DATA 201 · Day 3 · Tue 9/8/2026

Collision Course

Your first merge conflict, on purpose — so the first one that happens by accident, maybe during Exam 1, isn't a surprise.

⏲ ~20 min, start of class

WhyNobody's first conflict should be during an exam

By today your team has been through one full weekly sync-and-PR cycle (Day 1–2 homework, due Sunday). That's enough git muscle memory to safely break something on purpose. A merge conflict just means git found two changes to the same lines and can't guess which one you want — it's not an error, it's a question. Today's goal is to make that question feel routine before Exam 1 (Tue 10/6, two and a half weeks out).

01Set up the collision

Pair up with one teammate. You'll both edit the exact same line of the exact same file, on purpose.

Partner A

git checkout -b conflict-drill-a
Open (or create) scratch/conflict_practice.md
Edit the line that says FAVORITE PLOT: ___ — fill in your answer
git add . && git commit -m "A: favorite plot"
git push origin conflict-drill-a

Partner B

git checkout -b conflict-drill-b
Open the same file, same starting point
Edit the same line with a different answer
git add . && git commit -m "B: favorite plot"
git push origin conflict-drill-b

Both of you open a Pull Request into main. Partner A merges first — that one goes through clean. Partner B's PR now shows a conflict. That's the moment we're here for.

02Read the wreckage

Partner B: pull the now-updated main into your branch (git checkout main && git pull, then git checkout conflict-drill-b && git merge main). Open the file. It looks like this:

<<<<<<< HEAD
FAVORITE PLOT: scatter plot
=======
FAVORITE PLOT: heatmap
>>>>>>> conflict-drill-b

Everything between <<<<<<< HEAD and ======= is what's already on main (Partner A's). Everything between ======= and >>>>>>> is yours. Git is not confused — it's handing you the decision because only a human can make it.

03Resolve it together

Decide

Talk it out. Keep one answer, keep both, or write a new one — your call as a pair.

Edit

Delete all three marker lines (<<<<<<<, =======, >>>>>>>) and leave only the final text.

Save & stage

git add scratch/conflict_practice.md

Commit the merge

git commit -m "resolve favorite-plot conflict"

Push & merge

git push origin conflict-drill-b, then the PR merges clean.

Debrief (whole class, ~5 min)

What did the three marker lines actually mean, in your own words?
What would have avoided this conflict entirely? (Hint: talking to your partner before both of you touched the same line.)
Was it as scary as you expected before you'd seen one?
This is exactly what can happen on Exam 1 if two teammates edit the same cell — now you'll recognize the markers on sight instead of panicking.