Configuration Process

TurboGears 2 provides a configuration system that attempts to be both extremely flexible for power users and very simple to use for standard projects.

Overview

The application configuration is separated from the deployment specific information. In TurboGears 2.3.10 there is a config module, containing several configuration specific python files – these are done in python (not as INI files), because they actually setup the TurboGears 2.3.10 application and its associated WSGI middleware. Python provides an incredibly flexible config system with all kinds of tools to keep you from having to repeat yourself. But it comes with some significant drawbacks, python is more complex than INI, and is less declarative so can be less obvious.

But we believe these drawbacks are more than overcome by the power and flexibility of python based configuration for the app because these files are intended to be edited only by application developers, not by those deploying the application. We’ve also worked hard to create an environment that is generally declarative.

At the same time the deployment level configuration is done in simple .ini files, in order to make it totally declarative, and easy for deployers who may not be python programmers.

Configuration in the INI files

A TurboGears quickstarted project will contain a couple of .ini files which are used to define what WSGI app ought to be run, and to store end-user created configuration values, which is just another way of saying that the .ini files should contain deployment specific options.

By default TurboGears provides a development.ini, test.ini, and production.ini files. These are standard ini file formats.

These files are standard INI files, as used by PasteDeploy. The individual sections are marked off with []‘s.

See also

Configuration file format and options are described in great detail in the Paste Deploy documentation.

If want to add some configuration option (let’s say an administrator’s email) here is how you would do so. First you would edit your development.ini file and go to the end of the [app:main] section.

You can then choose a sensible name for your configuration key and add it to the section:

mail.from.administrator = someemail@somedomain.com

This would make sure this variable is now part of the configuration and can be accessed from anywhere in your code. For example let’s imagine that you wanted to get this config option from a controller’s code:

import tg
admin_emailfrom = tg.config.get('mail.from.administrator', 'notconfigured@nodomain.com')

If the person who deployed your application forgot to add the variable to his config file he would get the default value provided as the second argument of the get() call.

Warning

If you set a value like enable_subsystem = false, it will be loaded into python as the string ‘false’ which if used in a conditional will give you a very wrong result

The correct way of loading boolean values for your use is

from paste.deploy.converters import asbool
if asbool(config['enable_subsystem']):
   ... sub systems is enabled...

Configuration Milestones

Since TurboGears 2.3 the configuration process got divided in various milestones, each of those milestones is bound to an advancement in the framework setup process.

Whenever a milestone is reached all the registered callbacks are fired and the configuration process can continue. If the milestone is already passed when a callback is registered, the callback gets instantly fired.

Note

The tg.config object is available at import time but until the configuration file is parsed, it only contains the system defaults. If you need to perform startup time setup based on the supplied configuration, you should do so in a milestone.

Milestones are available through the tg.configuration.milestones module, the currently provided milestones are:

  • milestones.config_ready - Configuration file has been loaded and is

    available in tg.config

  • milestones.renderers_ready - Renderers have been registered and all

    of them are available

  • milestones.environment_loaded - Full environment have been loaded

    but application has not been created yet.

Registering an action to be executed whenever a milestone is reach can be done using tg.configuration.milestones._ConfigMilestoneTracker.register() method of each milestone. The registered action takes no parameters.

Milestones are much like Hooks but they are only related to the configuration process. The major difference is that while an hook can fire multiple times a milestone can be reached only once.

Milestones and Hooks order of execution

The order of execution of the milestones and hooks provided during the application startup process is:

  • milestones.config_ready
  • startup Hook
  • milestones.renderers_ready
  • milestones.environment_loaded
  • before_config Hook
  • after_config Hook

The config module

Tip

A good indicator of whether an option should be set in the config directory code vs. the configuration file is whether or not the option is necessary for the functioning of the application. If the application won’t function without the setting, it belongs in the appropriate config/ directory file. If the option should be changed depending on deployment, it belongs in the ini files.

Our hope is that 90% of applications don’t need to edit any of the config module files, but for those who do, the most common file to change is app_config.py:

from tg.configuration import AppConfig
import wiki20
from wiki20 import model
from wiki20.lib import app_globals, helpers

base_config = AppConfig()
base_config.renderers = []

base_config.package = wiki20

#Set the default renderer
base_config.default_renderer = 'kajiki'
base_config.renderers.append('kajiki')

#Configure the base SQLALchemy Setup
base_config.use_sqlalchemy = True
base_config.model = wiki20.model
base_config.DBSession = wiki20.model.DBSession

app_cfg.py exists primarily so that middleware.py and environment.py can import and use the base_config object.

The base_config object is an AppConfig() instance which allows you to access its attributes like a normal object, or like a standard python dictionary.

One of the reasons for this is that AppConfig() provides some defaults in its __init__. But equally important it provides us with several methods that work on the config values to produce the two functions that set up your TurboGears app.

We’ve taken care to make sure that the entire setup of the TurboGears 2.3.10 framework is done in code which you as the application developer control. You can easily customize it to your needs. If the standard config options we provide don’t do what you need, you can subclass and override AppConfig to get exactly the setup you want.

The base_config object that is created in app_cfg.py should be used to set whatever configuration values that belong to the application itself and are required for all instances of this app, as distinct from the configuration values that you set in the development.ini or production.ini files that are intended to be editable by those who deploy the app.

As part of the app loading process the base_config object will be merged in with the config values from the .ini file you’re using to launch your app, and placed in tg.config.

As we mentioned previously, in addition to the attributes on the base_config object there are a number of methods which are used to setup the environment for your application, and to create the actual TurboGears WSGI application, and all the middleware you need.

You can override base_config‘s methods to further customize your application’s WSGI stack, for various advanced use cases, like adding custom middleware at arbitrary points in the WSGI pipeline, or doing some unanticipated (by us) application environment manipulation.

And we’ll look at the details of how that all works in the advanced configuration section of this document.

Configuring your application

Here’s are some of the more general purpose configuration attributes:

Configuration Attributes

The configuration object has a number of attributes that automate the majority of what you need to do with the config object. These shortcuts eliminate the need to provide your own setup methods for configuring your TurboGears application.

To see the list of available configuration options refer to AppConfig.

Advanced Configuration

Sometimes you need to go beyond the basics of setting configuration options. We’ve created a number of methods that you can use to override the way that particular pieces of the TurboGears 2.3.10 stack are configured. The basic way you override the configuration within app.cfg looks something like this:

from tg.configuration import AppConfig
from tw2.core.middleware import TwMiddleware

class MyAppConfig(AppConfig):

    def add_tosca2_middleware(self, app):

        app = TwMiddleware(app,
            default_engine=self.default_renderer,
            translator=ugettext,
            auto_reload_templates = False
            )

        return app
base_config = MyAppConfig()

# modify base_config parameters below

The above example shows how one would go about overridding the ToscaWidgets2 middleware. See the AppConfig for more ideas on how you could modify your own custom config