Using hooks
===========

What is a hook?
---------------

One way to customize Bazaar's behaviour is with *hooks*.  Hooks allow you to
perform actions before or after certain Bazaar operations.  The operations
include ``commit``, ``push``, ``pull``, and ``uncommit``.
For a complete list of hooks and their parameters, see `Hooks
<../user-reference/bzr_man.html#hooks>`_ in the User Reference.

Most hooks are run on the client, but a few are run on the server.  (Also
see the `bzr-push-and-update`_ plugin that handles one special case of
server-side operations.)

.. _bzr-push-and-update: https://launchpad.net/bzr-push-and-update/

Using hooks
-----------

To use a hook, you should `write a plugin <#writing-a-plugin>`_.  Instead of
creating a new command, this plugin will define and install the hook.  Here's
an example::

    from bzrlib import branch


    def post_push_hook(push_result):
        print "The new revno is %d" % push_result.new_revno


    branch.Branch.hooks.install_named_hook('post_push', post_push_hook,
                                     'My post_push hook')

To use this example, create a file named ``push_hook.py``, and stick it in
``plugins`` subdirectory of your configuration directory.  (If you have never
installed any plugins, you may need to create the ``plugins`` directory).

That's it!  The next time you push, it should show "The new revno is...".
Of course, hooks can be much more elaborate than this, because you have the
full power of Python at your disposal.  Now that you know how to use hooks,
what you do with them is up to you.

The plugin code does two things.  First, it defines a function that will be
run after ``push`` completes.  (It could instead use an instance method or
a callable object.)  All push hooks take a single argument, the
``push_result``.

Second, the plugin installs the hook.  The first argument ``'post_push'``
identifies where to install the hook.  The second argument is the hook
itself.  The third argument is a name ``'My post_push hook'``, which can be
used in progress messages and error messages.

Debugging hooks
---------------

To get a list of installed hooks, use the hidden ``hooks`` command::

    bzr hooks