CS304: Django

Admin

Congratulations to Sonali, public speaking finalist at Ruhlman!
Functionality for Hwk5 is graded and entered into Sakai. Remaining 10 points next week.
Draft versions: comments on 9/17 where I got a CS304-S14-P3-foo document. Took me about 5 hours for 9 of them. Please submit one ASAP if you want feedback.
Exam solution distributed (on paper). Graded, I hope, by Thursday. There are 27, and at 20 minutes each, only 9 hours of grading! ... Starting Friday, I'll give what feedback I can on the Alpha versions. Definitely by end of reading period.
Presentations on Thursday. Most people can stay late. I'll make up a schedule based on that. Invite your friends!
Thank you, all of you who are here, for your focus and attention even after it wouldn't matter to your grade. Please don't leave now....

Plan

Introduce Django.
Address those questions
Describe some important infrastructure:
- virtualenv
- pip
- SQLite
Let Sarah Xu take over
Grab bag of other topics, but probably not. Maybe only atpworldtour.com, schedule for Madrid Open, 2015

Django

it is the entire web server. We saw that last time.
Django opens its own port, so that it doesn't conflict with Apache
It runs continuously. Use ^C in the shell to kill it
All the code is in a directory that provides configuration info and software for the same runserver command.
Comes with its own packages for templating and such, though you replace them with other packages.

Django questions

Address your questions

Ports

With Django, as with node.js, you're not extending an existing HTTP server, as we are with Apache or Tomcat. You're responsible for the entire server. You open up a port to do so.

For today, you'll each be running on your own Mac, so there's no trouble. We can all use port 8000 (the Django default).

Note that this is one reason, among several, that I haven't switched to Django for this course. I want to figure out a smooth way to integrate a Django-powered site with Apache-powered Tempest (and other servers).

Django Tutorial Resource

The following is a good start: Django Book. It's out of date, but still an excellent introduction to the ideas and infrastructure, which I think is sadly lacking from other tutorials.

Background Knowledge: Virtualenv

Separate from Django but crucial in the same way that plumbing is separate from opening a store.

Virtualenv is a way to allow you, the user, to install a set of Python packages for a particular project.

You don't need access to shared, root-controlled directories such as /usr/local/lib/python2.7/.
Each project can have its own, possibly conflicting, set of Python packages.
Each project has its own PYTHONPATH, determining where Python loads packages from, when in that project.
Setting the PYTHONPATH and other environment variables requires source-ing a script in that directory.

This is really cool!

On your classroom Mac, start a terminal
Use the virtualenv command; it should exist and tell you its usage.
Do virtualenv kittens or some other project name. This creates a kittens directory with a bunch of stuff in it for your kittens project.
Do cd kittens and ls. Look around. Look in bin.

Do printenv VIRTUAL_ENV and printenv PATH. Do which python Do:

python
>>> import sys, pprint
>>> pprint.pprint(sys.path)
>>> quit

Do source bin/activate. (Maybe in a second shell.)
Repeat the printing of VIRTUAL_ENV, and PATH and its effect on PYTHONPATH.

Background Knowledge: Pip

Pip is another bit of Python plumbing. It allows you to

download and install the latest version of a package or module,
for your version of Python,
together with any pre-requisite packages and modules (dependencies)

With virtualenv, pip installs to the activated project, rather than to system-wide directories. Usually, you couldn't use it; now you can!

In an activated project, do pip install django
do an ls on the lib/python2.7/site-packages directory.
run python, and do import django

Background Knowledge: SQLite

SQLite is a SQL-based DBMS in a single file. This is nice because you can easily copy your database from place to place, it's easily locked, etc. This is the default back-end DBMS for Django.

Turning it Over

I'm now going to turn the class over to Sarah Xu, who took 304 last year and did an independent study using Django this term.

She wrote this handout for us.

She also provided this zip file for us.

Summary

Django and frameworks like it are common in web databases.
The concepts and components are widespread and important:
- URL mappings, factoring code from URL patterns
- URL parsing, using path/to/view/foo/bar to
- Templating, particularly with inheritance, which is hard to do with what we know so far.
- Views as functions in a namespace, as opposed to different scripts that have to be loaded, parsed, and then run in another process. Thus, we get a multi-threaded app, like Java Servlets
- Models as a set of objects that map to relational database tables. That mapping can be automatic. So, you don't have to know SQL for most of your database interactions.
Worries:
- You have to worry about writing thread-safe code. The Django core is thread-safe. That's why the request and response are arguments, not globals. Whether you write thread-safe views is another question. See Does Django have any tread-safety issues?
- Deployment. Django isn't necessarily the right tool for retrieving CSS and JavaScript files, so have to play well with something like Apache.
- Ports: Django is great if it's the only project or app on the web server, but not so good in an environment like ours.