Mercurial and Heptapod short tutorial for FluidDyn

Mercurial is a free, distributed source control management tool. It’s is a great tool and if you are doing research (coding and/or writing papers), you should use a version control software! It seems to me that Mercurial is a good solution for researchers (in particular it is in my opinion simpler and nicer to learn and use than Git).

Mercurial couples very well with the programs TortoiseHG and Meld (if you can, just install them, especially Meld).

There are a lot of tutorials and documentations about Mercurial (for example the official Mercurial tutorial). In this page, I focus on what is needed to use and develop FluidDyn.

Heptapod is a friendly fork of GitLab Community Edition supporting Mercurial.


With TortoiseHG (simple for Windows)

Download the installer from

Set-up Mercurial

You need to create a file ~/.hgrc. For a good starting point, you can use the command:

hg config --edit

An example of configuration file:

username=myusername <>
editor=emacs -nw
tweakdefaults = True

hgext.extdiff =
# only to use Mercurial with GitHub and Gitlab
hggit =
# specific extension for FluidDyn dev
hgfluiddyn =
# more advanced extensions (really useful for FluidDyn dev)
churn =
shelve =
rebase =
absorb =
evolve =
topic =

cmd.meld =

The line starting with hggit is optional and enables the extension hg-git. This extension is useful to work on projects using Git, for example hosted on Github and Gitlab.

The extensions churn, shelve, rebase, absorb, evolve and topic are very useful for more advanced users. Note that evolve and topic comes from the package hg-evolve.


We wrote a specific Mercurial extension for FluidDyn development called hg-fluiddyn. All FluidDyn contributors / developers / maintainers should install and activate it!

With Mercurial installed with conda-app (as explained above), just run:

conda run -n _env_mercurial pip install hg-fluiddyn

and activate the extension with the line hgfluiddyn = in the file ~/.hgrc.


For development of FluidDyn packages, the evolve and topic extensions have to be installed and activated! If they are not activated, the hg-fluiddyn extension will warn you!

Get help

To get help on Mercurial, one can start with:

hg help

or for a specific command (here clone):

hg help clone

Simple workflow

To make a copy of an existing repository:

hg clone

To get a summary of the working directory state:

hg summary

or just hg sum.

To show changed files in the working directory:

hg status

or just hg st.

If you add new files or if you deleted files:

hg add name_of_the_file

hg remove name_of_the_file

This command is also very usefull:

hg addre

Each time you did some consistent changes:

hg commit


hg commit -m "A message explaining the commit"

I would advice to run after a commit command hg st to check that you did what you wanted to do. If you are unhappy with the commit, you can amend it with another commit with:

hg commit --amend

To push the state of your working repository to your repository on the web:

hg push

The inverse command (pull all commits from the remote repository) is:

hg pull

Get the last version of a code

First pull all the changesets from the remote repository:

hg pull

Then update the code to the tip:

hg update

or just hg up. You can also directly do:

hg pull -u

Read the history

You can get a list of the changesets with:

hg log --graph

or just hg log -G. With the --graph or -G option, the revisions are shown as an ASCII art.

Update the code to an old revision

Use hg up 220 to update to the revision 220. We can use a tag, bookmark, topic name or branch name instead of a number. To get a clean copy, add the option -C (beware).

Create a repository from a directory

Create a new repository in the given directory by doing:

hg init

Merge-Request based workflow with hg-evolve

We now use a Merge-Request (MR) based workflow for the development of FluidDyn packages.


GitLab’s “merge requests” are equivalent to GitHub’s “pull requests”.


In contrast to the standard workflow in Github and Gitlab, you don’t need to fork the repository to create Merge Requests.

Instead, you need to become a “developer” of the project. The developers have the permission to push changesets (i.e. “commits”) in a topic in the main repository (for example To acquire the “developer” role, please send a message in an issue or if needed, create a dedicated issue.

Topics are used in Mercurial for “lightweight branches” (like Git branches). If you are unfamiliar with Mercurial topics, you can read this tutorial, but what follows should be sufficient for FluidDyn development. The principle is that you first create a topic (with hg topic name_of_my_topic). Once a topic is activated, the changesets created belong to this topic. The new changesets gathered in a topic can be pushed into the main repository. Even after having been pushed into the main repository, they stay in the draft phase (which means they can be modified, as opposed to public changesets. Run hg help phases for more info).

To list the topics:

hg topics

To activate a topic already created:

hg up the_name_of_the_topic

To deactivate the topic and come back to the tip of the default branch:

hg up default

To get the list of the changesets in the active topic (very useful):

hg stack

Developers have to create Merge Requests to get things merged in the targeted branch (which is usually default for FluidDyn packages). Let’s present an example. A FluidDyn developer can do (here, we use ssh but you can also use https):

hg clone ssh://
hg topic fix_something
hg commit -m "Fix a bug related to ..."
hg push

Mercurial is going to print an URL to create the associated MR. Once created, the MR should then be reviewed by a “maintainer”. Only maintainers have the right to merge a MR, i.e. to publish changesets. The maintainer can tell you how to modify your MR and s-he can also directly modify the changesets of the MR!

We strongly advice to install and activate the evolve, rebase and absorb extensions locally (see the example of .hgrc above). This gives a very nice user experience for the MRs, with the ability to modify a MR with hg absorb and safe history editing.


hg absorb is very useful during code review. Let say that a developer submitted a PR containing few commits. As explained in this blog post, hg absorb is a mechanism to automatically and intelligently incorporate uncommitted changes into prior commits. Edit the files to take into account the remarks of the code review and just run:

hg absorb
hg push

and the PR is updated!


If you are asked to “rebase” your MR, it should work with the following commands:

hg pull
hg up name_of_my_topic
hg rebase
hg push

Documenting the merge-request with towncrier

Notable changes are announced in the changes page during a release. A news fragment is required to ensure to describe the MR. To do that, install towncrier (as of today, July 2020, the git master version is required) and create a new fragment:

pip install
towncrier create <MERGE_REQUEST>.<TYPE>

where <MERGE_REQUEST> should be the merge-request number and <TYPE> should be one of added, changed, deprecated, removed, fixed or security. Once created, the contents have to be edited in manually, committed and pushed along with the MR. Read more about creating news fragments.

Working with hggit and Github

To clone a git repository:

hg clone git+ssh://

or just:

hg clone

Git branches are represented as Mercurial bookmarks so such commands can be useful:

hg log --graph

hg up master

hg help bookmarks

# list the bookmarks
hg bookmarks

# put the bookmark master where you are
hg book master

# deactivate the active bookmark (-i like --inactive)
hg book -i


bookmarks, bookmark and book correspond to the same mercurial command.


If a bookmark is active, hg pull -u or hg up will move the bookmark to the tip of the active branch. You may not want that so it is important to always deactivate an unused bookmark with hg book -i or with hg up master.

Do not forget to place the bookmark master as wanted.

Github mirror for FluidDyn maintainers

For fluiddyn maintainers, we can add in the file .hg/hgrc something like:

default = ssh://
github = git+ssh://

And in ~/.hgrc:

update_github = !hg pull && hg up default && hg book master && hg push github -B master && hg book master -d

A quite complicated example with hg-git

We open a PR:

hg pull
hg up master
hg book fix/a_bug
# Modify/add/remove files
hg commit -m "A commit message"
hg push -B fix/a_bug

We want to change something in the commit of the PR. We first try hg absorb. Let’s say that we are in a situation for which it does not work:

# Modify/add/remove files
hg commit -m "A different commit message" --amend
# clean up Git commit map after history editing
hg git-cleanup
hg pull
hg push -B fix/a_bug --force

Delete a bookmark in a remote repository (close a remote Git branch)

With Mercurial, we can do:

hg bookmark --delete <bookmark name>
hg push --bookmark <bookmark name>

Unfortunately, it does not work for a remote Git repository (with hg-git). We have to use a Git client, clone the repository with Git and do something like:

# this deletes the branch locally
git branch --delete <branch name>
# this deletes the branch in the remote repository
git push origin --delete <branch name>