Merging and Conflicts

Bring a branch back into main with git merge, and learn how to fix merge conflicts calmly

Merging a branch

Your obstacle-avoidance branch works. Time to bring it into main. Merging always works the same way: switch to the branch you want to merge into, then merge the other branch in.

git switch main
git merge obstacle-avoidance

Updating 9d3f6a8..4e8a1c2
Fast-forward
 sensor.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 sensor.py

sensor.py is now on main. Delete the branch, since its work is done:

git branch -d obstacle-avoidance

Fast-forward or merge commit?

You’ll see two kinds of merge.

Fast-forward: if main hasn’t changed since you made the branch, Git simply slides the main label forward to the branch’s latest commit. No new commit is needed. That’s what happened above.

Merge commit: if both branches have new commits, Git combines them and makes a special merge commit with two parents:

gitGraph
    commit id: "Add README"
    branch add-buzzer
    commit id: "Add buzzer"
    checkout main
    commit id: "Tune motor speed"
    merge add-buzzer

When this happens, Git opens your editor with a message like Merge branch 'add-buzzer'. Just save and close it.

Merge conflicts

Most of the time Git merges automatically, even when both branches changed the same file, as long as they changed different lines. But if both branches changed the same lines, Git can’t know which version you want. That’s a merge conflict, and you have to choose.

Conflicts sound scary but they’re completely normal. Let’s make one on purpose.

On main, set the robot’s speed. Add this line to the end of main.py, then commit:

SPEED = 50

git commit -am "Set speed to 50"

(-a stages every tracked file that has changed, so you can skip git add. It doesn’t add new, untracked files.)

Now make a branch from the commit before that one, and change the same line differently:

git switch -c faster HEAD~1

Add SPEED = 80 to the end of main.py and commit:

git commit -am "Set speed to 80"

Now try to merge:

git switch main
git merge faster

Auto-merging main.py
CONFLICT (content): Merge conflict in main.py
Automatic merge failed; fix conflicts and then commit the result.

Fixing the conflict

Open main.py. At the end you’ll find this:

<<<<<<< HEAD
SPEED = 50
=======
SPEED = 80
>>>>>>> faster

Git has put both versions in the file between conflict markers:

• Between <<<<<<< HEAD and ======= is the version from the branch you’re on (main)
• Between ======= and >>>>>>> faster is the version from the branch you’re merging in

To fix it, edit the file so it looks exactly how you want, and delete all three marker lines. You can keep one side, the other, or write something new:

SPEED = 65  # a compromise

Then tell Git it’s fixed by staging the file, and commit:

git add main.py
git commit -m "Merge branch 'faster'"

That’s it, conflict resolved. Run git log --oneline --graph to see the two lines of history joining together.

Your editor can help

VS Code highlights conflicts in colour and adds buttons above each one: Accept Current Change, Accept Incoming Change and Accept Both Changes. It’s the same thing as editing by hand, just quicker.

Getting out of a merge

If a merge goes wrong and you’d rather start again, cancel it:

git merge --abort

Everything goes back to how it was just before you ran git merge.

Try it yourself

1. Merge obstacle-avoidance into main and delete the branch.
2. Create the SPEED conflict above and resolve it.
3. Create the conflict again with a new value, then cancel it with git merge --abort.
4. Challenge: make two branches that change different lines of main.py, merge both into main and notice there’s no conflict.

Common issues

• Problem: You committed the conflict markers by mistake, and now the code won’t run
• Solution: Remove the <<<<<<<, ======= and >>>>>>> lines, then commit again. Search for <<<<<<< before committing a merge.

• Why: Git trusts you when you run git add; it doesn’t check the markers are gone.

• Problem: error: Merging is not possible because you have unmerged files
• Solution: Finish the merge in progress (fix the files, git add, git commit) or run git merge --abort.

• Why: You’re still in the middle of an earlier merge.

• Problem: You merged the wrong way round
• Solution: Remember: switch to the branch that should receive the changes first, then git merge <other-branch>.
• Why: git merge always brings the named branch into the branch you’re on.

Check your understanding

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