PasteDeploy Application Factory for Django¶

Paste is an umbrella project for widely used WSGI-related packages, as well as the name of a meta-framework part of that project. One of these projects is PasteDeploy, which offers a very flexible configuration mechanism (based on INI files) to set up your application which can also give you full control over this initialisation by means of Python code. It’s not only used to set up the application in deployment mode, but also in development mode.

The way it works is very simple: You define a callable which takes all the configuration options as arguments and returns a WSGI application object (which in this case would be a Django application). These callables are called “Application Factories”, and this library provides one you can use in most cases or even extend when you do something more complicated.

The following is a minimal configuration file which uses the Application Factory provided by django-pastedeploy-settings. You can also download a complete example.

[DEFAULT]
debug = false
django_settings_module = your_django_project.settings

[app:main]
use = egg:django-pastedeploy-settings

It does not define any option that can be used by Django or your application, apart from debug. Note this option is lower case: That’s the de-facto spelling for this in the WSGI world. django-pastedeploy-settings will automatically set Django’s DEBUG to that value.

Sections with the app: prefix contain the settings for WSGI applications. In the context of INI files, these definitions are referred to as options. Whenever you reference a PasteDeploy file, you have to specify the application to be used, or else it’ll default to main (if it exists). All the options defined in the specified app:* section will be converted to Django settings, with the values converted from JSON (except for the options use and paste.app_factory which are only used by PasteDeploy).

The DEFAULT section is special. The options defined here can be used as variables to be substituted in other sections, as well as some meta options for Paste, django-pastedeploy-settings or other 3rd party software. Except for debug, the values set in this section aren’t parsed as JSON strings; they’ll always be raw strings and therefore don’t need to be quoted.

You can have more than one set of settings for your Django application. If, for example, you wanted to be able to use your application in development and deployment mode, you could use a configuration like this:

[DEFAULT]
debug = false
django_settings_module = your_django_project.settings

[app:main]
use = egg:django-pastedeploy-settings

[app:development]
use = main
set debug = true

Because we need to toggle the value of DEBUG from the configuration file, you must remove this variable from your settings module. If you have options which depend on this value, you can still refer to it like this:

[DEFAULT]
debug = false
django_settings_module = your_django_project.settings

[app:main]
use = egg:django-pastedeploy-settings
TEMPLATE_DEBUG = ${debug}

[app:development]
use = main
set debug = true

Or, you can override them on a per application basis:

[DEFAULT]
debug = false
django_settings_module = your_django_project.settings

[app:main]
use = egg:django-pastedeploy-settings
TEMPLATE_DEBUG = false

[app:development]
use = main
set debug = true
# TEMPLATE_DEBUG will be false unless we override it:
TEMPLATE_DEBUG = true

You can then use the values the same way you’ve been doing it, with Django’s settings object or the old way (importing your settings module directly):

from django.conf import settings

print settings.DEBUG

This mechanism can be used to complement your settings module or replace it completely (as long as you don’t use unsupported settings, which must still be set in Python code). The author believes it’s best to move it all to the convenient INI file, except for those settings which are not really settings, but a crucial element of your application (e.g., TEMPLATE_LOADERS, MIDDLEWARE_CLASSES, FILE_UPLOAD_HANDLERS, INSTALLED_APPS).

Unsupported settings¶

Only those settings whose values can be represented with JSON can be defined in a INI file, which covers the vast majority of settings in Django. The following are examples of settings whose values cannot be represented in JSON:

FILE_UPLOAD_PERMISSIONS (octal number).
LANGUAGES (iterable containing results from function calls).

If you need to use them, you would have to define them in your settings module or create your own factory to convert the values by yourself.

Variable substitution¶

You can create variables by defining them as options in the DEFAULT section of your configuration file. You’ll then be able to refer to these values by using the syntax ${variable_name}. To escape a string which otherwise would be interpreted as a variable, use the syntax $${not_a_variable_name}.

For example:

[DEFAULT]
debug = false
django_settings_module = your_django_project.settings
admin_email = user@example.com

[app:main]
use = egg:django-pastedeploy-settings
DEFAULT_FROM_EMAIL = "${admin_email}"

# The following will result in the string "${django} " (without quotes).
EMAIL_SUBJECT_PREFIX = "$${django} "

The syntax to refer to variables is specific to django-pastedeploy-settings. The syntax supported by PasteDeploy is %(variable_name)s, and it’s discouraged by the developers of this library because:

It’s not a good idea to mix the two syntaxes in your files, for maintainability reasons.
Given the way PasteDeploy handles variable substitution, the options defined in the DEFAULT section may not always be ready to be used as variables when they’re referenced. With our approach, you don’t need to worry about this.

Implicit Variables¶

There are a couple of variables defined by PasteDeploy which you can refer to in your configuration.

One of them is here, which is the absolute path to the directory that contains the INI file. You can use it like this:

# (...)

[app:main]
use = egg:django-pastedeploy-settings
MEDIA_ROOT = "${here}/media"

# (...)

The other variable is __file__, which is the absolute path to the INI file. It’s not very useful in the context of these files, but can be useful when using custom factories.

Serving Your Application¶

Serving your application is a piece of cake now that you use PasteDeploy. It’s simpler than using Django’s mechanisms because there’s no need to import os and set an environment variable.

Production Server¶

The following is a sample WSGI script for mod_wsgi:

from paste.deploy import loadapp

application = loadapp("config:/path/to/your/config.ini")

And the following is a sample script for FastCGI:

from paste.deploy import loadapp
from flup.server.fcgi_fork import WSGIServer

app = loadapp("config:/path/to/your/config.ini")
WSGIServer(app).run()

You might want to check the deployment documentation for the other Python frameworks (e.g., Pylons). They’ve been using WSGI heavily since day one, so it’s likely you’ll get ideas on how to meet your special needs, should you have any.

Development Server¶

PasteDeploy makes it easy to use any WSGI-compatible server for development too, and some Python-based servers (e.g., Gunicorn, PasteScript) make it even easier thanks to their built-in integration with PasteDeploy. So now you have the choice of sticking to Django’s manage renserver or use a different one.

There are a few WSGI servers that are very convenient for development of WSGI application and PasteScript is by far the most widely used one. Unlike Django’s, it is multi-threaded and therefore suitable for AJAX interfaces. Like Django’s, it’s able to reload the application when you change something in your code. It’s also so robust that it’s often the server of choice for people deploying with FastCGI.

Once you have installed PasteScript (e.g., easy_install PasteScript), you need to configure the server in your configuration file by adding the following section anywhere:

[server:main]
use = egg:Paste#http
port = 8080

And then you’ll be able to run the server:

cd /path/to/your/project
paster serve --reload config.ini

paster will load the application defined in app:main. If you want to use a different one, you’d need to set it explicitly, e.g.:

paster serve --reload config.ini#develop

If you don’t want to type that long command all the time, you could just execute that file directly.

Configure logging¶

You can configure logging from the same PasteDeploy configuration file by adding all the sections recognized by Python’s built-in logging mechanisms.

A full development configuration file could look like this:

[server:main]
use = egg:Paste#http
port = 8000

[app:main]
use = config:base-config.ini
set debug = true

# ===== LOGGING

[loggers]
keys = root,yourpackage

[handlers]
keys = global,yourpackage

[formatters]
keys = generic

# Loggers

[logger_root]
level = WARNING
handlers = global

[logger_yourpackage]
qualname = coolproject.module
handlers = yourpackage
propagate = 0

# Handlers

[handler_global]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = generic

[handler_yourpackage]
class = handlers.RotatingFileHandler
args = ("%(here)s/logs/coolpackage.log", )
level = NOTSET
formatter = generic

# Formatters

[formatter_generic]
format = %(asctime)s,%(msecs)03d %(levelname)-5.5s [%(name)s] %(message)s
datefmt = %Y-%m-%d %H:%M:%S

Making manage work again¶

You’ll find that your manage command will be broken after moving settings over to a PasteDeploy configuration file. The fix is really simple, just put the following at the top of your manage script:

from paste.deploy import loadapp

loadapp("config:/path/to/your/configuration.ini")

If the URI varies depending on whether you’re in a development environment or some other condition, you have two ways of setting this URI:

Using a relative path.
Introducing some form of variable substitution, such as creating manage.py from a template (potentially generated by a build system like Buildout) or using an environment variable.

Multiple configuration files¶

As we’ve seen so far, PasteDeploy configuration files can be extended in a cascade like fashion. This can also be done across files.

You could have the following base configuration file:

# base-config.ini

[DEFAULT]
debug = false

[app:base]
use = egg:django-pastedeploy-settings
EMAIL_PORT = 25

[app:debug]
use = base
set debug = true

And then override it for development:

# develop.ini

[server:main]
use = egg:Paste#http
port = 8080

[app:main]
use = config:base-config.ini#debug
EMAIL_PORT = 1025

This way, you could also run paster as:

paster serve --reload develop.ini

Using custom factories¶

If you need to perform a one-off routine when your application is started up (i.e., before any request) or wrap your Django application with WSGI middleware, you can write your own PasteDeploy application factory:

from django_pastedeploy_settings import get_configured_django_wsgi_app

def make_application(global_config, **local_conf):

    # Do something before importing Django and your settings have been applied.

    app = get_configured_django_wsgi_app(global_config, **local_conf)

    # Do something right after your application has been set up (e.g., add WSGI middleware).

    return app

global_config is a dictionary that contains all the options in the DEFAULT section, while local_conf will contain all the options in the app:* section. The values in both dictionaries are the raw strings defined in your INI file, not the decoded JSON values, so if you want to use such values, you should do it via django.conf.settings after calling get_configured_django_wsgi_app(). If you need to use any of those values before calling get_configured_django_wsgi_app(), you’d have to decode them yourself (keeping in mind that not all values are encoded in JSON because not all are Django settings).

PasteDeploy offers two options to use application factories in a configuration file:

Setuptools entry point: If you add the following to your setup.py file:

setup(
    "yourdistribution",
    # (...)
    entry_points="""
    # -*- Entry points: -*-
    [paste.app_factory]
    main = yourpackage.module:make_application
    """,
    )

you’d be able to use the factory as:

# (...)
[app:main]
use = egg:yourdistribution
# (...)

If you can’t or don’t want to define an entry point, you can use it like this:

# (...)
[app:main]
paste.app_factory = yourpackage.module:make_application
# (...)

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