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# Sage Development Process

This section is a concise overview of the Sage development process. In it, we will see how to make changes to the Sage source code and record them in the git revision control system. In the following section on Collaborative Development with Git-Trac we will look at communicating these changes back to the Sage project.

We also have a handy one-page “cheat sheet” of commonly used git commands that you can print out and leave on your desk. You can alternatively fork and create a pull request at github which will automatically fetch your code and open a ticket on our trac server.

## Configuring Git

One way or another, `git` is what Sage uses for tracking changes. So first, open a shell (for instance, Terminal on Mac) and check that `git` works:

    [user@localhost]$ git
    usage: git [--version] [--help] [-C <path>] [-c name=value]
    ...
    The most commonly used git commands are:
       add        Add file contents to the index
    ...
       tag        Create, list, delete or verify a tag object signed with GPG

    'git help -a' and 'git help -g' lists available subcommands and some
    concept guides. See 'git help <command>' or 'git help <concept>'
    to read about a specific subcommand or concept.

Don’t worry about the giant list of subcommands. You really only need a handful for effective development, and we will walk you through them in this guide. If you got a “command not found” error, then you don’t have git installed. Now is the time to install it; see Setting Up Git for instructions.

Because we also track who does changes in Sage with git, you must tell git how you want to be known. This only needs to be done once:

    [user@localhost]$ git config --global user.name "Your Name"
    [user@localhost]$ git config --global user.email you@yourdomain.example.com

If you have multiple accounts / computers use the same name on each of them. This name/email combination ends up in commits, so do it now before you forget!

## Obtaining the Sage Source Code

Obviously one needs the Sage source code to develop. You can use your local installation of Sage, or (to start without Sage) download it from github which is a public read-only mirror (=faster) of our internal git repository:

    [user@localhost]$ git clone git://github.com/sagemath/sage.git
    Cloning into 'sage'...
    [...]
    Checking connectivity... done.

This creates a directory named `sage` containing the sources for the current stable and development releases of Sage. You will need to compile Sage in order to use it.

(For the experts, note that the repository at git.sagemath.org is where development actually takes place .)

## Branching Out

In order to start modifying Sage, we want to make a branch of Sage. A branch is a copy (except that it doesn’t take up twice the space) of the Sage source code where you can store your modifications to the Sage source code and which you can upload to trac tickets.

It is easy to create a new branch, just check out (switch to) the branch from where you want to start (that is, `master`) and use the `git branch` command:

    [user@localhost sage]$ git checkout master
    [user@localhost sage]$ git branch last_twin_prime
    [user@localhost sage]$ git checkout last_twin_prime

You can list all branches using:

    [user@localhost]$ git branch
      master
    * last_twin_prime

The asterisk shows you which branch you are on. Without an argument, the `git branch` command just displays a list of all local branches with the current one marked by an asterisk. Also note that `git branch` creates a new branch, but does not switch to it. For this, you have to use `git checkout`:

    [user@localhost sage]$ git checkout master
    Switched to branch 'master'
    Your branch is up-to-date with 'github/master'.
    [user@localhost sage]$ git branch
    * master
      last_twin_prime
    [user@localhost sage]$ git checkout last_twin_prime
    Switched to branch 'last_twin_prime'

Note that, unless you explicitly upload (“push”) a branch to remote git repository, the local branch will only be on your computer and not visible to anyone else.

To avoid typing the new branch name twice you can use the shortcut `git checkout -b my_new_branch` to create and switch to the new branch in one command.

## The History

It is always a good idea to check that you are making your edits on the version that you think you are on. The first one shows you the topmost commit in detail, including its changes to the sources:

    [user@localhost sage]$ git show

To dig deeper, you can inspect the log:

    [user@localhost sage]$ git log

By default, this lists all commits in reverse chronological order. If you find your branch to be in the wrong place, you can use the `git reset --hard` command to reset it to something else; see Reset and Recovery for details.

## Editing the Source Code

Once you have your own branch, feel free to make any changes as you like. Subsequent chapters of this developer guide explain how your code should look like to fit into Sage, and how we ensure high code quality throughout.

Status is probably the most important git command. It tells you which files changed, and how to continue with recording the changes:

    [user@localhost sage]$ git status
    On branch master
    Changes not staged for commit:
      (use "git add <file>..." to update what will be committed)
      (use "git checkout -- <file>..." to discard changes in working directory)

        modified:   some_file.py
        modified:   src/sage/primes/all.py

    Untracked files:
      (use "git add <file>..." to include in what will be committed)

        src/sage/primes/last_pair.py

    no changes added to commit (use "git add" and/or "git commit -a")

To dig deeper into what was changed in the files you can use:

    [user@localhost sage]$ git diff some_file.py

to show you the differences.

## Rebuilding Sage

Once you have made any changes you of course want to build Sage and try out your edits. As long as you only modified the Sage library (that is, Python and Cython files under `src/sage/...`) you just have to run:

    [user@localhost sage]$ ./sage -br

to rebuild the Sage library and then start Sage. This should be quite fast. If you made changes to third-party packages then you have to run:

    [user@localhost sage]$ make

as if you were installing Sage from scratch. However, simply running `make` will only recompile packages that were changed, so it shoud be much faster than compiling Sage the first time. Rarely there are conflicts with other packages, or with the already-installed older version of the package that you changed, in that case you do have to recompile everything using:

    [user@localhost sage]$ make distclean && make

Also, don’t forget to run the tests (see Doctesting the Sage Library) and build the documentation (see The Sage Manuals).

## Commits (Snapshots)

Whenever you have reached your goal, a milestone towards it, or just feel like you got some work done you should commit your changes. A commit is just a snapshot of the state of all files in the repository (the program you are working on).

Unlike with some other revision control programs, in git you first need to stage the changed files, which tells git which files you want to be part of the next commit:

    [user@localhost sage]$ git status
    # On branch my_branch
    # Untracked files:
    #   (use "git add <file>..." to include in what will be committed)
    #
    #       src/sage/primes/last_pair.py
    nothing added to commit but untracked files present (use "git add" to track)

    [user@localhost sage]$ git add src/sage/primes/last_pair.py
    [user@localhost sage]$ git status
    # On branch my_branch
    # Changes to be committed:
    #   (use "git reset HEAD <file>..." to unstage)
    #
    #   new file:   src/sage/primes/last_pair.py
    #

Once you are satisfied with the list of staged files, you create a new snapshot with the `git commit` command:

    [user@localhost sage]$ git commit
    ... editor opens ...
    [my_branch 31331f7] Added the very important foobar text file
     1 file changed, 1 insertion(+)
      create mode 100644 foobar.txt

This will open an editor for you to write your commit message. The commit message should generally have a one-line description, followed by an empty line, followed by further explanatory text:

    Added the last twin prime

    This is an example commit message. You see there is a one-line
    summary followed by more detailed description, if necessary.

You can then continue working towards your next milestone, make another commit, repeat until finished. As long as you do not `git checkout` another branch, all commits that you make will be part of the branch that you created.

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