"Monkey patching" is used (and misused) a lot in Ruby. It is a way of adding and overriding methods to existing classes without having to rewrite the entire class.

This post looks to demonstrate how monkey patching works in Ruby, as well as some best practices found from across the community.

Source code can be found here

We will create the project directory demo-monkey-patching-in-ruby and use Bundler to initialize the project:

$ mkdir demo-monkey-patching-in-ruby
$ cd demo-monkey-patching-in-ruby
# Create the required folders and files
# Our recursive creation for the extensions
$ mkdir -p lib/core_extensions/monkey_patching
$ touch lib/monkey_patching.rb lib/core_extensions/monkey_patching/basic.rb
# Make the test folder
$ mkdir spec
$ touch spec/monkey_patching_spec.rb
# Initialise Bundler project
$ bundler init
# RSpec added for testing
$ bundler add rspec --group "development,test"
# Pry added for debugging
$ bundler add pry --group "development,test"
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At this stage, our project is now ready to start working with.

Within the lib/monkey_patching.rb file, let's add the following code:

class MonkeyPatching
  def hello
    'world'
  end

  def yell
    'gaaah!'
  end

  def extend
    'extended'
  end
end
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This class is contrived for the sake of our demonstration, but in this post we will be aiming for the following:

Before we write out our extension file, it is best to do a quick pivot to understand why we will opt to use prepend instead of include for the purpose of monkey patching in Ruby.

Dave Allie has an excellent post demonstrating the ancestral chain of classes when using include vs prepend.

It is best to read the post through, but as a simple example, here is the relevant ancestral demonstration:

# For `include`
module IncludeExt
end

class IncludeExample
  include IncludeExt
end

IncludeExample.ancestors
# => [IncludeExample, IncludeExt, Object, Kernel, BasicObject]

# For `prepend`
module PrependExt
end

class PrependExample
  prepend PrependExt
end

IncludeExample.ancestors
# => [PrependExample, PrependExt, Object, Kernel, BasicObject]
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The tl;dr from the post most relevant to monkey patching: Use prepend when overwriting/extending an existing class.

If we prepend a module to a class with methods of the same name, we will overwrite the base definition. If we use the super keyword within a prepend, we can call to the original definition and extend its behavior. We will demonstrate both overwriting as well as extending.

Let's write out our spec into spec/monkey_patching.rb so that we can see what our aims our. The approach in these specs are contrived but designed to colocate the examples into one test file:

require 'monkey_patching'
require 'core_extensions/monkey_patching/basic'

RSpec.describe MonkeyPatching do
  context 'no monkey patching' do
    it 'should return "world"' do
      expect(subject.hello).to eq('world')
    end

    it 'should return our gasp when we yell' do
      expect(subject.yell).to eq('gaaah!')
    end

    it 'should return "extended" when we call to extend' do
      expect(subject.extend).to eq('extended')
    end
  end

  context 'with monkey patching' do
    before do
      MonkeyPatching.prepend CoreExtensions::MonkeyPatching::Basic
    end

    xit 'should return "people"' do
      expect(subject.hello).to eq('people')
    end

    xit 'should return our gasp when we yell' do
      expect(subject.yell).to eq('gaaah!')
    end

    xit 'should return "I am extended" when we call to extend' do
      expect(subject.extend).to eq('I am extended')
    end
  end
end
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In our specs, we split up the examples into two contexts:

The "With monkey patching" context has a before hook to prepend the module CoreExtensions::MonkeyPatching::Basic that we have included in our file before the tests.

I am also currently using xit to denote the tests that we will skip and work on later.

As things currently stand, if we run bundler exec -- rspec spec/monkey_patching_spec.rb then we should get the following output:

$ bundler exec -- rspec spec/monkey_patching_spec.rb
...***

Pending: (Failures listed here are expected and do not affect your suite's status)

  1) MonkeyPatching with monkey patching should return "people"
     # Temporarily skipped with xit
     # ./spec/monkey_patching_spec.rb:24

  2) MonkeyPatching with monkey patching should return our gasp when we yell
     # Temporarily skipped with xit
     # ./spec/monkey_patching_spec.rb:28

  3) MonkeyPatching with monkey patching should return "I am extended" when we call to extend
     # Temporarily skipped with xit
     # ./spec/monkey_patching_spec.rb:32
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Our basic expectations for our standard MonkeyPatching class all pass, and our three tests that are marked xit are not yet run.

If we change our fourth test 'should return "people"' to it and run our tests again, we will see our fourth test fails as subject.hello still returns world.

$ bundler exec -- rspec spec/monkey_patching_spec.rb
...F**

Pending: (Failures listed here are expected and do not affect your suite's status)

  1) MonkeyPatching with monkey patching should return our gasp when we yell
     # Temporarily skipped with xit
     # ./spec/monkey_patching_spec.rb:28

  2) MonkeyPatching with monkey patching should return "I am extended" when we call to extend
     # Temporarily skipped with xit
     # ./spec/monkey_patching_spec.rb:32


Failures:

  1) MonkeyPatching with monkey patching should return "people"
     Failure/Error: MonkeyPatching.prepend CoreExtensions::MonkeyPatching::Basic

     NameError:
       uninitialized constant CoreExtensions
     # ./spec/monkey_patching_spec.rb:21:in `block (3 levels) in <top (required)>'

Finished in 0.00441 seconds (files took 0.12602 seconds to load)
6 examples, 1 failure, 2 pending

Failed examples:

rspec ./spec/monkey_patching_spec.rb:24 # MonkeyPatching with monkey patching should return "people"
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To remedy this, let's move onto writing some code for our extension to fix this problem.

The file lib/core_extensions/monkey_patching/basic.rb is where we will write our monkey patching code.

The naming convention used here comes from a Rails convention brought up in this blog post by Justin Weiss where states that the Rails convention is to write our monkey patches to the file-naming convention lib/core_extensions/<class_name>/<group>.rb. Feel free to adjust as you see fit for your own project.

Within the file, add the following:

module CoreExtensions
  module MonkeyPatching
    module Basic
      def hello
        'people'
      end

      def extend
        "I am #{super}"
      end
    end
  end
end
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Our code here enables us to fulfill all three of our objectives.

At this stage, we are ready to go back to our test file and change all of our xit tests to it.

require 'monkey_patching'
require 'core_extensions/monkey_patching/basic'

RSpec.describe MonkeyPatching do
  context 'no monkey patching' do
    it 'should return "world"' do
      expect(subject.hello).to eq('world')
    end

    it 'should return our gasp when we yell' do
      expect(subject.yell).to eq('gaaah!')
    end

    it 'should return "extended" when we call to extend' do
      expect(subject.extend).to eq('extended')
    end
  end

  context 'with monkey patching' do
    before do
      MonkeyPatching.prepend CoreExtensions::MonkeyPatching::Basic
    end

    it 'should return "people"' do
      expect(subject.hello).to eq('people')
    end

    it 'should return our gasp when we yell' do
      expect(subject.yell).to eq('gaaah!')
    end

    it 'should return "I am extended" when we call to extend' do
      expect(subject.extend).to eq('I am extended')
    end
  end
end
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If we now run RSpec, we should see the following output:

$ bundler exec -- rspec spec/monkey_patching_spec.rb
......

Finished in 0.00611 seconds (files took 0.15551 seconds to load)
6 examples, 0 failures
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Perfect! Our monkey patching code has been successfully applied to our MonkeyPatching class.

We can look deeper at this using binding.pry to see what is happening within our MonkeyPatching class.

Let's adjust our first test to add binding.pry to our test:

require 'pry' # require pry gem
require 'monkey_patching'
require 'core_extensions/monkey_patching/basic'

RSpec.describe MonkeyPatching do
  context 'no monkey patching' do
    it 'should return "world"' do
      expect(subject.hello).to eq('world')
    end

    it 'should return our gasp when we yell' do
      expect(subject.yell).to eq('gaaah!')
    end

    it 'should return "extended" when we call to extend' do
      expect(subject.extend).to eq('extended')
    end
  end

  context 'with monkey patching' do
    before do
      MonkeyPatching.prepend CoreExtensions::MonkeyPatching::Basic
    end

    it 'should return "people"' do
      binding.pry # LINE CHANGED HERE
      expect(subject.hello).to eq('people')
    end

    it 'should return our gasp when we yell' do
      expect(subject.yell).to eq('gaaah!')
    end

    it 'should return "I am extended" when we call to extend' do
      expect(subject.extend).to eq('I am extended')
    end
  end
end
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Now when we run our tests, our debugger will open up in the terminal. We can use the ancestors method to see what is happening within our MonkeyPatching class.

[1] pry(#<RSpec::ExampleGroups::MonkeyPatching::WithMonkeyPatching>)> MonkeyPatching.ancestors
=> [CoreExtensions::MonkeyPatching::Basic,
 MonkeyPatching,
 Object,
 PP::ObjectMixin,
 Kernel,
 BasicObject]
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Here we can note that CoreExtensions::MonkeyPatching::Basic is the first ancestor of MonkeyPatching. This is because we have added our monkey patching code to the top of our class.

The super ancestor of CoreExtensions::MonkeyPatching::Basic is MonkeyPatching, so this is what helps us to access the original implementation of extend.

Below are some more debugging calls to show what is returned by binding.pry to aid in demonstrating what our MonkeyPatching subject has as return values when those methods are invoked:

[2] pry(#<RSpec::ExampleGroups::MonkeyPatching::WithMonkeyPatching>)> subject.extend
=> "I am extended"
[3] pry(#<RSpec::ExampleGroups::MonkeyPatching::WithMonkeyPatching>)> subject.hello
=> "people"
[4] pry(#<RSpec::ExampleGroups::MonkeyPatching::WithMonkeyPatching>)> subject.yell
=> "gaaah!"
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Today's post demonstrated how to use the prepend method to add monkey patching code to a class.

Monkey patching can be useful for augmenting functionality to classes in Ruby, but you must implement them with caution to ensure that you don't break the original functionality or make the implementation of monkey patches confusing and messy.

Photo credit: joshstyle