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. The contrib directory includes an extension to mimic some git commands under Mercurial. This is named hgit, and will be used as an example here.

To load an extension, you add it to your .hgrc file. You can either specify an absolute path: 

[extensions]
hgit=/usr/local/lib/hgit

Mercurial can also scan the default python library path for a file named 'hgit': 

[extensions]
hgit=

hg help will now show the new commands provided by the hgit extension.

To write your own extension, your python module needs to provide a dict with entries describing each command, and a callback named reposetup. reposetup 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