Paste Deployment

author:

Ian Bicking <ianb@colorstudy.com>

Documents:

• Paste Deployment News
• paste.deploy.loadwsgi -- Load WSGI applications from config files
• paste.deploy.config -- Configuration and Environment middleware
• paste.deploy.converters -- Conversion helpers for String Configuration

Introduction

Paste Deployment is a system for finding and configuring WSGI applications and servers. For WSGI application consumers it provides a single, simple function (loadapp) for loading a WSGI application from a configuration file or a Python Egg. For WSGI application providers it only asks for a single, simple entry point to your application, so that application users don't need to be exposed to the implementation details of your application.

The result is something a system administrator can install and manage without knowing any Python, or the details of the WSGI application or its container.

Paste Deployment currently does not require other parts of Paste, and is distributed as a separate package.

To see updates that have been made to Paste Deploy see the news file.

Paste Deploy is released under the MIT license.

Status

Paste Deploy has passed version 1.0. Paste Deploy is an actively maintained project. As of 1.0, we'll make a strong effort to maintain backward compatibility (this actually started happening long before 1.0, but now it is explicit). This will include deprecation warnings when necessary. Major changes will take place under new functions or with new entry points.

Note that the most key aspect of Paste Deploy is the entry points it defines (such as paste.app_factory). Paste Deploy is not the only consumer of these entry points, and many extensions can best take place by utilizing the entry points instead of using Paste Deploy directly. The entry points will not change; if changes are necessary, new entry points will be defined.

Installation

First make sure you have either setuptools or its modern replacement distribute installed. For Python 3.x you need distribute as setuptools does not work on it.

Then you can install Paste Deployment using pip by running:

pip install PasteDeploy

If you want to track development, do:

git clone https://github.com/Pylons/pastedeploy
cd pastedeploy
pip install -e .

This will install the package locally, and will load the files in the checkout. You can also simply install PasteDeploy==dev.

For downloads and other information see the PyPI PasteDeploy page.

A complementary package is Paste Script. To install that, use pip install PasteScript (or pip install PasteScript==dev).

From the User Perspective

In the following sections, the Python API for using Paste Deploy is given. This isn't what users will be using (but it is useful for Python developers and useful for setting up tests fixtures).

The primary interaction with Paste Deploy is through its configuration files. The primary thing you want to do with a configuration file is serve it. To learn about serving configuration files, see the ``paster serve` command <https://pastescript.readthedocs.io/en/latest/#paster-serve>`_.

The Config File

A config file has different sections. The only sections Paste Deploy cares about have prefixes, like app:main or filter:errors -- the part after the : is the "name" of the section, and the part before gives the "type". Other sections are ignored.

The format is a simple INI format: name = value. You can extend the value by indenting subsequent lines. # is a comment.

Typically you have one or two sections, named "main": an application section ([app:main]) and a server section ([server:main]). [composite:...] signifies something that dispatches to multiple applications (example below).

Here's a typical configuration file that also shows off mounting multiple applications:

[composite:main]
use = egg:Paste#urlmap
/ = home
/blog = blog
/wiki = wiki
/cms = config:cms.ini

[app:home]
use = egg:Paste#static
document_root = %(here)s/htdocs

[filter-app:blog]
use = egg:Authentication#auth
next = blogapp
roles = admin
htpasswd = /home/me/users.htpasswd

[app:blogapp]
use = egg:BlogApp
database = sqlite:/home/me/blog.db

[app:wiki]
use = call:mywiki.main:application
database = sqlite:/home/me/wiki.db

I'll explain each section in detail now:

[composite:main]
use = egg:Paste#urlmap
/ = home
/blog = blog
/cms = config:cms.ini

That this is a composite section means it dispatches the request to other applications. use = egg:Paste#urlmap means to use the composite application named urlmap from the Paste package. urlmap is a particularly common composite application -- it uses a path prefix to map your request to another application. These are the applications like "home", "blog", "wiki" and "config:cms.ini". The last one just refers to another file cms.ini in the same directory.

Next up:

[app:home]
use = egg:Paste#static
document_root = %(here)s/htdocs

egg:Paste#static is another simple application, in this case it just serves up non-dynamic files. It takes one bit of configuration: document_root. You can use variable substitution, which will pull variables from the section [DEFAULT] (case sensitive!) with markers like %(var_name)s. The special variable %(here)s is the directory containing the configuration file; you should use that in lieu of relative filenames (which depend on the current directory, which can change depending how the server is run).

Then:

[filter-app:blog]
use = egg:Authentication#auth
next = blogapp
roles = admin
htpasswd = /home/me/users.htpasswd

[app:blogapp]
use = egg:BlogApp
database = sqlite:/home/me/blog.db

The [filter-app:blog] section means that you want an application with a filter applied. The application being filtered is indicated with next (which refers to the next section). The egg:Authentication#auth filter doesn't actually exist, but one could imagine it logs people in and checks permissions.

That last section is just a reference to an application that you probably installed with pip install BlogApp, and one bit of configuration you passed to it (database).

Lastly:

[app:wiki]
use = call:mywiki.main:application
database = sqlite:/home/me/wiki.db

This section is similar to the previous one, with one important difference. Instead of an entry point in an egg, it refers directly to the application variable in the mywiki.main module. The reference consist of two parts, separated by a colon. The left part is the full name of the module and the right part is the path to the variable, as a Python expression relative to the containing module.

So, that's most of the features you'll use.

Basic Usage

The basic way you'll use Paste Deployment is to load WSGI applications. Many Python frameworks now support WSGI, so applications written for these frameworks should be usable.

The primary function is paste.deploy.loadapp. This loads an application given a URI. You can use it like:

from paste.deploy import loadapp
wsgi_app = loadapp('config:/path/to/config.ini')

There's two URI formats currently supported: config: and egg:.

config: URIs

URIs that being with config: refer to configuration files. These filenames can be relative if you pass the relative_to keyword argument to loadapp().

Note

Filenames are never considered relative to the current working directory, as that is an unpredictable location. Generally when a URI has a context it will be seen as relative to that context; for example, if you have a config: URI inside another configuration file, the path is considered relative to the directory that contains that configuration file.

Config Format

Configuration files are in the INI format. This is a simple format that looks like:

[section_name]
key = value
another key = a long value
    that extends over multiple lines

All values are strings (no quoting is necessary). The keys and section names are case-sensitive, and may contain punctuation and spaces (though both keys and values are stripped of leading and trailing whitespace). Lines can be continued with leading whitespace.

Lines beginning with # (preferred) or ; are considered comments.

Applications

You can define multiple applications in a single file; each application goes in its own section. Even if you have just one application, you must put it in a section.

Each section name defining an application should be prefixed with app:. The "main" section (when just defining one application) would go in [app:main] or just [app].

There's two ways to indicate the Python code for the application. The first is to refer to another URI or name:

[app:myapp]
use = config:another_config_file.ini#app_name

# or any URI:
[app:myotherapp]
use = egg:MyApp

# or a callable from a module:
[app:mythirdapp]
use = call:my.project:myapplication

# or even another section:
[app:mylastapp]
use = myotherapp

It would seem at first that this was pointless; just a way to point to another location. However, in addition to loading the application from that location, you can also add or change the configuration.

The other way to define an application is to point exactly to some Python code:

[app:myapp]
paste.app_factory = myapp.modulename:app_factory

You must give an explicit protocol (in this case paste.app_factory), and the value is something to import. In this case the module myapp.modulename is loaded, and the app_factory object retrieved from it.

See Defining Factories for more about the protocols.

Configuration

Configuration is done through keys besides use (or the protocol names). Any other keys found in the section will be passed as keyword arguments to the factory. This might look like:

[app:blog]
use = egg:MyBlog
database = mysql://localhost/blogdb
blogname = This Is My Blog!

You can override these in other sections, like:

[app:otherblog]
use = blog
blogname = The other face of my blog

This way some settings could be defined in a generic configuration file (if you have use = config:other_config_file) or you can publish multiple (more specialized) applications just by adding a section.

Global Configuration

Often many applications share the same configuration. While you can do that a bit by using other config sections and overriding values, often you want that done for a bunch of disparate configuration values. And typically applications can't take "extra" configuration parameters; with global configuration you do something equivalent to "if this application wants to know the admin email, this is it".

Applications are passed the global configuration separately, so they must specifically pull values out of it; typically the global configuration serves as the basis for defaults when no local configuration is passed in.

Global configuration to apply to every application defined in a file should go in a special section named [DEFAULT]. You can override global configuration locally like:

[DEFAULT]
admin_email = webmaster@example.com

[app:main]
use = ...
set admin_email = bob@example.com

That is, by using set in front of the key.

Composite Applications

"Composite" applications are things that act like applications, but are made up of other applications. One example would be a URL mapper, where you mount applications at different URL paths. This might look like:

[composite:main]
use = egg:Paste#urlmap
/ = mainapp
/files = staticapp

[app:mainapp]
use = egg:MyApp

[app:staticapp]
use = egg:Paste#static
document_root = /path/to/docroot

The composite application "main" is just like any other application from the outside (you load it with loadapp for instance), but it has access to other applications defined in the configuration file.

Other Objects

In addition to sections with app:, you can define filters and servers in a configuration file, with server: and filter: prefixes. You load these with loadserver and loadfilter. The configuration works just the same; you just get back different kinds of objects.

Filter Composition

There are several ways to apply filters to applications. It mostly depends on how many filters, and in what order you want to apply them.

The first way is to use the filter-with setting, like:

[app:main]
use = egg:MyEgg
filter-with = printdebug

[filter:printdebug]
use = egg:Paste#printdebug
# and you could have another filter-with here, and so on...

Also, two special section types exist to apply filters to your applications: [filter-app:...] and [pipeline:...]. Both of these sections define applications, and so can be used wherever an application is needed.

filter-app defines a filter (just like you would in a [filter:...] section), and then a special key next which points to the application to apply the filter to.

pipeline: is used when you need apply a number of filters. It takes one configuration key pipeline (plus any global configuration overrides you want). pipeline is a list of filters ended by an application, like:

[pipeline:main]
pipeline = filter1 egg:FilterEgg#filter2 filter3 app

[filter:filter1]
# ...

Getting Configuration

If you want to get the configuration without creating the application, you can use the appconfig(uri) function, which is just like the loadapp() function except it returns the configuration that would be used, as a dictionary. Both global and local configuration is combined into a single dictionary, but you can look at just one or the other with the attributes .local_conf and .global_conf.

egg: URIs

Python Eggs are a distribution and installation format produced by setuptools and distribute that adds metadata to a normal Python package (among other things).

You don't need to understand a whole lot about Eggs to use them. If you have a <python:distutils> setup.py script, just change:

from distutils.core import setup

to:

from setuptools import setup

Now when you install the package it will be installed as an egg.

The first important part about an Egg is that it has a specification. This is formed from the name of your distribution (the name keyword argument to setup()), and you can specify a specific version. So you can have an egg named MyApp, or MyApp==0.1 to specify a specific version.

The second is entry points. These are references to Python objects in your packages that are named and have a specific protocol. "Protocol" here is just a way of saying that we will call them with certain arguments, and expect a specific return value. We'll talk more about the protocols later.

The important part here is how we define entry points. You'll add an argument to setup() like:

setup(
    name='MyApp',
    # ...
    entry_points={
        'paste.app_factory': [
            'main=myapp.mymodule:app_factory',
            'ob2=myapp.mymodule:ob_factory'],
        },
    )

This defines two applications named main and ob2. You can then refer to these by egg:MyApp#main (or just egg:MyApp, since main is the default) and egg:MyApp#ob2.

The values are instructions for importing the objects. main is located in the myapp.mymodule module, in an object named app_factory.

There's no way to add configuration to objects imported as Eggs.

Defining Factories

This lets you point to factories (that obey the specific protocols we mentioned). But that's not much use unless you can create factories for your applications.

There's a few protocols: paste.app_factory, paste.composite_factory, paste.filter_factory, and lastly paste.server_factory. Each of these expects a callable (like a function, method, or class).

paste.app_factory

The application is the most common. You define one like:

def app_factory(global_config, **local_conf):
    return wsgi_app

The global_config is a dictionary, and local configuration is passed as keyword arguments. The function returns a WSGI application.

paste.composite_factory

Composites are just slightly more complex:

def composite_factory(loader, global_config, **local_conf):
    return wsgi_app

The loader argument is an object that has a couple interesting methods. get_app(name_or_uri, global_conf=None) return a WSGI application with the given name. get_filter and get_server work the same way.

A more interesting example might be a composite factory that does something. For instance, consider a "pipeline" application:

def pipeline_factory(loader, global_config, pipeline):
    # space-separated list of filter and app names:
    pipeline = pipeline.split()
    filters = [loader.get_filter(n) for n in pipeline[:-1]]
    app = loader.get_app(pipeline[-1])
    filters.reverse() # apply in reverse order!
    for filter in filters:
        app = filter(app)
    return app

Then we use it like:

[composite:main]
use = <pipeline_factory_uri>
pipeline = egg:Paste#printdebug session myapp

[filter:session]
use = egg:Paste#session
store = memory

[app:myapp]
use = egg:MyApp

paste.filter_factory

Filter factories are just like app factories (same signature), except they return filters. Filters are callables that take a WSGI application as the only argument, and return a "filtered" version of that application.

Here's an example of a filter that checks that the REMOTE_USER CGI variable is set, creating a really simple authentication filter:

def auth_filter_factory(global_conf, req_usernames):
    # space-separated list of usernames:
    req_usernames = req_usernames.split()
    def filter(app):
        return AuthFilter(app, req_usernames)
    return filter

class AuthFilter(object):
    def __init__(self, app, req_usernames):
        self.app = app
        self.req_usernames = req_usernames

    def __call__(self, environ, start_response):
        if environ.get('REMOTE_USER') in self.req_usernames:
            return self.app(environ, start_response)
        start_response(
            '403 Forbidden', [('Content-type', 'text/html')])
        return ['You are forbidden to view this resource']

paste.filter_app_factory

This is very similar to paste.filter_factory, except that it also takes a wsgi_app argument, and returns a WSGI application. So if you changed the above example to:

class AuthFilter(object):
    def __init__(self, app, global_conf, req_usernames):
        # ...

Then AuthFilter would serve as a filter_app_factory (req_usernames is a required local configuration key in this case).

paste.server_factory

This takes the same signature as applications and filters, but returns a server.

A server is a callable that takes a single argument, a WSGI application. It then serves the application.

An example might look like:

def server_factory(global_conf, host, port):
    port = int(port)
    def serve(app):
        s = Server(app, host=host, port=port)
        s.serve_forever()
    return serve

The implementation of Server is left to the user.

paste.server_runner

Like paste.server_factory, except wsgi_app is passed as the first argument, and the server should run immediately.

Outstanding Issues

• Should there be a "default" protocol for each type of object? Since there's currently only one protocol, it seems like it makes sense (in the future there could be multiple). Except that paste.app_factory and paste.composite_factory overlap considerably.

• ConfigParser's INI parsing is kind of annoying. I'd like it both more constrained and less constrained. Some parts are sloppy (like the way it interprets [DEFAULT]).

• config: URLs should be potentially relative to other locations, e.g., config:$docroot/.... Maybe using variables from global_conf?

• Should other variables have access to global_conf?

• Should objects be Python-syntax, instead of always strings? Lots of code isn't usable with Python strings without a thin wrapper to translate objects into their proper types.

• Some short-form for a filter/app, where the filter refers to the "next app". Maybe like:

[app-filter:app_name]
use = egg:...
next = next_app

[app:next_app]
# ...