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TurboGears 2 Configuration

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

Like TurboGears 1, the application configuration is separated from the deployment specific information. In TurboGears 2.1 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 2.1 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.

All of this is similar to Pylons and to TurboGears 1, but slightly different from both.

Differences from TurboGears 1

In turbogears 1.x branches, the application specific configuration variables were kept in a .ini file packaged inside the egg. For better control over those variables, TurboGears 2.1 is now using a python module that contains code.

The advantage of this new method is that the configuration can contain complex python objects without adding a dependency on ConfigObj (which was used in TG1).

One disadvantage of the new configuration system is that it does not evaluate values in the .ini files therefore all values are considered strings. This is especially important when using boolean attributes and numbers as you need to convert them before use inside your project. This will be fixed in TurboGears 2.2 see ticket #2240

Differences from Pylons

TurboGears 2.1 has done quite a bit of work to simplify the config module in a standard Pylons quickstart, and to make the configuration in those files as declarative as possible. This makes it easier to make small updates to the config, and allows us to move some of the code into the framework.

This is particularly important as it allows the framework to evolve and change the middleware stack without forcing developers to constantly update their code with every release.

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.

Let’s take a closer look at the development.ini file:

[DEFAULT]
debug = true
# Uncomment and replace with the address which should receive any error reports
#email_to = you@yourdomain.com
smtp_server = localhost
error_email_from = paste@localhost

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.

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 supplied configuration, you should do so in middleware.make_app() or in lib/app_globals.py.

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...

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, Bunch
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 = 'genshi'
base_config.renderers.append('genshi') 

#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 2.1 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 (also known as pylons.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.

Mimetypes

By default, only json/application and text/html are defined mimetypes. If you would like to use additional mime-types you must register them with your application’s config. You can accomplish this by adding the following code your your app_cfg.py file:

base_config.mimetype_lookup = {'.ext':'my-mimetype'}

Startup and Shutdown

TurboGears allows you to attach callables both to the startup of the server, and the shut down. The startup is called before the environment is loaded, and the shutdown code runs when the python process shuts down. Heres an example of “hello, world” using startup and shutdown in your app_cfg.py file:

def on_startup():
    print 'hello, startup world'

def on_shutdown():
    print 'hello, shutdown world'

# ... (base_config init code)

base_config.call_on_startup = [on_startup]
base_config.call_on_shutdown = [on_shutdown]

Static Files

base_config.serve_static – automatically set to True for you. Set to False if you have set up apache, or nginx (or some other server) to handles static files.

Request Extensions

base_config.disable_request_extensions – by default this is false. This means that TG will take the request, and strip anything off the end of the last element in the URL that follows ”.”. It will then take this information, and assign an appropriate mime-type and store the data in the tg.request.response_type and tg.request.response_ext variables. By enabling this flag, you disable this behavior, rendering TG unable to determine the mime-type that the user is requesting automatically.

Stand Alone

base_config.stand_alone – set this to False if you don’t want error handling, HTTP status code error pages, etc. This is intended for the case where you’re embedding the TG app in some other WSGI app which handles these things for you.

Authentication Character Set

Set base_config.sa_auth.charset to define the character encoding for your user’s login. This is especially important if you expect your users to have non-ascii usernames and passwords. To set it to utf-8, your add this to your app_config.py file.:

base_config.sa_auth.charset = 'utf-8'

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 2.1 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 class definition below for more ideas on how you could modify your own custom config

AppConfig General Options

class tg.configuration.AppConfig(minimal=False, root_controller=None)

Class to store application configuration.

This class should have configuration/setup information that is necessary for proper application function. Deployment specific configuration information should go in the config files (e.g. development.ini or deployment.ini).

AppConfig instances have a number of methods that are meant to be overridden by users who wish to have finer grained control over the setup of the WSGI environment in which their application is run.

This is the place to configure custom routes, transaction handling, error handling, etc.

add_core_middleware(app)

Add support for routes dispatch, sessions, and caching. This is where you would want to override if you wanted to provide your own routing, session, or caching middleware. Your app_cfg.py might look something like this:

from tg.configuration import AppConfig
from routes.middleware import RoutesMiddleware
from beaker.middleware import CacheMiddleware
from mysessionier.middleware import SessionMiddleware

class MyAppConfig(AppConfig):
    def add_core_middleware(self, app):
        app = RoutesMiddleware(app, config['routes.map'])
        app = SessionMiddleware(app, config)
        app = CacheMiddleware(app, config)
        return app
base_config = MyAppConfig()
add_error_middleware(global_conf, app)

Add middleware which handles errors and exceptions.

make_load_environment()

Return a load_environment function.

The returned load_environment function can be called to configure the TurboGears runtime environment for this particular application. You can do this dynamically with multiple nested TG applications if necessary.

setup_helpers_and_globals()

Add helpers and globals objects to the config.

Override this method to customize the way that app_globals and helpers are setup.

setup_tg_wsgi_app(load_environment=None)

Create a base TG app, with all the standard middleware.

load_environment
A required callable, which sets up the basic evironment needed for the application.
setup_vars
A dictionary with all special values necessary for setting up the base wsgi app.

More Configuration Options

These configuration options have been broken into sub pages for easier digestion.