How does File.expand_path work? - ruby

Question

I'm not sure I understand the order in which operations are done with File.expand_path. Below is an example pry session:
[1] pry(main)> File.expand_path('.')
=> "/Users/max/Dropbox/work/src/github.com/mbigras/foobie"
[2] pry(main)> File.expand_path('..')
=> "/Users/max/Dropbox/work/src/github.com/mbigras"
[3] pry(main)> File.expand_path('..', "cats")
=> "/Users/max/Dropbox/work/src/github.com/mbigras/foobie"
[4] pry(main)> File.expand_path('..', __FILE__)
=> "/Users/max/Dropbox/work/src/github.com/mbigras/foobie"
[5] pry(main)> File.expand_path('../lib', __FILE__)
=> "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/lib"
[7] pry(main)> File.expand_path('./lib')
=> "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/lib"
[8] pry(main)> File.expand_path('./lib', __FILE__)
=> "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/(pry)/lib"
[9] pry(main)>
[1] makes sense, I'm expanding the path of the current working directory.
[2] makes sense, I'm expanding the path of the parent directory of the cwd
[3] doesn't make sense, I accept from reading another answer that for some reason ruby implicitly takes the File.dirname of the second arg and in the case of File.dirname('cats') it expands to the cwd . because 'cats' isn't nested. But then why doesn't File.expand_path('..', '.') have the same result?
[18] pry(main)> File.expand_path('..', 'cats')
=> "/Users/max/Dropbox/work/src/github.com/mbigras/foobie"
[19] pry(main)> File.dirname('cats')
=> "."
[20] pry(main)> File.expand_path('..', '.')
=> "/Users/max/Dropbox/work/src/github.com/mbigras"
[4] doesn't make sense but for the same reason as [3]. In this case the "random string" is "(pry)" because p __FILE__ #=> "(pry)" while inside a pry session.
[5] doesn't make sense, why would File.expand_path go to seemingly noone's parent directory and then magically come back to the cwd and decide to go into lib
[7] makes sense, but doesn't help me understand [5]
[8] doesn't make sense, why is the "random string" now wedged between the cwd . and lib
From the docs:
File.expand_path("../../lib/mygem.rb", __FILE__)
#=> ".../path/to/project/lib/mygem.rb"
So first it resolves the parent of __FILE__, that is bin/, then go to the parent, the root of the project and appends lib/mygem.rb.
The order of operations doesn't really add up to me.
Take File.dirname(__FILE__)
Go to the parent which is the root
append lib/mygem.rb
Steps 2 and 3 don't help. Why are we going to the parent? Why did we even do Step 1 in the first place? Why is there ../..? Doesn't that mean go two levels up from the current working directory?
Would love some guiding principles to understand these examples.
Edit to add the Gold:
File.expand_path goes to the first parameter from the directory specified by the second parameter (Dir.pwd if not present). - Eric Duminil

Answer 1

Theory
I think you missed a .. while reading the answer you link to.
No File.dirname is ever done implicitely by expand_path.
File.expand_path('../../Gemfile', __FILE__)
# ^^ Note the difference between here and
# vv there
File.expand_path('../Gemfile', File.dirname(__FILE__))
What confused me at first was that the second parameter is always considered to be a directory by File.expand_path, even if it doesn't exist, even if it looks like a file or even if it is an existing file.
File.expand_path goes to the first parameter from the directory specified by the second parameter (Dir.pwd if not present).
Examples
[3]
File.expand_path('..', "cats")
It is executed in the current directory, which is "/Users/max/Dropbox/work/src/github.com/mbigras/foobie".
Is cats an existing file, an existing directory or a non-existent directory?
It doesn't matter to File.expand_path : "cats" is considered to be an existing directory, and File.expand_path starts inside it.
This command is equivalent to launching :
File.expand_path('..')
inside the "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/cats" directory.
So expand_path goes back one directory, and lands back to :
"/Users/max/Dropbox/work/src/github.com/mbigras/foobie"
[4]
Same thing. It is equivalent to File.expand_path('..') from the (probably) non-existing :
"/Users/max/Dropbox/work/src/github.com/mbigras/foobie/(pry)"
So it is :
"/Users/max/Dropbox/work/src/github.com/mbigras/foobie"
[5]
Going from [4], it just goes to the subfolder lib.
[8]
Starting from "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/(pry)"
, it just goes to the subfolder lib.
Once again, File.expand_path never checks if the corresponding folders and subfolders exist.