Hooks

Introduction to Hooks

Hooks allow developers to tie into different pieces of the framework and an application.

A hook can be defined anywhere, be it internally in the application, or in a plugin, extension, etc. Once a hook is defined, functions can be registered to that hook so that when the hook is called, all functions registered to that hook will be run. By defining a hook, you are saying that you are going to honor that hook somewhere in your application. Using descriptive hook names are good for clarity. For example, pre_database_connect is obviously a hook that will be run before a database connection is attempted.

The most important thing to remember when defining hooks for your application is to properly document them. Include whether anything is expected in return or what, if any, arguments will be passed to the hook functions when called.

API Reference:

Configuration

Application Meta Options

The following options under App.Meta modify hook handling:

Option

Description

define_hooks

List of hook labels to define hooks by.

hooks

List of hooks to be registered.

Working with Hooks

The HookManager provides mechanisms for defining, registering, and executing hooks.

Example: Working with Hooks
Example: Working with Hooks
from cement import App
def some_function():
pass
with App('myapp') as app:
# list all defined hooks
app.hook.list()
# define a hook
app.hook.define('my_example_hook')
# test if a hook is defined
app.hook.defined('my_example_hook')
# register a function to a hook
app.hook.register('my_example_hook', some_function)
# run a hook
for res in app.hook.run('my_example_hook'):
# do something with res...
pass

Defining a Hook

A hook can be defined anywhere. However it is generally recommended to define the hook as early as possible. A hook definition simply gives a label to the hook, and allows the developer (or third-party plugin developers) to register functions to that hook. Its label is arbitrary.

The most convenient way to define a hook is via CementApp.Meta.define_hooks:

Example: Defining Hooks via App
Example: Defining Hooks via App
from cement import App
class MyApp(App):
class Meta:
label = 'myapp'
define_hooks = [
'my_example_hook',
]

Alternatively, hooks can be defined by extensions or plugins via the hook manager directly:

Example: Defining Hooks via Extensions/Plugins
Example: Defining Hooks via Extensions/Plugins
def load(app):
app.hook.define('my_example_hook')

Registering Functions to a Hook

A hook is just an identifier, but the functions registered to that hook are what get run when the hook is called. Registering a hook function should also be done early on in the runtime process, any time after the application has been created, after the hook is defined, and before the hook is run. Note that every hook is different, and therefore should be clearly documented by the owner of the hook (application developer, plugin developer, etc).

The most convenient way to register a hook function is via CementApp.Meta.hooks:

Example: Registering Functions to a Hook
Example: Registering Functions to a Hook
from cement import App
def my_hook_function(app):
# do something with app
pass
class MyApp(App):
class Meta:
label = 'myapp'
hooks = [
('pre_setup', my_hook_function),
]

Alternatively, hooks can be registered by extensions or plugins via the hook manager directly:

Example: Registering Hooks via Extensions/Plugins
Example: Registering Hooks via Extensions/Plugins
def my_hook_function(app):
# do something with app
pass
def load(app):
app.hook.register('pre_setup', my_hook_function)

Hook Parameters and Return Values

What you receive from a hook (arguments, keyword arguments), and what you return from your hook function depends on what the developer who owns the hook has defined. Each hook is different, and the nature of the hook determines whether you need to accept arguments, or return anything.

It is important that the owner of the hook (application/plugin developer, etc) properly document the usage of the hook including the *args, **kwargs it is sending as well as what it is expecting in return.

Running a hook

Now that a hook is defined, and functions have been registered to that hook all that is left is to run it. Keep in mind, you don't want to run a hook until after the application load process... meaning, after all plugins and other code are loaded. If you receive an error that the hook doesn't exist, then you are likely trying to register a hook too soon before the hook is defined. Likewise, if it doesn't seem like your hook is running and you don't see it mentioned in --debug output, you might be registering your hook after the hook has already run.

Example: Running a Hook
Example: Running a Hook
from cement import App
class MyApp(App):
class Meta:
label = 'myapp'
define_hooks = ['my_example_hook']
with MyApp() as app:
for res in app.hook.run('my_example_hook'):
# do something with res
pass

As you can see we iterate over the hook, rather than just calling app.hook.run() by itself. This is necessary because app.hook.run() yields the results from each hook function as they are run. Hooks can be run anywhere after the hook is defined, and hooks are registered to that hook.

Controlling Hook Run Order

Sometimes you might have a very specific purpose in mind for a hook, and need it to run before or after other functions in the same hook. For that reason there is an optional weight parameter that can be passed when registering a hook function.

Example: Controlling Hook Run Order
cli
Example: Controlling Hook Run Order
from cement import App
def func1(app):
print('Inside func1()')
def func2(app):
print('Inside func2()')
def func3(app):
print('Inside func3()')
class MyApp(App):
class Meta:
label = 'myapp'
hooks = [
('pre_setup', func1, 0),
('pre_setup', func2, 100),
('pre_setup', func3, -99),
]
with MyApp() as app:
app.run()
cli
$ python tmp/myapp.py
Inside func3()
Inside func1()
Inside func2()

As you can see, it doesn’t matter what order we register the hook, the weight runs them in order from lowest to highest based on their weight value.

Alternatively, with a plugin or extension:

Example: Controlling Hook Run Order via Extension/Plugin
Example: Controlling Hook Run Order via Extension/Plugin
def func1():
pass
def load(app):
app.hook.register('my_example_hook', func1, weight=10)

Cement Framework Hooks

Cement defines a number of hooks that tie into the framework.

Hook Name

Description

pre_setup

Run first when App.setup() is called. The application object is passed as an argument. Nothing is expected in return.

post_setup

Run last when App.setup() is called. The application object is passed as an argument. Nothing is expected in return.

pre_run

Run first when App.run() is called. The application object is passed as an argument. Nothing is expected in return.

post_run

Run last when App.run() is called. The application object is passed as an argument. Nothing is expected in return.

pre_argument_parsing

Run after App.run() is called, just before argument parsing happens. The application object is passed as an argument to these hook functions. Nothing is expected in return.

post_argument_parsing

Run after App.run() is called, just after argument parsing happens. The application object is passed as an argument to these hook functions. Nothing is expected in return.This hook is generally useful where the developer needs to perform actions based on the arguments that were passed at command line, but before the logic of App.run() happens.

pre_render

Run first when App.render() is called. The application object, and data dictionary are passed as arguments. Must return either the original data dictionary, or a modified one.

post_render

Run last when App.render() is called. The application object, and rendered output text are passed as arguments. Must return either the original output text, or a modified version.

pre_close

Run first when App.close() is called. This hook should be used by plugins and extensions to do any 'cleanup' at the end of program execution. Nothing is expected in return.

post_close

Run last when App.close() is called. Most use cases need pre_close, however this hook is available should anyone need to do anything after all other cleanup operations.

signal

Run when signal handling is enabled, and the defined signal handler callback is executed. This hook should be used by the application, plugins, and extensions to perform any actions when a specific signal is caught. Nothing is expected in return.