Functions and Classes in Python

Branches¶

Overview:

Teaching: 5 min
Exercises: 10 min

Questions

What are branches?
How should I use branches?
Once I've made my changes in the branch how do I merge them back?

Objectives

Know that branches can be used to compartmentalise different pieces of work.
Understand how and when to create branches.

If using git is like a time machine, then branches are parallel universes. You can use branches to to try out experiments, to isolate development of features or topics and, as we'll see later, they are used to collaborate with other repositories.

In some version control systems branching and (especially) merging are expensive operations. In git they are very easy and fast. In fact branching is just creating an alias to point to a particular commit.

git branch¶

We can use the git branch command to see what branches we have. Initially we just have the one branch, by default called master

$ git branch
* master

We can create a new branch simply by appending the name

$ git branch crazy-idea

Now we have

$ git branch
  crazy-idea
* master

git checkout¶

The asterisk indicates which branch we are currently on. To change the the new branch we use

git checkout crazy-idea

You could also do the last two steps in the one command

git checkout -b crazy-idea

git checkout can be used in a number of contexts, here we have used it to checkout or switch to a different branch or create and switch to a branch. You may have also seen it used to checkout a file(s) from an earlier state of the repository, while each of the bevhaious are related they can seem very different.

Remember you can always get help on commands e.g. git checkout --help, and beware blindly following advice from google.

Deleting a branch¶

If we add a new file, commit it, then change back to the master branch we see that the new file is no longer in the working directory.

If we decide that the branch is not needed we can delete it. If there are commits on that branch git will warn us

$ git branch -d crazy-idea 
error: The branch 'crazy-idea' is not fully merged.
If you are sure you want to delete it, run 'git branch -D crazy-idea'.
$ git branch -D crazy-idea
Deleted branch crazy-idea (was b85e41a).

git merge¶

Merging is the partner to branching. Suppose we have finished developemnt on a new feature in a branch and we want the code in the master branch, we do this using git merge

First make sure we are on the branch we want to merge in to

$ git checkout master

Then use git merge with the name of the branch we are merging in

$ git merge new-feature 
Updating f7a92c7..bf7762c
Fast-forward
 new-feature.md | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 new-feature.md

We can go ahead and delete the feature branch now. Git won't complain since the changes have been merged.

Merge conflicts¶

When git can automatically figure out how to merge it does, but it is sometimes the case that the same file has different edits in the same place. When this happens it is called a conflict and the output looks like this

$ git merge feature2
Auto-merging new-feature.md
CONFLICT (content): Merge conflict in new-feature.md
Automatic merge failed; fix conflicts and then commit the result.

Git has edited the file mentioned and added markers to draw your attention to where the conflicts are. It looks like

$ cat new-feature.md 
# Implementing the new feature

<<<<<<< HEAD
Here's how it looks in master
=======
Here's how it looks in feature2
>>>>>>> feature2

What we need to do now is use a text editor to change the file to the state we want then git add then git commit it.

If you are doing a lot of merging, say you have become the maintainer or release manager of a piece of software you should look in to the more sophisticated merge tools which can be configured with git mergetool.

Excercises¶

1

Create and use a branch

examine what branches you have

git branch

create a new branch called experiment1

git branch experiment1

switch to your branch

git checkout experiment1

add a new file and commit it

nano new.md
git add new.md
git commit -m "new file containing vital info"

switch back to the master branch

git checkout master

delete the experiment1 branch

git branch -D experiment1

2

Merge a feature

create a new branch called feature1 and check it out

git checkout -b feature1

add a new file and commit it

nano new.md
git add new.md
git commit -m "implement the new feature"

switch back to the master branch

git checkout master

merge in your feature branch

git merge feature1

3

Create and resolve a merge conflit

Edit the same line in the same file in both the master and feature branches and commit it in both

# on the master branch
nano new.md
git add new.md
git commit -m "feature work in master"

# on the feature1 branch
git checkout feature1
nano new.md
git add new.md
git commit -m "more feature development"

Attempt to merge feature1 in to master

git checkout master
git merge feature1

Resolve the conflict

nano new.md
git add new.md
git commit -m "resolved conflict"

Key Points:

Branches are used to perform allow you to perform different tasks separately.
You can create a branch using git branch <branch-name> or git checkout -b <branch-name>.
Use git checkout <branch-name> to switch between branches.
Use git merge <branch-name> to merge changes in <branch-name> to the <current-branch>
Use branches for development and test them to ensure that your master branch is working!