20. Tools to do Reproducible Science

You should strive to make your experiments, simulations and data analyses reproducible, that is, anyone should be able to check what you did, and reproduce it. You should be able to understand what you did, even years later.

This means that you should:

  • keep track of exactly how you selected your materials (not only the end result)

  • keep track of the precise recipes for the data analyses

This is very difficult to achieve! Here are some possible strategies:

  1. keep a detailed lab notebook (few people manage to do it properly)

  2. write computer scripts that automate the processing pipelines.

  3. give up, hope you have not made mistakes, and will not need to check or rerun the experiment.

Although 3 is still the most widespread strategy among scientists, I cannot recommend it!

20.1. Two tools: literate programming and version control

Literate programming mixes code and text to produce a human readable document. It goes way beyond simply documenting one’s code with comments. For example:


  1. There are problems with tools such as Excel, E-Prime, …

    • they are impossible to check thoroughly.

    • the files are saved in binary format (not human readable)

    • the compatibility is not assured between successive versions.

  2. In literate programming tools, text is often written in the markup language Markdown. You should definitely learn it! (see https://www.markdownguide.org/getting-started). It is good enough to produce scientific reports (LaTeX not needed!).

Version control systems (e.g. git, svn, mercurial,…) are useful for two reasons:

  1. They allow you keep track of the complete history of a project. While your scripts or documents improve, it keeps cached copies of the previous versions. This is much, much better than M using a numbering scheme and multiple copies of the same file or directory (myfile001.doc, myfiles002.doc…).

  2. They facilitate collaborating on projects (documents and programs) with other people.

  • Bad: use email’s attachments to exchange successive versions of a file.

  • Good: collaborate on a project by using a shared git repository

To go further;

20.2. Lesson 101: how to compare files or directories

The most basic task is to compare two files. Your text editor may already have such a function built in — for example, in Emacs, it is accessible through the command ediff or the menu Tools/Compare).

You can also compare two files on the command-line with the command diff:

diff file1 file2

But I recommend another command, meld (which can be downloaded from http://meldmerge.org), as it has a more user-friendly output:

meld file1 file2

On Mac, meld can be difficult to install, you can rather use opendiff or filemerge (See https://eclecticlight.co/2018/04/14/comparing-files-filemerge-opendiff-and-bbedit/).

These tools also work on directories. To quickly check if there is any difference between two directories:

diff -r -q dir1 dir2


meld dir1 dir2

Note: If you compare text files that contain natural language, I recommend wdiff which ignores changes in whitespaces (line breaks, etc.). For latex files, latexdiff produces a formatted output that clearly shows the textual differences.

20.3. Introducing git

A version control system keeps track of the history of changes to a project, allows one to explore ideas by maintaining several parallel versions of the code.

In these lectures, we use Git. Although Git is a complex beast, you will only need to know a few commands (git clone, git pull, git init, git add, git status, git commit and git push)

20.3.1. Creating a local repository

A local repository is simply a folder where you have ran the command git init:

mkdir project
cd project
git init
Initialized empty Git repository in /home/pallier/cours/Python/version_control/git-test/.git/

Often, you will work on a local copy of a remote repository, imported either from another directory or from the Internet using git clone:

git clone https://github.com/chrplr/pyepl_examples

Note: If, when you create a repository you already know that you want to share it on the internet, I recommend to first create a repository on http://github.com or http://bitbucket.org, and then clone it on your local hard drive. In this way, the internet location will be automatically added to the list of remote repositories under the name origin.

Importanly, with git, you can still do version control locally, and only transfer your changes to the remote repository whenever you want, or never, because git is a decentralized version control system and all repositories are equal.

20.3.2. Adding files to the local repository

While working on the projectdirectory, you can tag files to track using the git add command:

echo 'essai1' > readme.txt  # create a file "readme.txt"; you can also use an editor like atom
git add readme.txt

To check which files are currently being tracked (or staged in git’s terminology), use the command `git status``:

git status
# On branch master
# Initial commit
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#       new file:   readme.txt

Note that you can add entire directories, for example:

git add .

It is possible to prevent certain files to be tracked (see https://help.github.com/articles/ignoring-files)).

20.3.3. Creating a commit (a.k.a. committing)

Once you are satisfied with the files in your working directory, you can take a snapshot, that is make a permanent copy of all the tracked files. This operation is also called commiting your changes:

git commit -m 'my first attempt'
[master (root-commit) a7a3a47] First commit
1 file changed, 1 insertion(+)
create mode 100644 readme.txt

This saves a snapshot (or commit) of the staged files in the hidden directory .git at the root of your project. Unless you delete this directory, this version of your files is saved there forever and will always be accessible.

Note: Before commiting, it is always useful to check which files are tracked and which are not, with git status.

20.3.4. Modifying the project

Let us now modify the file readme.txt in the working directory:

echo 'line2' >> readme.txt

The command git status allows us to check the state of the files in the working directory:

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

git add readme.txt
git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#       modified:   readme.txt

Let us create a new file, `readme2.txt``:

echo 'trial2' >readme2.txt
readme2.txt  readme.txt
git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#       modified:   readme.txt
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#       readme2.txt

We now add readme2.txt to the repository:

git add readme2.txt
git commit
[master a7e25a1] First revision; added readme2.txt
2 files changed, 2 insertions(+), 1 deletion(-)
create mode 100644 readme2.txt

Let us consult the history of the project:

git log
commit a7e25a158ce52a75c62381420f7dc375de631b1b
Author: Christophe Pallier <christophe@pallier.org>
Date:   Mon Aug 27 10:49:54 2012 +0200

First revision; added readme2.txt

commit a7a3a47edfae9d7c720356b691000a81ded73906
Author: Christophe Pallier <christophe@pallier.org>
Date:   Mon Aug 27 10:47:32 2012 +0200

First commit

git status
# On branch master
nothing to commit (working directory clean)

20.3.5. Renaming a file

To rename a tracked file, you should use git mv rather then just `mv``:

git mv file.ori file.new

20.3.6. Recovering a file deleted by accident

Let us delete readme2.txt “by accident”:

rm readme2.txt # oops
git status
# On branch master
# Changes not staged for commit:
#   (use "git add/rm <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#       deleted:    readme2.txt
no changes added to commit (use "git add" and/or "git commit -a")

To recover it:

git checkout -- readme2.txt
readme2.txt  readme.txt
cat readme2.txt

20.3.7. Checking for changes

Let us now modify readme2.txt and then compare the file in the current directory from the ones in the last commit:

echo 'line2 of 2' > readme2.txt
git diff
diff --git a/readme2.txt b/readme2.txt
index 33d1e15..e361691 100644
--- a/readme2.txt
+++ b/readme2.txt
@@ -1 +1 @@
+line2 of 2
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:   readme2.txt
no changes added to commit (use "git add" and/or "git commit -a")

You prefer meld, you can use

git difftool -t meld

Compare the working version of a f ile with the one in the last commit ———————————————————————

git diff HEAD

20.3.8. Inspecting the history of the project

git log

For a graphical view of the history of the project:


20.3.9. Branches

One interest of git is that it is possible to create several branches to make independent developments and merge them later.

To create a new branch:

git checkout -b [new_branch_name]

To list all branches:

git branch -a

To switch to an existing branch:

git checkout [branch_name]

To compare two branches

git difftool -d branch1..branch2

To compare a specific file:

git difftool branch1..branch2 -- filename

To merge a branch to the master branch:

git checkout master
git merge [branch_name]

20.4. Working with remotes

To add a remote repository

git remote add -f nameforremote path/to/repo_b.git
git remote update

To list the remotes

git remote -v

To compare the current branch with one in a remote

git diff master remotes/b/master

To see branches on remotes

git branch -r

(To see local branches: git branch -l, all branches, git branch -a)

20.4.1. Downloading the most recent changes from the distant repository

If you imported your repository from the internet with git clone, you can import the recent changes with:

git fetch
git merge

20.4.2. Comparing the local working direcoty with a remote

If you want to compare the current working directory with the distant remote origin/master.

git fetch origin master
git diff --summary FETCH_HEAD
git diff --stat FETCH_HEAD

20.4.3. Pushing your changes to the distant repository

You can send your modified repository (after commiting) to the original remote internet repository:

git push

20.4.4. Handling very large files (e.g. data)

git-annex allows you to leave large files in some of the repositories and keep only links in others.

See https://writequit.org/articles/getting-started-with-git-annex.html and https://git-annex.branchable.com/walkthrough/

20.4.5. Resources to learn more about Git

To learn more about git, check out:

To understand the inner workings of git, the following documents are useful:

Finally, the comprehensive documentation is the Git Book