Branches

Branches are incredibly important to why git is cool and powerful.

They are an easy and cheap way of making a second version of your software, which you work on in parallel, and pull in your changes when you are ready.

import os

top_dir = os.getcwd()
git_dir = os.path.join(top_dir, "learning_git")
working_dir = os.path.join(git_dir, "git_example")
os.chdir(working_dir)
%%bash
git branch # Tell me what branches exist
* main
%%bash
git checkout -b experiment # Make a new branch
Switched to a new branch 'experiment'
%%bash
git branch
* experiment
  main
%%writefile Wales.md
Mountains In Wales
==================

* Pen y Fan
* Tryfan
* Snowdon
* Glyder Fawr
* Fan y Big
* Cadair Idris
Overwriting Wales.md
%%bash
git commit -am "Add Cadair Idris"
[experiment 63c432f] Add Cadair Idris
 1 file changed, 2 insertions(+), 1 deletion(-)
%%bash
git checkout main # Switch to an existing branch
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
%%bash
cat Wales.md
Mountains In Wales
==================

* Pen y Fan
* Tryfan
* Snowdon
* Fan y Big
* Glyder Fawr
%%bash
git checkout experiment
Switched to branch 'experiment'
cat Wales.md
Mountains In Wales
==================

* Pen y Fan
* Tryfan
* Snowdon
* Glyder Fawr
* Fan y Big
* Cadair Idris

Publishing branches

To let the server know there’s a new branch use:

%%bash
git push -u origin experiment
To github.com:alan-turing-institute/github-example.git
 ! [rejected]        experiment -> experiment (non-fast-forward)
error: failed to push some refs to 'github.com:alan-turing-institute/github-example.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.
---------------------------------------------------------------------------
CalledProcessError                        Traceback (most recent call last)
Input In [11], in <cell line: 1>()
----> 1 get_ipython().run_cell_magic('bash', '', 'git push -u origin experiment\n')

File /opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/site-packages/IPython/core/interactiveshell.py:2358, in InteractiveShell.run_cell_magic(self, magic_name, line, cell)
   2356 with self.builtin_trap:
   2357     args = (magic_arg_s, cell)
-> 2358     result = fn(*args, **kwargs)
   2359 return result

File /opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/site-packages/IPython/core/magics/script.py:153, in ScriptMagics._make_script_magic.<locals>.named_script_magic(line, cell)
    151 else:
    152     line = script
--> 153 return self.shebang(line, cell)

File /opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/site-packages/IPython/core/magics/script.py:305, in ScriptMagics.shebang(self, line, cell)
    300 if args.raise_error and p.returncode != 0:
    301     # If we get here and p.returncode is still None, we must have
    302     # killed it but not yet seen its return code. We don't wait for it,
    303     # in case it's stuck in uninterruptible sleep. -9 = SIGKILL
    304     rc = p.returncode or -9
--> 305     raise CalledProcessError(rc, cell)

CalledProcessError: Command 'b'git push -u origin experiment\n'' returned non-zero exit status 1.

We use --set-upstream origin (Abbreviation -u) to tell git that this branch should be pushed to and pulled from origin per default.

If you are following along, you should be able to see your branch in the list of branches in GitHub.

Once you’ve used git push -u once, you can push new changes to the branch with just a git push.

If others checkout your repository, they will be able to do git checkout experiment to see your branch content, and collaborate with you in the branch.

%%bash
git branch -r
  origin/experiment
  origin/main

Local branches can be, but do not have to be, connected to remote branches They are said to “track” remote branches. push -u sets up the tracking relationship. You can see the remote branch for each of your local branches if you ask for “verbose” output from git branch:

%%bash
git branch -vv
* experiment a4378e5 [origin/experiment] Add Cadair Idris
  main       85b2797 [origin/main] Merge branch 'main' of github.com:alan-turing-institute/github-example

Find out what is on a branch

In addition to using git diff to compare to the state of a branch, you can use git log to look at lists of commits which are in a branch and haven’t been merged yet.

%%bash
git log main..experiment
commit a4378e56a723f2ba662262f94108a74be5f896f0
Author: Turing Developer <developer@example.com>
Date:   Mon Nov 8 14:06:02 2021 +0000

    Add Cadair Idris

Git uses various symbols to refer to sets of commits. The double dot A..B means “ancestor of B and not ancestor of A”

So in a purely linear sequence, it does what you’d expect.

%%bash
git log --graph --oneline HEAD~9..HEAD~5
*   34f8bbb Merge branch 'main' of github.com:alan-turing-institute/github-example
|\  
| * 289afed Add Scotland
* | 0c976b0 Add wales
|/  
* 82b4fa0 Add Helvellyn
* 6ff088f Include lakes in the scope

But in cases where a history has branches, the definition in terms of ancestors is important.

%%bash
git log --graph --oneline HEAD~5..HEAD
* a4378e5 Add Cadair Idris
*   85b2797 Merge branch 'main' of github.com:alan-turing-institute/github-example
|\  
| * ed30178 Add another Beacon
* | bc04a83 Add Glyder
|/  
*   ecc3206 Merge branch 'main' of github.com:alan-turing-institute/github-example
|\  
| * 8cd684e Add a beacon
* b8fb6c5 Translating from the Welsh

If there are changes on both sides, like this:

%%bash
git checkout main
Your branch is up to date with 'origin/main'.
Switched to branch 'main'
%%writefile Scotland.md
Mountains In Scotland
==================

* Ben Eighe
* Cairngorm
* Aonach Eagach
Overwriting Scotland.md
%%bash
git diff Scotland.md
diff --git a/Scotland.md b/Scotland.md
index 9613dda..bf5c643 100644
--- a/Scotland.md
+++ b/Scotland.md
@@ -3,3 +3,4 @@ Mountains In Scotland
 
 * Ben Eighe
 * Cairngorm
+* Aonach Eagach
%%bash
git commit -am "Commit Aonach onto main branch"
[main 537950c] Commit Aonach onto main branch
 1 file changed, 1 insertion(+)

Then this notation is useful to show the content of what’s on what branch:

%%bash
git log --left-right --oneline main...experiment
< 537950c Commit Aonach onto main branch
> a4378e5 Add Cadair Idris

Three dots means “everything which is not a common ancestor” of the two commits, i.e. the differences between them.

Merging branches

We can merge branches, and just as we would pull in remote changes, there may or may not be conflicts.

%%bash
git branch
git merge experiment
  experiment
* main
Merge made by the 'recursive' strategy.
 Wales.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
%%bash
git log --graph --oneline HEAD~3..HEAD
*   2365c66 Merge branch 'experiment'
|\  
| * a4378e5 Add Cadair Idris
* | 537950c Commit Aonach onto main branch
|/  
* 85b2797 Merge branch 'main' of github.com:alan-turing-institute/github-example
* ed30178 Add another Beacon

Cleaning up after a branch

%%bash
git branch  # list branches
  experiment
* main
%%bash
git branch -d experiment  # delete a branch
Deleted branch experiment (was a4378e5).
%%bash
git branch # current branch
* main
%%bash
git branch --remote  # list remote branches
  origin/experiment
  origin/main
%%bash
git push --delete origin experiment 
# Remove remote branch. Note that you can also use the GitHub interface to do this.
To github.com:alan-turing-institute/github-example.git
 - [deleted]         experiment
%%bash
git branch --remote  # list remote branches
  origin/main
%%bash
git push
To github.com:alan-turing-institute/github-example.git
   85b2797..2365c66  main -> main
%%bash
git branch  # current branch
* main 2365c66 [origin/main] Merge branch 'experiment'

A good branch strategy

  • A production or main branch: the current working version of your code

  • A develop branch: where new code can be tested

  • feature branches: for specific new ideas

  • release branches: when you share code with others

    • Useful for applying bug fixes to older versions of your code

Grab changes from a branch

Make some changes on one branch, switch back to another, and use:

git checkout <branch> <path>

to quickly grab a file from one branch into another. This will create a copy of the file as it exists in <branch> into your current branch, overwriting it if it already existed. For example, if you have been experimenting in a new branch but want to undo all your changes to a particular file (that is, restore the file to its version in the main branch), you can do that with:

git checkout main test_file

Using git checkout with a path takes the content of files. To grab the content of a specific commit from another branch, and apply it as a patch to your branch, use:

git cherry-pick <commit>