.ordered_example_groups.map { |g| g.run(reporter) } # ... ensure @configuration.hooks.run(:after, :suite) end end end

RSpec::Core::CommandLine.run

Loading Spec Files

Loading Spec FilesRSpec::Core::Configuration.load_spec_files

def load_spec_files files_to_run.uniq.each { |f| load f } @spec_files_loaded = true end

Loading Spec FilesRSpec::Core::Configuration.load_spec_files

def load_spec_files files_to_run.uniq.each { |f| load f } @spec_files_loaded = true end

Building Example Groups

describe TokyoRailsMeetup do let(:meetup) { described_class.new } describe '#start' do context 'without a chef' do it 'gets everyone drunk' do meetup.start expect(meetup.everyone_wasted?) .to be_true end end end end

Loading a Typical Spec File

Where Does describe Come From?RSpec::Core::DSL

def self.expose_example_group_alias(name) example_group_aliases << name ! (class << RSpec; self; end). __send__(:define_method, name) # ... ! expose_example_group_alias_globally(name) if exposed_globally? # defines on Module end

Disabling Module Monkey Patching

RSpec.configure do |config| config.expose_dsl_globally = false end !RSpec.describe TokyoRailsMeetup do let(:meetup) { described_class.new } describe '#start' do # ... end end

Only Top Level describe Blocks Are Affected

Opt-in as of RSpec 2.11

Building Example GroupsHierarchies of Example Groups and Examplesclass ExampleGroup def self.example_group(*args, &example_group_block) # ... child = subclass(self, args, &example_group_block) children << child child end ! def self.examples @examples ||= [] end # ... end

Building Example GroupsHierarchies of Example Groups and Examples

Running the Examples

Running the ExamplesRSpec::Core::Example.run

def run(example_group_instance, reporter) # ... begin run_before_each @example_group_instance. instance_exec(self, &@example_block) rescue Exception => e set_exception(e) ensure run_after_each end end

Making Expectations

it 'is a grand old time' do meetup.start meetup.should be_hoppin end !it 'gets everyone drunk' do meetup.start expect(meetup.everyone_wasted?).to be_true end

The Deprecated should Syntax

Making Expectations

The Deprecated should Syntax

How should is Monkey Patched

class Configuration # ... def syntax=(values) if Array(values).include?(:expect) Expectations::Syntax.enable_expect else Expectations::Syntax.disable_expect end ! if Array(values).include?(:should) Expectations::Syntax.enable_should else Expectations::Syntax.disable_should end end end

RSpec::Expectations::Configuration

def enable_should(syntax_host=::Object.ancestors.last) # ... syntax_host.module_exec do def should(matcher=nil, message=nil, &block) # ... end ! def should_not(matcher=nil, message=nil, &block) # ... end end end

RSpec::Expectations::Syntax

Making Expectations

The New expect(…).to Syntax

Making Expectations

How expect is Implemented

def enable_expect(syntax_host=::RSpec::Matchers) # ... syntax_host.module_exec do def expect(*target, &target_block) # ... end end end

Disabling should Monkey Patching

RSpec.configure do |config| config.expect_with :rspec do |c| c.syntax = [:should, :expect] c.syntax = :expect end end !it 'is a grand old time' do meetup.start meetup.should be_hoppin end

RSpec.configure do |config| config.expect_with :rspec do |c| c.syntax = [:should, :expect] c.syntax = :expect end end !it 'is a grand old time' do meetup.start meetup.should be_hoppin endexpect(meetup).to be_hoppin

should Emits Deprecation Warning as of RSpec 3.0

Bringing it All Together

Load the Specs & Build Example Groups, then Run

Want to Learn More about RSpec?

• http://modocache.io/rspec-under-the-covers • Expectations in RSpec 3.0 • RSpec Output Formatting • Shared Examples in RSpec

• Follow me on Twitter and GitHub at @modocache • First person to tweet me gets an Atom invite! #swag

• Myron Marston’s blog: http://myronmars.to/n/dev-blog

rspec 3.0: under the covers

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