Writing Mercurial Extensions

Mercurial features an extension mechanism for adding new commands.

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 MercurialApi (but beware of API changes).

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

1. Writing your own extension

1.1. File Layout

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

1.2. Command table

To write your own extension, your python module can provide an optional dict named cmdtable with entries describing each command.

1.2.1. 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).

1.2.2. 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 ommit the "hg newcommand" part and only the options and parameter subsstring is needed).

1.2.3. Example cmdtable

cmdtable = {
    # "command-name": (function-call, options-list, help-string)
    "print-parents": (printparents,
                     [('s', 'short', None, 'print short form'),
                      ('l', 'long', None, 'print long form')],
                     "hg print-parents [options] node")
}

1.3. Command function signatures

Functions that implement new commands always receive a ui and usually a repo parameter. Please see the MercurialApi for information on how to use these. 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 commands should be imported from mercurial and the extension name should be added to commands.norepo like this:

from mercurial import commands
...
commands.norepo += " mycommand"

For examples of norepo see the source code to the RcpathExtension (direct link to RcpathExtension/rcpath.py extension source) or the ConvertExtension (direct link to convert extension source).

1.4. 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:

Paragraphs:

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::

  verbatim
    text
     !!

We have field lists:

:key1: value1
:key2: value2

Bullet lists:

- foo
- bar

Enumerated lists:

1. foo
2. bar

Inline markup: *bold*, ``monospace``. Mark Mercurial commands as :hg:`command` to make a nice link to the corresponding documentation. We'll expand the support if new constructs can be parsed without too much trouble.

1.5. Communicating with the user

Besides the ui methods listed in MercurialApi, 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.

1.6. 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.

1.6.1. ui setup

Extensions can implement an optional callback named uisetup. uisetup is called when the extension is first loaded and receives a ui object:

def uisetup(ui):
    # ...

1.6.2. Extension setup

Extensions can implement an optional callback named extsetup. It is called after all the extension are loaded, and can be useful in case one extension optionally depends on another extension. Signature:

def extsetup():
    # ...

Mercurial version 8e6019b16a7d and later (that is post-1.3.1) will pass a ui argument to extsetup.

1.6.3. Command table setup

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

1.6.4. 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 us the same ui object and hence different hooks must be configured in different setup functions.

1.7. 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):
    print("Pre-update hook triggered")

def updatehook(ui, repo, **kwargs):
    print("Update hook triggered")

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.

2. Example extension

   1 #!/usr/bin/env python
   2 
   3 '''printparents
   4 
   5 Prints the parents of a given revision.
   6 '''
   7 
   8 from mercurial import hg
   9 
  10 # every command must take a ui and and repo as arguments.
  11 # opts is a dict where you can find other command line flags
  12 #
  13 # Other parameters are taken in order from items on the command line that
  14 # don't start with a dash.  If no default value is given in the parameter list,
  15 # they are required.
  16 #
  17 # For experimenting with Mercurial in the python interpreter:
  18 # Getting the repository of the current dir:
  19 #    >>> from mercurial import hg, ui
  20 #    >>> repo = hg.repository(ui.ui(), path = ".")
  21 
  22 def printparents(ui, repo, node, **opts):
  23     # The doc string below will show up in hg help
  24     """Print parent information"""
  25 
  26     # repo can be indexed based on tags, an sha1, or a revision number
  27     ctx = repo[node]
  28     parents = ctx.parents()
  29 
  30     if opts['short']:
  31         # the string representation of a context returns a smaller portion of the sha1
  32         ui.write("short %s %s\n" % (parents[0], parents[1]))
  33     elif opts['long']:
  34         # the hex representation of a context returns the full sha1
  35         ui.write("long %s %s\n" % (parents[0].hex(), parents[1].hex()))
  36     else:
  37         ui.write("default %s %s\n" % (parents[0], parents[1]))
  38 
  39 cmdtable = {
  40     # cmd name        function call
  41     "print-parents": (printparents,
  42                      # see mercurial/fancyopts.py for all of the command
  43                      # flag options.
  44                      [('s', 'short', None, 'print short form'),
  45                       ('l', 'long', None, 'print long form')],
  46                      "[options] REV")
  47 }

If cmdtable or reposetup is not present, your extension will still work. This means that an extension can work "silently", without making new functionality directly visible through the command line interface.

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