“Defining paths relative to a file name has always hurt my soul, with __dir__ we can restore order in the Universe.” - by @fxn.

This post about the method in Ruby Programming Language Kernel#__dir__ and restoring order in the Universe.

Let’s take a look at the method Kernel#__dir__.

__dir__ => string

Returns the canonicalized absolute path of the directory of the file from which this method is called. It means symlinks in the path is resolved. If __FILE__ is nil, it returns nil. The return value equals to File.dirname(File.realpath(__FILE__)).

This method was introduced in Ruby since version 2.0.0 (NEWS for Ruby 2.0.0).

More info about development of Kernel#__dir__ here: Feature #1961.

Since Ruby 2.0.0 it is time to define relative paths with __dir__ and refactor all definitions of relative paths with __FILE__.

Why is __dir__ better than __FILE__ for a definition of path?

Imagine there is folder project/ that contains files: Rakefile, application.rb. Inside the file Rakefile need to get the path to the file application.rb. We can do it by using __FILE__:

File.expand_path('../application.rb', __FILE__)

There is the definition of path to the file application.rb by using __dir__:

File.expand_path('application.rb', __dir__)

By using __dir__ we get rid of redundant ../. Looks better, isn’t it? The result of these definitions is the same, but if we need number 5, we don’t type 3+2 or 1+4.

__FILE__ is definitely better than File.expand_path('Rakefile', __dir__) when inside the file Rakefile need to get the path to the file Rakefile(the current file).

It is important not to do and get rid of needless things in a code!

Examples of refactoring with __dir__:

-APP_PATH = File.expand_path('../../config/application', __FILE__)
+APP_PATH = File.expand_path('../config/application', __dir__)

-$:.unshift File.expand_path("..", __FILE__)
+$:.unshift __dir__

 namespace :isolated do
   task adapter => "test:env:#{adapter}" do
-    dir = File.dirname(__FILE__)
-    Dir.glob("#{dir}/test/cases/**/*_test.rb").all? do |file|
-      sh(Gem.ruby, "-w", "-I#{dir}/lib", "-I#{dir}/test", file)
+    Dir.glob("#{__dir__}/test/cases/**/*_test.rb").all? do |file|
+     sh(Gem.ruby, "-w", "-I#{__dir__}/lib", "-I#{__dir__}/test", file)
     end || raise("Failures")
   end
 end

-Dir[File.dirname(__FILE__) + "/stubs/*.rb"].each { |file| require file }
+Dir[File.expand_path("stubs/*.rb", __dir__)].each { |file| require file }

-$:.unshift(File.dirname(__FILE__) + "/lib")
-$:.unshift(File.dirname(__FILE__) + "/fixtures/helpers")
-$:.unshift(File.dirname(__FILE__) + "/fixtures/alternate_helpers")
+$:.unshift File.expand_path("lib", __dir__)
+$:.unshift File.expand_path("fixtures/helpers", __dir__)
+$:.unshift File.expand_path("fixtures/alternate_helpers", __dir__)

 def self.base_root
-  File.dirname(__FILE__)
+  __dir__
 end

-source_root File.expand_path("../templates", __FILE__)
+source_root File.expand_path("templates", __dir__)

When I was investigating Rails’s sources, I found one wonderful commit related to the using of __dir__ for definition of relative path.

rails$ git show 5b8738c2df003a96f0e490c43559747618d10f5f

commit 5b8738c2df003a96f0e490c43559747618d10f5f
Author: Xavier Noria <fxn@hashref.com>
Date:   Sat Mar 5 08:09:20 2016 +0100

    define APP_PATH with __dir__

    Defining paths relative to a file name has always hurt my soul,
    with __dir__ we can restore order in the Universe.

diff --git a/railties/lib/rails/generators/rails/app/templates/bin/rails b/railties/lib/rails/generators/rails/app/templates/bin/rails
index 80ec808..513a2e0 100644
--- a/railties/lib/rails/generators/rails/app/templates/bin/rails
+++ b/railties/lib/rails/generators/rails/app/templates/bin/rails
@@ -1,3 +1,3 @@
-APP_PATH = File.expand_path('../../config/application', __FILE__)
+APP_PATH = File.expand_path('../config/application', __dir__)
 require_relative '../config/boot'
 require 'rails/commands'

I got inspiration from this commit and decided to “restore order” in Rails, Jekyll, and own projects.

I made 2 patches and they are already merged:

Also, I got applause from @fxn for my Pull Request in Rails:

Hope you got inspiration from my little story that related to __dir__ and you will find time for providing a patch with definitions of relative paths with __dir__ to projects that you often use, it will help to “restore order in the Universe”. I am sure that you will get applause too if you do it. ;)