Mercurial features an extension mechanism for adding new commands. It allows you to create new features and use them directly from the main hg command line.
Using the hgk extension
The contrib directory includes an extension to add support for a port of gitk under Mercurial. This is named hgk.py, and will be used as an example here.
To load an extension, you add it to your .hgrc file. You can specify an absolute path:
[extensions] hgk=/usr/local/lib/hgk.py
Mercurial can also scan the default python library path for a file named 'hgit':
[extensions] hgk=
In version 0.7, this extension is named "hgit" (without the .py) and does not provide the view command. You must invoke hgk directly.
hg help will now show the new commands provided by the hgk extension:
$ hg help status show changed files in the working directory ... tag add a tag for the current tip or a given revision tags list repository tags tip show the tip revision unbundle apply a changegroup file undo undo the last commit or pull update update or merge working directory verify verify the integrity of the repository version output version and copyright information view start interactive history viewer $ hg view
Writing your own extension
To write your own extension, your python module can provide an optional dict named cmdtable with entries describing each command, and an optional callback named reposetup. The reposetup callback is called after the main Mercurial repository initialization, and can be used to setup any local state the extension might need. Below is an example extension to help demonstrate how things work:
1 #!/usr/bin/env python
2
3 from mercurial import hg
4
5 # every command must take a ui and and repo as arguments.
6 # opts is a dict where you can find other command line flags
7 #
8 # Other parameters are taken in order from items on the command line that
9 # don't start with a dash. If no default value is given in the parameter list,
10 # they are required.
11 def print_parents(ui, repo, node, **opts):
12 # The doc string below will show up in hg help
13 """Print parent information"""
14
15 # repo.lookup can lookup based on tags, an sha1, or a revision number
16 node = repo.lookup(node)
17 parents = repo.changelog.parents(node)
18
19 if opts['short']:
20 # hg.short will return a smaller portion of the sha1
21 print "short %s %s" % (hg.short(parents[0]), hg.short(parents[1]))
22 elif opts['long']:
23 # hg.hex will return the full sha1
24 print "long %s %s" % (hg.hex(parents[0]), hg.hex(parents[1]))
25 else:
26 print "default %s %s" % (hg.short(parents[0]), hg.short(parents[1]))
27
28 cmdtable = {
29 # cmd name function call
30 "print-parents": (print_parents,
31 # see mercurial/fancyopts.py for all of the command
32 # flag options.
33 [('s', 'short', None, 'print short form'),
34 ('l', 'long', None, 'print long form')],
35 "hg print-parents [options] node")
36 }
37
38 def reposetup(ui, repo):
39 pass
40 # extension specific setup can go here
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.
Where to put extensions in the source tree
As of a change shortly after the 0.7 release, the recommended location for installing extensions in the source tree is the hgext directory. If you put a file in there called foo.py, you will need to refer to it in the hgrc file as a qualified package name, hgext.foo.
The contents of the hgext directory will be installed by the top-level setup.py script along with the rest of Mercurial.