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Plug applications and extensions in a TurboGears2 project

Project description

About Pluggable Apps

tgext.pluggable permits to plug extensions and applications inside a TG projects much like the Django Apps.


tgext.pluggable can be installed both from pypi or from bitbucket:

pip install tgext.pluggable

should just work for most of the users

Plugging Apps

In your application config/ import plug:

from tgext.pluggable import plug

Then at the end of the file call plug for each pluggable application you want to enable (package_name must be already installed in your python environment):

plug(base_config, 'package_name')

The plug function accepts various optional arguments, for example if the plugged application exposes a controller you can mount it in a different place specifying a different appid:

plug(base_config, 'package_name', 'new_app_id')

If you want to mount the pluggable application in a subcontroller you can used a dotted appid, like subcontroller.appid. Note that subcontroller must exist in RootController.

Other options include:

  • plug_helpers (True/False) -> Enable helpers injection

  • plug_models (True/False) -> Enable models plugging

  • plug_controller (True/False) -> Mount pluggable app root controller

  • plug_bootstrap (True/False) -> Enable websetup.bootstrap plugging

  • plug_statics (True/False) -> Enable plugged app statics

  • rename_tables (True/False) -> Rename pluggable tables by prepending appid.

Relations with Plugged Apps Models

There are cases when you might need to create a relationship or a foreign key with a model which is defined by a pluggable application. As pluggable application models are loaded after loading your application they are not available at the time your app models are imported.

tgext.pluggable provides some utilities to make easier to create relations with models defined by pluggable applications.

The first step you might want to take is setting the global_models=True parameter to the plug call, this will make all the models declared by the pluggable application available to you:

plug(base_config, 'package_name', global_models=True)

After the specified pluggable application is plugged, the models will be available inside your code through the tgext.pluggable.app_model object.

Then you can create foreign keys to the desired model using the tgext.pluggable.LazyForeignKey class and declare relations using the lazy version of sqlalchemy.orm.relation:

from tgext.pluggable import app_model, LazyForeignKey

class AdditionalInfo(DeclarativeBase):
    __tablename__ = 'sample_model'

    uid = Column(Integer, primary_key=True)
    data = Column(Unicode(255), nullable=False)

    plugged_model_id = Column(Integer, LazyForeignKey(lambda:app_model.PluggedModel.uid))
    plugged_model = relation(lambda: app_model.PluggedModel)


tgext.pluggables provides a bunch of utilities to work with partials. Partials in tgext.pluggable can be declared as a function or TGController subclass method that has an @expose decorator. Those partials can lately be rendered with:

${h.call_partial('module:function_name', arg1='Something')}

In the case of a class method:

${h.call_partial('module.Class:method', arg1='Something')}

The quickstarted pluggable application provides an example partial:

from tg import expose

def something(name):
    return dict(name=name)

which can be rendered using:

${h.call_partial('plugappname.partials:something', name='Partial')}

Replacing Templates

tgext.pluggable provides a function to replace templates. This is useful when you want to override the template that an application you plugged in is exposing. To override call replace_template inside your application config:

from tgext.pluggable import replace_template

replace_template(base_config, 'myapp.templates.about', 'myapp.templates.index')

replace_template will work even with tgext.pluggable partials, but won’t work with templates rendered directly calling the render method.

Calls to replace_template must be performed before the application has started.

Patching Templates

tgext.pluggable provides a function to patch templates, the result of a template rendering will be passed through a list of operations which will make possible to alter the rendering result.

This behavior is much inspired by Deliverance meant for much simpler use cases. The most common usage is for small changes to templates of plugged applications. For advanced manipulations using replace_template is suggested as it’s both faster and easier to maintain.

Template patching is enabled by using the load_template_patches function:

from tgext.pluggable import load_template_patches


Supposing your project is inside a Python distribution named myapp this will load the template_patches.xml file from the root of the distribution and will apply all the specified patches.

To load template patches from a python module (or pluggable) use:

load_template_patches(base_config, 'plugname')

You can use previous expression even to load patches from your own application in case the distribution automatic detection failed.

Template patching format is an xml file in the form of:

  <patch template="tgext.crud.templates.get_all">
    <content selector="#crud_content > h1" template="myapp.templates.replacements.crud_title" />
    <append  selector="#crud_content > h1" template="myapp.templates.replacements.crud_subtitle" />
    <prepend selector="#crud_content > h1" template="myapp.templates.replacements.crud_superscript" />
    <replace selector="#crud_btn_new > .add_link" template="" />

Each action listed inside the patch will be performed whenever the specified template is rendered, the template associated to the action will be used as the content of the templacement and the same data available to the patched template will be available to the action template too. Available actions are:

  • content - replaces the content of tags identified by the selector

  • append - appends after the tags identified by the selector

  • prepend - prepends before the tags identified by the selector

  • replace - replaces the tags identified bt the selector.

Creating Pluggable Apps

tgext.pluggable provides a quickstart-pluggable command to create a new pluggable application:

$ gearbox quickstart-pluggable plugtest
Enter package name [plugtest]:

The quickstarted application will provide an example on how to use models, helpers, bootstrap, controllers and statics.

In the previous example the pluggable application can be enabled inside any TurboGears using:

plug(base_config, 'plugtest')

After enabling the plugtest application you should run gearbox setup-app inside your TurboGears project to create the sample model. Then you can access the sample application page though http://localhost:8080/plugtest

The plugme Entry Point

Pluggable applications are required to implement a plugme(app_config, options) entry point which will be called when plugging the application.

The plugme action is called before TurboGears configuration has been loaded so that it is possible to register more pluggables inside the plugme hook. This way a pluggable can plug any dependency it requires just by calling tgext.pluggable.plug inside its own plugme function.

Any options passed to the plug call will be available inside the options dictionary, other parts of the pluggable applications like controllers, models and so on will be imported after the call to plugme so that plugme can set any configuration options that will drive the behavior of the other parts.

Keep in mind that as plugme is called before loading the TurboGears configuration if you need to perform something based on any configuration file option you must register a setup from the plugme call and perform them there.

Changing Static Files Behavior

By default every pluggable application serves all the static files available inside its public directory as they are. This is performed by a WSGI application which is in charge of serving the static files. Since version 0.2.1 it is now possible to replace this application or apply any WSGI middleware to it through the static_middlewares option.

For example you can enable SCSS inside your pluggable application by defining a plugme function like:

from tgext.scss import SCSSMiddleware

def plugme(app_config, options):
    return dict(appid='plugtest', global_helpers=False, static_middlewares=[SCSSMiddleware])

Accessing Application Models from Pluggable Apps

When creating a pluggable application you might often need to access to some models that have been declared inside the target application where the pluggable app will be mounted.

The most common use case for this is referencing the User, Group and Permission models. To do this tgext.pluggable provides an app_model object which wraps the application model and is initialized before loading the pluggable app.

This makes possible to access target application models referencing them as app_model.User or app_model.Group and so on. While you can guess that the primary key for those models is known (for the app_model.User object for example you might consider referencing to it as app_model.User.user_id) it is best practice to call the primary_key function provided by tgext.pluggable to get a reference to its column.

This way it is possibile to declare relations to models which are not provided by your pluggable app:

from tgext.pluggable import app_model, primary_key

user_id = Column(Integer, ForeignKey(primary_key(app_model.User)))
user = relation(app_model.User)

Pluggable Relative Urls

It is possible to generate an url relative to a pluggable mount point using the plug_url(pluggable, path, params=None, lazy=False) this function is also exposed inside the application helpers when a pluggable is used. For example to generate an url relative to the plugtest pluggable it is possible to call plug_url:

plug_url('plugtest', '/')

To perform redirects inside a pluggable app the plug_redirect(pluggable, path, params=None) function is provided. This function exposes the same interface as plug_url but performs a redirect much like tg.redirect.


tgext.pluggable provides some utilities for to manage text translations inside pluggables. When tg.i18n.ugettext or tg.i18n.lazy_ugettext are used they will lookup for translations inside the Application and when not available will fallback to the translations provided by the pluggable itself.

Messages extration and catalog creation/update for the pluggable work as in TurboGears using Babel. Just run inside the pluggable application the python extract_messages , python init_catalog -l LANG and python compile_catalog commands to create a catalog for LANG.

Just distribute the catalogs with your pluggable application to make them available and translated in applications that use it.

Managing Migrations

It is possible to initialize a migrations repository for a pluggable application. This makes possible to evolve the database at later times for each pluggable application using the alembic migration library for SQLAlchemy.

Create Migration Repository

To be able to manage migrations the pluggable has to be initialized with a migration repository to perform so, the author of the pluggable application has to run:

$ gearbox migrate-pluggable plugtest init

Then to create migration scripts run:

$ gearbox migrate-pluggable plugtest create 'Add column for user_name'

A file named like will be available inside the migration/versions directory of the pluggable application. Remember to add this directory to your distribution package to make it available to users of your pluggable application

Using Migrations

If the pluggable application your are using supports migrations it is possible to apply them using the upgrade and downgrade commands.

It is possible to run upgrade to move forward:

$ gearbox migrate-pluggable plugtest upgrade
22:11:28,029 INFO  [alembic.migration] Running upgrade None -> 3ca22a16fdcc

Or downgrade to revert a migration:

$ gearbox migrate-pluggable plugtest downgrade
22:15:24,004 INFO  [alembic.migration] Running downgrade 3ca22a16fdcc -> None

The versioning commands support being called on all the pluggables enabled inside your application by specifying all as the pluggable name. This will load your application to detect the plugged apps and will run the specified command for each one of them:

$ gearbox migrate-pluggable all db_version
22:15:54,104 INFO  [tgext.pluggable] Plugging plug1
22:15:54,105 INFO  [tgext.pluggable] Plugging plug2
22:15:54,106 INFO  [tgext.pluggable] Plugging plug3
Migrating plug1, plug3, plug2

plug1 Migrations
    Repository '/tmp/PLUGS/plug1/migration'
    Configuration File 'development.ini'
    Versioning Table 'plug1_migrate'
22:15:54,249 INFO  [alembic.migration] Context impl SQLiteImpl.
22:15:54,249 INFO  [alembic.migration] Will assume transactional DDL.
Current revision for sqlite:////tmp/provaapp/devdata.db: 4edef05cc346 -> 1ae930148d69 (head), fourth migration

plug3 Migrations
    Repository '/tmp/PLUGS/plug3/migration'
    Configuration File 'development.ini'
    Versioning Table 'plug3_migrate'
22:15:54,253 INFO  [alembic.migration] Context impl SQLiteImpl.
22:15:54,254 INFO  [alembic.migration] Will assume transactional DDL.
Current revision for sqlite:////tmp/provaapp/devdata.db: 15819683bb72 -> 453f571f41e4 (head), test migration

plug2 Migrations
    Repository '/tmp/PLUGS/plug2/migration'
    Configuration File 'development.ini'
    Versioning Table 'plug2_migrate'
22:15:54,258 INFO  [alembic.migration] Context impl SQLiteImpl.
22:15:54,259 INFO  [alembic.migration] Will assume transactional DDL.
Current revision for sqlite:////tmp/provaapp/devdata.db: 154b4f69cbd1 -> 2c8c79324a5e (head), third migration

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