Version Control with Git

6. Conflicts

As soon as people can work in parallel, someone is going to step on someone else’s toes.

This will even happen with a single person: if we are working on a piece of software on both our laptop and a server in the lab, we could make different changes to each copy.

These situations are called conflicts Version control helps us manage these conflicts by giving us tools to resolve overlapping changes.

To see how we can resolve conflicts, we must first create one. The file rainfall_conversion.py currently looks like this in both partners’ copies of our climate-analysis repository:

$ cat rainfall_conversion.py

"""A library to perform rainfall unit conversions"""

def inches_to_mm(inches):
    """Convert inches to milimetres.

    Arguments:
    inches -- the rainfall inches
    """
    mm = inches * 25.4
    return mm

Developer A - Modify and Push

Conflicts

Let’s add a line to the bottom of Developer A’s copy only:

$ nano rainfall_conversion.py
$ cat rainfall_conversion.py

"""A library to perform rainfall unit conversions"""

def inches_to_mm(inches):
    """Convert inches to milimetres.

    Arguments:
    inches -- the rainfall inches
    """
    mm = inches * 25.4
    return mm

# TODO(Developer B): Add function to convert from inches to centimetres

and then push the change to GitHub:

$ git add rainfall_conversion.py
$ git commit -m "Developer A added a ToDo"

[master 5ae9631] Developer A added a ToDo
 1 file changed, 1 insertion(+)

$ git push origin master

Counting objects: 5, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 357 bytes | 0 bytes/s, done.
Total 3 (delta 2), reused 0 (delta 0)
To https://github.com/js-robinson/climate-analysis.git
   3973c44..479a5d2  master -> master

Developer B - Modify and push without pull

Now let’s have Developer B make a different change to their copy without updating (pulling) from GitHub:

$ nano rainfall_conversion.py
$ cat rainfall_conversion.py

"""A library to perform rainfall unit conversions"""

def inches_to_mm(inches):
    """Convert inches to milimetres.

    Arguments:
    inches -- the rainfall inches
    """
    mm = inches * 25.4
    return mm

# TODO(Developer A): Add function to convert from inches to centimetres

We can commit the change locally:

$ git add rainfall_conversion.py
$ git commit -m "Developer B added a different line"

[master 07ebc69] Developer B added a different line
 1 file changed, 1 insertion(+)

but Git won’t let us push it to GitHub:

$ git push origin master

To https://github.com/js-robinson/climate-analysis.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'https://github.com/js-robinson/climate-analysis.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

Conflicts #2

Git detects that the changes made in one copy overlap with those made in the other and stops us from trampling on our collaborators work.

What we have to do is pull the changes from GitHub, merge them into the copy we’re currently working in, and then push that.

Developer B - Pull and resolve

Let’s start by pulling:

$ git pull origin master

remote: Counting objects: 3, done.
remote: Compressing objects: 100% (1/1), done.
remote: Total 3 (delta 2), reused 3 (delta 2), pack-reused 0
Unpacking objects: 100% (3/3), done.
From https://github.com/js-robinson/climate-analysis
   3973c44..479a5d2  master     -> origin/master
Auto-merging rainfall_conversion.py
CONFLICT (content): Merge conflict in rainfall_conversion.py
Automatic merge failed; fix conflicts and then commit the result.

git pull tells us there’s a conflict, and marks that conflict in the affected file:

$ cat rainfall_conversion.py

"""A library to perform rainfall unit conversions"""

def inches_to_mm(inches):
    """Convert inches to milimetres.

    Arguments:
    inches -- the rainfall inches
    """
    mm = inches * 25.4
    return mm

<<<<<<< HEAD
# TODO(Developer A): Add function to convert from inches to centimetres
=======
# TODO(Developer B): Add function to convert from inches to centimetres
>>>>>>> dabb4c8c450e8475aee9b14b4383acc99f42af1d

Our change—the one in HEAD—is preceded by <<<<<<<. Git has then inserted ======= as a separator between the conflicting changes and marked the end of the content downloaded from GitHub with >>>>>>>. (The string of letters and digits after that marker identifies the revision we’ve just downloaded.)

It is now up to us to edit this file to remove these markers and reconcile the changes.

We can do anything we want: keep the change made in the local repository, keep the change made in the remote repository, write something new to replace both, or get rid of the change entirely.

Let’s replace both so that the file looks like this:

$ cat rainfall_conversion.py

"""A library to perform rainfall unit conversions"""

def inches_to_mm(inches):
    """Convert inches to milimetres.

    Arguments:
    inches -- the rainfall inches
    """
    mm = inches * 25.4
    return mm

# TODO(all): Hire a new developer

Developer B - Add, Commit and Push

To finish merging, we add rainfall_conversion.py to the changes being made by the merge and then commit:

$ git add rainfall_conversion.py
$ git status

On branch master
Your branch and 'origin/master' have diverged,
and have 1 and 1 different commit each, respectively.
  (use "git pull" to merge the remote branch into yours)

All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

        modified:   rainfall_conversion.py

$ git commit -m "Merging changes from GitHub"

[master 2abf2b1] Merging changes from GitHub

Now we can push our changes to GitHub:

$ git push origin master

Counting objects: 10, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 697 bytes, done.
Total 6 (delta 2), reused 0 (delta 0)
To https://github.com/js-robinson/climate-analysis.git
   dabb4c8..2abf2b1  master -> master

Git keeps track of what we’ve merged with what, so we don’t have to fix things by hand again when Developer A who made the first change pulls again:

Developer A Pull

$ git pull origin master

remote: Counting objects: 10, done.        
remote: Compressing objects: 100% (4/4), done.        
remote: Total 6 (delta 2), reused 6 (delta 2)        
Unpacking objects: 100% (6/6), done.
From https://github.com/js-robinson/climate-analysis
 * branch            master     -> FETCH_HEAD
Updating dabb4c8..2abf2b1
Fast-forward
rainfall_conversion.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

we get the merged file:

$ cat rainfall_conversion.py

"""A library to perform rainfall unit conversions"""

def inches_to_mm(inches):
    """Convert inches to milimetres.

    Arguments:
    inches -- the rainfall inches
    """
    mm = inches * 25.4
    return mm

# TODO(all): Hire a new developer

We have resolved the conflict

We don’t need to merge again because Git knows someone has already done that.

Version control’s ability to merge conflicting changes is another reason users tend to divide their programs and papers into multiple files instead of storing everything in one large file. There’s another benefit too: whenever there are repeated conflicts in a particular file, the version control system is essentially trying to tell its users that they ought to clarify who’s responsible for what, or find a way to divide the work up differently.

IF TIME

Ignoring Things