The document discusses how RSpec achieves "zero monkey patching mode" in version 3.0. It describes how RSpec previously extensively used monkey patching but has worked to reduce this in recent versions. It details how RSpec loads spec files, builds example groups and hierarchies, runs examples, and makes expectations without monkey patching core objects or classes. Key aspects covered include the rspec executable, loading files, building example groups, running examples, and the expect-based syntax.

1. RSpec 3.0: Under the Covers
Achieving“Zero Monkey-Patching Mode”
Brian Gesiak
March 13th, 2014
Research Student, The University of Tokyo
@modocache

2. Today
• 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

3. Monkey Patching
How 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

4. Monkey Patching
How 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

5. Monkey Patching
How 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

6. Monkey Patching Root Objects
Go 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

7. Monkey Patching Root Objects
Go 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

8. Monkey Patching Root Objects
Go 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

9. Monkey Patching Root Objects
Go 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

10. Monkey Patching Root Objects
Go 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

11. 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

12. RSpec 3.0
Achieving“Zero Monkey-Patching Mode”
$ rspec meetup_spec.rb

13. The rspec Executable
rspec-core/exe/rspec
#!/usr/bin/env ruby
!
require 'rspec/core'
RSpec::Core::Runner.invoke

14. The rspec Executable
rspec-core/exe/rspec
#!/usr/bin/env ruby
!
require 'rspec/core'
RSpec::Core::Runner.invoke

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Loading Spec Files
RSpec::Core::Configuration.load_spec_files
def load_spec_files
files_to_run.uniq.each { |f| load f }
@spec_files_loaded = true
end

22. Loading Spec Files
RSpec::Core::Configuration.load_spec_files
def load_spec_files
files_to_run.uniq.each { |f| load f }
@spec_files_loaded = true
end

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. Building Example Groups
Hierarchies of Example Groups and Examples
class 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

38. Building Example Groups
Hierarchies of Example Groups and Examples
class 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

39. Building Example Groups
Hierarchies of Example Groups and Examples
class 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

40. Building Example Groups
Hierarchies of Example Groups and Examples
class 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

41. Building Example Groups
Hierarchies of Example Groups and Examples

42. Running the Examples
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

43. Running the Examples
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

44. Running the Examples
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

45. Running the Examples
RSpec::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

46. Running the Examples
RSpec::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

47. Running the Examples
RSpec::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

48. Running the Examples
RSpec::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

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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

55. How should is Monkey Patched
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

56. How should is Monkey Patched
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

57. How should is Monkey Patched
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

58. 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 New expect(…).to Syntax

59. 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 New expect(…).to Syntax

60. 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 New expect(…).to Syntax

61. How expect is Implemented
def enable_expect(syntax_host=::RSpec::Matchers)
# ...
syntax_host.module_exec do
def expect(*target, &target_block)
# ...
end
end
end
RSpec::Expectations::Syntax

62. How expect is Implemented
def enable_expect(syntax_host=::RSpec::Matchers)
# ...
syntax_host.module_exec do
def expect(*target, &target_block)
# ...
end
end
end
RSpec::Expectations::Syntax

63. How expect is Implemented
def enable_expect(syntax_host=::RSpec::Matchers)
# ...
syntax_host.module_exec do
def expect(*target, &target_block)
# ...
end
end
end
RSpec::Expectations::Syntax

64. 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

65. 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

66. 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

67. 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

68. 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
expect(meetup).to be_hoppin

69. 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
expect(meetup).to be_hoppin
should Emits Deprecation Warning as of RSpec 3.0

70. Bringing it All Together
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
Load the Specs & Build Example Groups, then Run

71. Bringing it All Together
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
Load the Specs & Build Example Groups, then Run

72. Bringing it All Together
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
Load the Specs & Build Example Groups, then Run

73. Bringing it All Together
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
Load the Specs & Build Example Groups, then Run

74. Bringing it All Together
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
Load the Specs & Build Example Groups, then Run

75. 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