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Commit Tracing In Pry (Part 2)

How Done Is This Post? #

Anything I write ranges from “super rough notes-as-i-go, don’t even expect full sentences” to “this post is done and proven to deliver what it promises to deliver”

  1. Scratch-pad, take-notes-as-I-go scratch pad notes here: Commit Tracing In Pry, notes
  2. Review post, clean up for first ‘share’ <– we are here
  3. I mkdir new directory, follow my own steps, clarify
  4. Integrate feedback/answer questions from others
  5. Final cleanup, post is considered done unless someone reaches out with further questions

I spent a few hours working struggling through commit tracing once. PLEASE go read the companion post, at least the first few main points: Commit Tracing in Pry take 1

I’ve finished that first pass, and now I want to do it quickly, concisely, from scratch, and timed to various steps. I’m making a resource I can hand off to an early-career software developer and have them gain immediate value.

Commit Tracing Overview #

Read Chelsea Troy’s post: Leveling Up Skill #6: Commit Tracing.

Why To trace commits in the Pry Gem #

Pry is a ubiquitous tool in the Ruby/Rails community, seemed like a good place to start.

Which Commit to Trace? #

Last time, I spent a long time exploring commits. Now I’ve traced a commit, and, having found a few good commits to execute this technique upon, you’ll get the enjoyable, smoother journey. Again, refer to part 1 of this series for my raw notes of explorations, mis-steps, and more.

Here’s the list of commits I explored. I mostly skimmed the commit titles, occasionally opening up the commit to look harder: Pry commits

We’ll start with this commit: Add Pry::Env

Step 1: Get Pry Source Code #

In whatever directory you’d like to work in (I have a commit-tracing directory) clone down the pry repo:

git clone
cd pry

Step 2: Check out the repo to the commit in question #

Based on the PR in question, you’ll have what you need to check the repo out to the right place, which is the commit of the changes we’re about to trace

git checkout 7ce5ca70bb4a1ff447269bdde4cb5d07b8932c8a
git checkout 7ce5ca # the first six characters of the commit sha is sufficient

Step 3: get the diffs for the prior commit #

$ git diff HEAD~

Open up the files in question:

Step 4: Open these files in your editor #

git diff HEAD~ --name-only 
git diff HEAD~ --name-only | xargs atom

Step 5: Copy all the diffs down by hand on paper #

This commit took me about 10 minutes. Do it. Do it. DO NOT SKIP THIS STEP WITHOUT WRITING IT OUT ON PAPER!

Step 6: Delete the lib code #

Run the tests. You could just run rspec and see all the tests run, or pass the path to the test in question:

rspec spec/env_spec.rb

Now that you’re running just that one test, can delete the lib code, and rebuild it from the tests.

THIS IS WHERE THE LEARNING HAPPENS! Everything up to here has been prep for the learning that’s about to happen.

Step 7: Make the tests pass #

Step 8: Delete the lib AND the spec code #

now do git reset --hard HEAD~, and rebuild the entire commit.

Do it at least twice, until you can do it from scratch, without looking at the paper.

Here’s the code I ended up with, without refering to any paper, writing tests and making them pass as I go:

class Pry
  module Env
    def self.[](key)
      value = ENV[key] 
      return if value == ''
RSpec.describe Pry::Env do
  describe "#[]" do
    let(:key) { "test-key" }
    after { ENV.delete(key) }
    context "when ENV is provided" do
      before { ENV[key] = 'test-value' }
      it "should return the value" do
        expect(described_class[key]).to eq 'test-value'
    context "when ENV is not provided" do
      it "should return nil" do
        expect(described_class[key]).to be_nil
    context "when ENV is provided but set to '' " do
      before { ENV[key] = '' }
      it "should return nil" do
        expect(described_class[key]).to be_nil

Cool. I’m done tracing this commit.

Checks for Understanding #

What is commit tracing? #

git stash not working, how to fix? #

If you are trying to move commits, you don’t care about saving your work, and you do something like:

$ git checkout 7ce5ca
error: The following untracked working tree files would be overwritten by checkout:
Please move or remove them before you switch branches.

What do you do?

git stash

and try again?

Not quite! Run git status, you’ll see they’re unaffected.

  1. Add the files to git: git add .
  2. stash them: git stash
  3. now checkout the new commit: git checkout 7ce5ca

How do you view what the current commit changed? #

git diff gives you nothing…

git diff HEAD~

What commit does HEAD~1 reference? What’s a common alias? #

HEAD   => wherever the repo "is", "right now"
    ~1 => "backwards" one commit

If you don’t pass a number, you can shorten this to:


How do you find the names of the changed files? #

git diff HEAD~1 --name-only

How can you pass the list of changed files to your editor to open them all at once? #

git diff HEAD~1 --name-only | xargs atom

xargs is a great tool. Look it up. (link to til/cli/xargs)

Why would you need after { ENV.delete(key) }? #

Answering this is an exercise for the reader

What does Rspec’s described_class mean? #

Answering this is an exercise for the reader