Python Web Frameworks – Application Object

In response to my previous post, someone asked “… what’s the point? How one way is better than the other?” My response was that I’m “not trying to judge these frameworks from the perspective of a developer (at least not completely), but rather from the POV of ‘if I were writing a framework, which features are essential, which nice to have, which I can disregard'” (emphasis added). Of course, I have my own biases as a developer as to what I consider “nice” or essential.

With that clarification out of the way, let’s look at the next feature: the WSGI callable generally known as the application object.

Django doesn’t want developers to be concerned with this artifact. A Django application is the collection of model, views (controllers in standard MVC terminology) and templates (views) that gets run by the server. For those interested, the Django WSGI callable is an instance of WSGIHandler (in django.core.handlers.wsgi).

As best as I can tell, Twisted Web expects the developer to write the application object. It will get called from the WSGIResponse run() method in twisted.web.wsgi. As its documentation says, Twisted Web is “a framework for doing things with the web” but is “not a ‘web framework’ in the same sense” as Django, so it doesn’t fit well in this comparison.

Pyramid‘s Router class (in pyramid.router) is what implements the WSGI callable. The Router instance gets created from the Configurator class’s make_wsgi_app() method. Select Router attributes and methods:

• logger
• root_factory
• routes_mapper
• request_factory
• registry (from args)
• handle_request()

In CherryPy, the Application class is the application object, although it’s not generallly used directly. Instead a user root object is mounted on a Tree which is a registry of these applications. Major attributes and methods:

• root (from args): root object (application handler)
• namespaces, config: for configuration
• wsgiapp(): a CPWSGIApp instance, to handle apps in a pipeline

Being a support library, Werkzeug doesn’t include an application object, but it has a sample in its Shortly example. Major attributes and methods:

• dispatch_request(): dispatcher
• jinja_env: template enviroment
• url_map: Map() taking list of Rule()’s
• error_404(): error handler
• render_template(): template renderer

Like Django, Web2py isn’t interested in having developers worry about the application object. Its WSGI callable is the standalone wsgibase() function, in gluon.main.

The web.py application class (yes, not capitalized) implements its WSGI callable. Major attributes and methods:

• autoreload (from args)
• init_mapping (from args)
• request()
• handle(): matches path to mapping
• run(): runs the developer server
• notfound(), internalerror(): error handlers

Flask‘s Flask class is its application object, derived from flask.helpers._PackageBoundObject. Selected attributes and methods (in addition to those in Werkzeug’s example above):

• static_url_path, static_folder (from args)
• instance_path (from args or auto-determined)
• view_functions
• before/after_xxx_funcs (dicts)
• logger
• run()
• route(): decorator for URL rules
• error_handler_spec (a dict)
• errorhandler(): decorator for error handler functions
  

Like Werkzeug, WebOb doesn’t include an application object, but it has a sample in its wiki example, most of it very specific to the example.

Bottle‘s Bottle class is its WSGI callable. Major attributes and methods:

• routes (list) and router (class Router instance)
• config (class ConfigDict instance)
• mount(): to mount an app at a given point
• match(): to search for matching routes
• route(): decorator to bind a function to a URL

Pesto‘s DispatcherApp class is its application object. Major attributes and methods:

• prefix (from args): a “mount” point
• matchpattern(), match(): matches URLs (match is decorator)
• urlfor(): handler/dispatcher to URL converter
• gettarget(): returns a four-part tuple from the URI
• status404_application(): error handler

Diva‘s Application class is the object of interest. Select attributes and methods:

• config (from kwargs), configure()
• routing_cfg and routing
• locale_dir (from args)
• template_dirs (from args)
• templates(): template loader
• prepare() and cleanup(): before and after methods

Summary: For developers who care about writing application code, Django and Web2py and to some extent Pyramid and CherryPy hide the existence of the WSGI callable, while other frameworks require that the programmer instantiate it or invoke it. If I were to write a framework, I’d want to make it easy on the developer as the former projects do, but wouldn’t keep it completely out of sight.

Python Web Frameworks – Development Server

One of the most frequent tasks of a web developer is running the app while it’s being written. So we’ll begin our examination of Python web frameworks by looking at the development servers provided and how does one run the app.

Django developers are used to invoking

python manage.py runserver

The runserver option runs Django’s development server which is invoked via the run() function in django.core.servers.basehttp. This in turn runs a WSGIServer which is derived from Python’s wsgiref’s simple_server.

Twisted Web has its own server based on its core reactor object. Here’s a simple usage example:

from twisted.web import server, resource
from twisted.internet import reactor

class HelloResource(resource.Resource):
    ...

reactor.listenTCP(8080, server.Site(HelloResource()))
reactor.run()

Pyramid uses Paste’s paste.httpserver which in turn is based on Python’s BaseHTTPServer. Sample usage:

from paste.httpserver import serve
from pyramid.config import Configurator

def hello_world(request):
   ... 

if __name__ == '__main__':
   config = Configurator()
   ...
   app = config.make_wsgi_app()
   serve(app, host='0.0.0.0')

In CherryPy, the normal server is cherrypy.engine which is invoked as:

cherrypy.engine.start()
cherrypy.engine.block()

cherrypy.engine is actually an instance of cherrypy.process.wspbus.Bus. A simpler alternative to the engine calls is to use:

cherrypy.quickstart(app())

Under the covers, CherryPy also defines a global server variable that is an instance of Server which implements the HTTP server, with help from cherrypy._cpwsgi_server.CPWSGIServer.

Werkzeug has a built-in server, used as follows:

from werkzeug.serving import run_simple
from myproject import make_app

app = make_app(...)
run_simple('localhost', 8080, app, use_reloader=True)

run_simple() invokes its own (werkzeug.serving) make_server which creates one of three types of XxxWSGIServer’s, which are derived from Python’s BaseHTTPServer.HTTPServer.

When Web2py is invoked, a GUI dialog is shown. The dialog is implemented in gluon.widget and its web2pyDialog.start method creates a main.HttpServer. gluon.main’s HttpServer class uses the Rocket server whose code is in gluon.rocket.

In web.py, to run an application you instantiate a web.application and invoke its run() method, e.g.:

app = web.application(urls, globals())
app.run()

This eventually runs a BaseHTTPServer.HTTPServer from the Python standard library.

Flask‘s WSGI application class (Flask) has a run() method:

run(host='127.0.0.1', port=5000, debug=None, **options)

This invokes the Werkzeug serving.run_simple function (see above).

WebOb doesn’t offer any server per se, but suggests using Paste’s httpserver.serve or wsgiref.simple_server.

Bottle has a standalone run() function:

run(app=None, server='wsgiref', host='127.0.0.1', port=8080,
    interval=1, reloader=False, quiet=False, plugins=None, **kargs)

This runs one of several web servers supported, by default, wsgiref, which uses the wsgiref.simple_server.

Pesto suggests using the Python wsgiref directly, e.g.:

import pesto
dispatcher = pesto.dispatcher_app()
...
httpd = make_server('', 8080, dispatcher)
httpd.serve_forever()

Diva uses Python’s WSGIServer from the standard library to implement its main() and serve() standalone functions. The former can be used as follows:

from diva.core import Application
from diva.server import main

class MyApp(Application): pass

main(MyApp())

Summary: With the exception of Twisted and CherryPy, all frameworks base their development servers on the standard library’s wsgiref or BaseHTTPServer, Paste or Rocket (which claims to be CherryPy-compatible). Lesson for prospective framework creators: Don’t write your own server. However, you may want to enhance your server with auto-reloading, threading, debugging. etc.

There are two approaches to running the app or server. Django and Web2py don’t want the developer to bother with writing app.run() or similar, the other frameworks expect the programmer to plug the various pieces together.

Best wishes for 2012 to all readers!

Python Web Frameworks – Candidates

I’ve been researching web frameworks, partly with a view to writing my own, and thought I’d share my findings. This first post will present the candidates briefly and subsequent posts will delve into particular features.

The candidates are presented based on the number of users as found on Ohloh, as a rough measure of popularity.  Each includes the number of downloads for the latest release on PyPI (sometimes misleading if a package had a recent release) and the salient parts of its advertised description (caveat emptor!). All of them are open source and written in Python, but some of them are WSGI libraries rather than full frameworks.

Django (843 users, 126,426 downloads): A “high-level … Web framework that encourages rapid development and clean, pragmatic design.”

Twisted Web (136 users, 14,413 downloads [both for Twisted]):  An “HTPP server that can be used as a library or run … stand-alone …, an HTML templating engine [and] an HTTP client library.”

Pyramid (aka Pylons on Ohloh, 125 users, 606 + 640 downloads): A “very general … web framework [designed] to make it easier for a developer to create an arbitrary web application.”

CherryPy (75 users, PyPI download info not available): A “minimalist … pythonic, object-oriented web framework.”

Werkzeug (31 users, 4225 downloads): A “WSGI utility library, … [it] is Simple … And Powerful.”

Web2py (21 users, 1157 downloads): A “full-stack framework for rapid development of fast, scalable, secure and portable database-driven web-based applications.”

Web.py (18 users, 11,708 downloads): A “web framework … that is as simple as it is powerful” (hmmm … see Werkzeug above).

Flask (16 users, 42,657 downloads): A “microframework … based on Werkzeug, Jinja 2 and good intentions.”

WebOb (16 users, 29,664 downloads): A “library that provides wrappers around the WSGI request environment, and an object to help create WSGI responses.”

Bottle (4 users, 163 downloads): A “fast, simple and lightweight WSGI micro web-framework.”

Pesto (no users, 421 downloads): Not a framework, but a library for “writing WSGI web applications.”

Diva (not on Ohloh or PyPI): This is Christopher Lenz’s  framework experimentation sandbox: “a lightweight web framework … built on top of WSGI and integrated with … Genshi.” My first, unpublished attempt at writing a framework was based on Diva.

Left Out: I had to draw the line somewhere. Compared to Richard Jones’ micro-framework battle, I included three of what he called “mega frameworks” and excluded three of his micro frameworks. For the record, here are the ones I left out: Aspen, Bobo, CubicWeb, Grok, Itty, Milla, Nagare, Nevow, Pump, Pyjamas, Pylons, Python Paste, Quixote, Spyce, Tornado, TurboGears, Webware, Zope 2 and Zope 3Bluebream. But that’s not all: see the Python.org wiki, WSGI.org and Wikipedia.

Unlike Richard, I will not attempt to develop an application with each framework. Instead, I’ll analyze specific features along the lines of Christopher’s Diva documentation.

Design Notes on Database Application Development

I recently took a brief look at web2py since its Database Abstraction Layer (DAL) sounded interesting. After following the tutorials in the Overview chapter, I ended up not delving into DAL.  Instead, I revisited Ruby on Rails which I had last looked circa 2005 when its support for PostgreSQL was rather weak. Although that has improved, the basic approach of Rails hasn’t and is very similar to web2py’s as well as Django.

The essence of these full stack frameworks is that the developer —or should I say programmer— knows how to design classes and that that design can be transformed into a relational database design mechanically and flawlessly. Maybe I’m biased because my first involvements in software engineering were in the areas of IT system design, but the latter premise seems like a very weak foundation for developing robust systems.

Maybe this also explains, in part, why NoSQL solutions have cropped up and found a following. It’s much easier —albeit not reliable or sound— to design a system when you “liberate” yourself from the “straitjacket” constraints of a database design (aka schema). However, in my experience, most systems beyond the blog/wiki/toy database examples used in tutorials benefit from specialized data analysis and modeling that result in a formal database design.

If the database precedes the application development, it appears the Rails/Django/web2py approach is back asswards. If the database is available during development, doesn’t it make more sense to re-use the database design effort in creating the application rather than try to repeat it with Python or Ruby classes?

The Pyrseas project, in its quest to rescue Andromeda, plans to implement a utility to help in database application development (tentatively named dbappgen but that may change). The utility will start off by connecting to a database and will allow the developer to create the application based on the tables and other objects present in it, not the other way around. If database changes are needed, they can be made there first, not by imposing some application-induced “migration” (as web2py does).

Since Pyrseas’ dbtoyaml utility already outputs a YAML/JSON description of a database, it may even be possible to conduct an application development exercise, at least in part, without connecting to the database. For example, if the developer invokes dbappgen moviesdb, the utility could first check whether a file named moviesdb.yaml (or with similar extensions) exists in the current directory and if so, read it and recreate in memory the PostgreSQL catalogs, thus allowing “offline” development.

In other news, due to the recent discussions regarding Python 3, my hopes that Werkzeug will be ported soon have been dashed. It appears it’s either use WebOb or write my own framework (which ironically is what Armin recommended).

Designing a (somewhat) RESTful Database User Interface

To continue exploring the design of a generic database user interface, let us turn our attention to a web interface. I’ll use the pattern of the Django admin application as the basis for specifying the behavior of a somewhat RESTful web interface for maintaining the movies database I’ve used in previous posts.

Home Page

The home page (canonically at http://www.example.com/) is comparable to the menu displayed initially by the command line interface (CLI). It will consist of an HTML table listing the logical entities together with options to add a new record (New) or edit an existing one (Edit). For now, the table will include a single entity: a film.

The latter option (or the entity link itself) will take the user to the List Records Page (see below). The New option will go to the

Add Record Page

This will be presented at http://www.example.com/film/new and is comparable to the Add interactions in the CLI. It will have an HTML form with fields (HTML text input or textarea elements) for each attribute and a “Save” button (HTML submit input element). Pressing the button will POST the new data to http://www.example.com/film/create.

List Records Page

This is of course comparable to the CLI List display. It will feature an HTML table listing all films. This is the index page for the entity so its URL will be http://www.example.com/film/.

We’ll worry about pagination later, but when we do the URL will have a query string to select a given page, e.g., http://www.example.com/film/?p=123.

Each row of the HTML table will have a link that leads to the

Record Edit Page

This will have a URL that identifies the film being edited, e.g., http://www.example.com/film/23969, where 23969 corresponds to a value in the id column of the film table in our database. This is equivalent to the CLI Update option.

The Edit page will have the same HTML form as the Add Record page, including the “Save” button, but the POST will go to http://www.example.com/film/save/23969. It will also have a “Delete” link that goes to the

Record Delete Confirmation Page

This will be presented at http://www.example.com/film/delete/23969 and is comparable to the confirmation requested by the CLI Delete. It will display an “Are you sure?” message together with details of the record to be deleted and a button titled “Yes, I’m sure.” The POST will use the same URL as the GET.

Form and Error Handling

Both the Add and Edit pages will validate the data entered. If there are any errors, the respective page will be redisplayed with a message(s) indicating what needs to be corrected. If a “Save” operation succeeds, we’ll redisplay the film listing.

Entity Handler

With this specification, if we translate each URL path above into a class method, we can outline a Python class that will act as the handler (or MVC controller, although Django calls this the view layer) for the HTTP requests. Note that the entity index URL (the listing) maps to the index method while the specific entity URL maps to the default method.

class FilmHandler(object):
    def new(self):
        "Displays a form to create a new film"
        pass

    def create(self, **formdata):
        "Saves the film submitted from ``new``"
        pass

    def index(self):
        "List all films"
        pass

    def default(self, id=None):
        "Displays a form for editing a film by id"
        pass

    def save(self, id=None, **formdata):
        "Saves the film submitted from ``default``"
        pass

    def delete(self, id=None):
        "Deletes an existing film by id"
        pass

There are many more details such as the “Model,” i.e., the interface to the database itself, and the MVC View (which Django calls the template layer). So visit again if you’re interested!

Update: Changed title and first paragraph to emphasize that the interface does not adhere strictly to the REST canon. See Oliver Charles’ detailed critique below.

User Interfaces for Databases

Pyrseas was started to improve on Andromeda. In addition to the schema version control I’ve been discussing in these posts, Andromeda provides two other capabilities: automations and web application programming.

Andromeda’s web programming allows you to generate a database application with very little actual programming, since Andromeda takes into account the primary key, foreign key and other information present in the YAML specification to generate much of the application code for you. This is another area that I’d like to explore for implementation in Pyrseas.

Administrative Applications

Nearly any database needs a user interface to add, retrieve, update and delete records, known by the unattractive acronym CRUD. There are a number of admin apps that provide such generic facilties. For example, for PostgreSQL we have pgAdmin which offers a GUI, and phpPgAdmin and Adminer which offer PHP web interfaces. These have a couple of limitations.

Admin apps almost always operate on a single table at a time. Interfaces for end users often require interacting with or viewing two or more tables on a single screen or page, e.g., customers with their invoices, accounts with their transactions.

By definition, admin apps are “raw” and cater to the needs of DBAs and programmers. After retrieving a customer record searching by name, the admin user can update any column, including the primary key and other columns that end users would normally be prevented from updating.

Django Admin

Django includes an appealing admin site. With a moderate dose of programming, you can develop an application suitable for end users. However, you’re at the mercy of Django’s constraints on “modeling” a database. Take for example the film_category table I used in my second post on version control:

CREATE TABLE film_category (
    film_id INTEGER NOT NULL REFERENCES film (id),
    category_id INTEGER NOT NULL
        REFERENCES category (category_id),
    last_update TIMESTAMP WITH TIME ZONE NOT NULL,
    PRIMARY KEY (film_id, category_id)
);

Django cannot use this table “as is” because it insists in having a single column as the primary key. So you end up adding an unnecessary “id” column together with a SEQUENCE (and needless to say, you incur the wrath of the SSSKA).

We could turn to SQLAlchemy, a Python-based ORM that understands that a primary key –albeit singular– can encompass more than one column, and some other web framework or even Django itself, and attempt to recreate the nice Django admin app. However, aside from the additional effort that would represent, we wouldn’t be taking advantage of the YAML database specification we can now store in our VCS.

This is a very broad, and some may say, ambitious topic, but as I did with version control, in future posts I’ll survey existing “art” and progress towards designing and implementing a generic end user interface for PostgreSQL. Reader feedback will be much appreciated.