Using hooks

What is a hook?

One way to customize Breezy’s behaviour is with hooks. Hooks allow you to perform actions before or after certain Breezy operations. The operations include commit, push, pull, and uncommit. For a complete list of hooks and their parameters, see Hooks in the User Reference.

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

Using hooks

To use a hook, you should write a plugin. Instead of creating a new command, this plugin will define and install the hook. Here’s an example:

from breezy 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.

To reduce the start-up time of Breezy it is also possible to “lazily” install hooks, using the breezy.hooks.install_lazy_named_hook function. This removes the need to load the module that contains the hook point just to install the hook. Here’s lazy version of the example above:

from breezy import hooks

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


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

Debugging hooks

To get a list of installed hooks (and available hook points), use the hidden hooks command:

brz hooks

Example: a merge plugin

Here’s a complete plugin that demonstrates the Merger.merge_file_content hook. It installs a hook that forces any merge of a file named *.xml to be a conflict, even if Breezy thinks it can merge it cleanly.

merge_xml.py:

"""Custom 'merge' logic for *.xml files.

Always conflicts if both branches have changed the file.
"""

from breezy.merge import PerFileMerger, Merger

def merge_xml_files_hook(merger):
    """Hook to merge *.xml files"""
    return AlwaysConflictXMLMerger(merger)

class AlwaysConflictXMLMerger(PerFileMerger):

    def file_matches(self, params):
        filename = self.get_filename(params, self.merger.this_tree)
        return filename.endswith('.xml')

    def merge_matching(self, params):
        return 'conflicted', params.this_lines

Merger.hooks.install_named_hook(
    'merge_file_content', merge_xml_files_hook, '*.xml file merge')

merge_file_content hooks are executed for each file to be merged. For a more a complex example look at the news_merge plugin that’s bundled with Breezy in the breezy/plugins directory.