Extension API

Extensions allow the creation of new features and using them directly from the main hg command line as if they were built-in commands. The extensions have full access to the *internal* API.

Use of Mercurial's internal API very likely makes your code subject to Mercurial's license. Before going any further, read the License page.

There are NO guarantees that third-party code calling into Mercurial's internals won't break from release to release. If you do use Mercurial's API for published third-party code, we expect you to test your code before each major Mercurial release. This will prevent various bug reports from your users when they upgrade their copy of Mercurial.

File Layout

Extensions are usually written as simple python modules. Larger ones are better split into multiple modules of a single package (see the convert extension). The package root module gives its name to the extension and implements the "cmdtable" and optional callbacks described below.

Command table

To write your own extension, your python module can provide an optional dict named "cmdtable" with entries describing each command. A command should be registered to the "cmdtable" by "@command" decorator.

Example using "@command" decorator (requires Mercurial 1.9):

from mercurial.i18n import _
cmdtable = {}
    from mercurial import registrar
    command = registrar.command(cmdtable)
except (AttributeError, ImportError):
    # Fallback to hg < 4.3 support
    from mercurial import cmdutil
    command = cmdutil.command(cmdtable)
    [('s', 'short', None, _('print short form')),
     ('l', 'long', None, _('print long form'))],
    _('[options] node'))
def printparents(ui, repo, node, **opts):

The cmdtable dictionary

The "cmdtable" dictionary uses as key the new command names, and, as value, a tuple containing:

  1. the function to be called when the command is used.
  2. a list of options the command can take.
  3. a command line synopsis for the command (the function docstring is used for the full help).

List of options

All the command flag options are documented in the mercurial/fancyopts.py sources.

The options list is a list of tuples containing:

  1. the short option letter, or "''" if no short option is available (for example, "o" for a "-o" option).
  2. the long option name (for example, "option" for a "--option" option).
  3. a default value for the option.
  4. a help string for the option (it's possible to omit the "hg newcommand" part and only the options and parameter substring is needed).

Command function signatures

Functions that implement new commands always receive a "ui" and usually a "repo" parameter. The rest of parameters are taken from the command line items that don't start with a dash and are passed in the same order they were written. If no default value is given in the parameter list they are required.

If there is no repo to be associated with the command and consequently no "repo" passed, then "norepo=True" should be passed to the "@command" decorator:

@command('mycommand', [], norepo=True)
def mycommand(ui, **opts):

For examples of "norepo", see the convert extension.

Command function docstrings

The docstring of your function is used as the main help text, shown by "hg help mycommand". The docstring should be formatted using a simple subset of reStructuredText markup. The supported constructs include:


This is a paragraph.
Paragraphs are separated
by blank lines.

A verbatim block is introduced with a double colon followed by an indented block. The double colon is turned into a single colon on display:

Some text::

We have field lists:

:key1: value1
:key2: value2

Bullet lists:

- foo
- bar

Enumerated lists:

1. foo
2. bar

Inline markup:

``*bold*``, ``monospace``, :hg:`command`

Mark Mercurial commands with ":hg:" to make a nice link to the corresponding documentation. We'll expand the support if new constructs can be parsed without too much trouble.

Communicating with the user

Besides the "ui" methods, like "ui.write(*msg)" or "ui.prompt(msg, default="y")", an extension can add help text for each of its commands and the extension itself.

The module docstring will be used as help string when "hg help extensionname" is used and, similarly, the help string for a command and the docstring belonging to the function that's wrapped by the command will be shown when "hg help command" is invoked.

Setup Callbacks

Extensions are loaded in phases. All extensions are processed in a given phase before the next phase begins. In the first phase, all extension modules are loaded and registered with Mercurial. This means that you can find all enabled extensions with "extensions.find" in the following phases.

Extension setup

There are two callbacks to be called when extensions are loaded, named "uisetup" and "extsetup". "uisetup" is called first for each extension, then "extsetup" is called. This means "extsetup" can be useful in case one extension optionally depends on another extension.

Both "uisetup" and "extsetup" receive a ui object with the local repository configuration:

def uisetup(ui):
    # ...
def extsetup(ui):
    # ...

Be aware that "uisetup" in NOT the function to configure a "ui" instance. It's called only once per process, not per "ui" instance. Also, any changes to the "ui" may be discarded because the "ui" here temporarily loaded local configuration. So, it's generally wrong to do 'ui.setconfig()' in these callbacks. Notable exception is setting "pre/post-<command>" hooks and extending "ui.__class__".

In Mercurial 1.3.1 or earlier, "extsetup" takes no argument.

Command table setup

After "extsetup", the "cmdtable" is copied into the global command table in Mercurial.

Ui instance setup

The optional "uipopulate" is called for each "ui" instance after configuration is loaded, where extensions can set up additional ui members, update configuration by "ui.setconfig()", and extend the class dynamically.

Typically there are three "ui" instances involved in command execution:

"req.ui" (or "repo.baseui")
Only system and user configurations are loaded into it.
Local repository configuration is loaded as well. This will be used at early dispatching stage where a repository isn't available.
The fully-loaded "ui" used after a repository is instantiated. This will be created from the "req.ui" per repository.

In command server and hgweb, this may be called more than once for the same "ui" instance.

(New in Mercurial 4.9)

Repository setup

Extensions can implement an optional callback named "reposetup". It is called after the main Mercurial repository initialization, and can be used to setup any local state the extension might need.

As other command functions it receives an "ui" object and a "repo" object (no additional parameters for this, though):

def reposetup(ui, repo):
    #do initialization here.

It is important to take into account that the "ui" object that is received by the "reposetup" function is not the same as the one received by the "uisetup" and "extsetup" functions. This is particularly important when setting up hooks as described in the following section, since not all hooks use the same "ui" object and hence different hooks must be configured in different setup functions.

Wrapping methods on the ui and repo classes

Because extensions can be loaded *per repository*, you should avoid using "extensions.wrapfunction()" on methods of the "ui" and "repo" objects. Instead, create a subclass of the specific class of the instance passed into the "*setup()" hook; e.g. use "ui.__class__" as the base class, then reassign your new class to "ui.__class__" again. Mercurial will then use your updated "ui" or "repo" instance only for repositories where your extension is enabled (or copies thereof, reusing your new class).

For example:

def uisetup(ui):
    class echologui(ui.__class__):
        def log(self, service, *msg, **opts):
            if msg:
                self.write('%s: %s\n' % (service, msg[0] % msg[1:]))
            super(echologui, self).log(service, *msg, **opts)
ui.__class__ = echologui

Configuring Hooks

Some extensions must use hooks to do their work. These required hooks can be configured manually by the user by modifying the "[hook]" section of their hgrc, but they can also be configured automatically by calling the "ui.setconfig('hooks', ...)" function in one of the setup functions described above.

The main difference between manually modifying the hooks section in the hgrc and using "ui.setconfig()" is that when using "ui.setconfig()" you have access to the actual hook function object, which you can pass directly to "ui.setconfig()", while when you use the hooks section of the hgrc file you must refer to the hook function by using the "python:modulename.functioname" idiom (e.g. "python:hgext.notify.hook").

For example:

# Define hooks -- note that the actual function name it irrelevant.
def preupdatehook(ui, repo, **kwargs):
    ui.write("Pre-update hook triggered\n")
def updatehook(ui, repo, **kwargs):
    ui.write("Update hook triggered\n")
def uisetup(ui):
    # When pre-<cmd> and post-<cmd> hooks are configured by means of
    # the ui.setconfig() function, you must use the ui object passed
    # to uisetup or extsetup.
    ui.setconfig("hooks", "pre-update.myextension", preupdatehook)
def reposetup(ui, repo):
    # Repository-specific hooks can be configured here. These include
    # the update hook.
    ui.setconfig("hooks", "update.myextension", updatehook)

Note how different hooks may need to be configured in different setup functions. In the example you can see that the "update" hook must be configured in the "reposetup" function, while the "pre-update" hook must be configured on the "uisetup" or the "extsetup" functions.

Marking compatible versions

Every extension should use the "testedwith" variable to specify Mercurial releases it's known to be compatible with. This helps us and users diagnose where problems are coming from:

testedwith = '2.0 2.0.1 2.1 2.1.1 2.1.2'

Do not use the "internal" marker in third-party extensions; we will immediately drop all bug reports mentioning your extension if we catch you doing this.

Similarly, an extension can use the "buglink" variable to specify how users should report issues with the extension. This link will be included in the error message if the extension produces errors:

buglink = 'https://bitbucket.org/USER/REPO/issues'

If an extension requires a minimum version of Mercurial, it can be declared with the "minimumhgversion" variable:

minimumhgversion = '4.6'

Older clients will print a warning that the extension requires a new version, instead of attempting to load it.

Wrap up: what belongs where?

You will find here a list of most common tasks, based on setups from the extensions included in Mercurial core.