web programming in django - emory university · web programming in django...

Web Programming in DjangoIntroduction to its Model-View-Controller Architecture

CS 370 SE Practicum, Cengiz Günay

(Some slides courtesy of Eugene Agichtein and the Internets)

Snowpacolypse ATL 2014

CS 370, Günay (Emory) Django MVC intro Spring 2014 1 / 5

Agenda

Warm-up project:

Good proposals! ,Tentatively due Feb 13th

Today:

Intro to Django & its Model-View-Controller frameworkRunning Django on your laptops

Surveys first. . .


Entry/Exit Surveys

Exit survey: Web ProgrammingTell me the steps between the server and client during an HTML webform display, submission, and response.To maintain user sessions, what pieces of persistent information mustbe stored on the client and the server side?

Entry survey: Django & MVCIn your opinion, what is the most useful feature of theModel-View-Controller approach?What parts of web programming are most tedious to you that youwould like it handled by a high-level web framework?


• high-level Python Web frameworkthat encourages rapid development and clean, pragmatic design.

• Django was designed to handle two challenges: – intensive deadlines

– stringent requirements of the experienced Web developers who wrote it.

• It lets you build high-performing, elegant Web applications quickly.

• Focuses on automating as much as possible

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Approach: Model-View-Controller

• Model: maintain state of application. Typically done with a database

• View: output to client, usually as HTML code, rendered by a web browser.

• Controller: interaction with user. handles user input, processes data and communicates with Model to save state.

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Formalized in 1979

MVC

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Django flow

• 1) Visitor’s browser asks for a URL.2) Django matches the request against its urls.py files.3) If a match is found, Django moves on to the view that’s associated with the URL. Views are generally found inside each app in the views.py file.4) The view generally handles all the database manipulation. It grabs data and passes it on.5) A template (specified in the view) then displays that data.

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Site design in Django: review

• Goal: make common Web-development tasks fast

• Step 1: Design your model: model.py (M)

– Automatically create DB, APIs, admin interface

• Step 2: Design your views: views.py (V)

• Step 3: Design application logic: urls.py (C)

• Step 4: Design your templates:

• Step 5: Enjoy free bells & whistles (caching, syndication, …)

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Who cares?

• Spotify uses Django for some of their web sites. • Instagram uses Django for their web application [10]

• Yandex uses Django for their weather forecast site, OpenID provider, blog hosting service, internal Wiki and internal Mail Archives.

• Transifex – an open source platform for localization. • Bucketlist – uses Django to let users store life goals. • The Public Broadcasting Service• The Berkeley Graduate School of Journalism• Universal Subtitles uses Django for its free and open source collaborative

subtitling site. • FreeNAS uses Django for its web interface in its 8.0 major release.• PBS's Merlin, a platform for managing member stations' video metadata• Google internal stuff (details unknown).• ….

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Example Django Project

• Tutorial: https://docs.djangoproject.com/en/dev/intro/tutorial01/

– https://docs.djangoproject.com/en/dev/intro/tutorial03/

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Django Models

• A model is the single, definitive source of data about your data.

• Each model is a Python class that subclasses django.db.models.Model.

• Each attribute of the model represents a database field.

• Django gives you an automatically-generateddatabase-access API

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Example: Person (first_name, last_name)

• Define Person:from django.db import models

class Person(models.Model):

first_name = models.CharField(max_length=30)

last_name = models.CharField(max_length=30)

• What happens automaticallyCREATE TABLE myapp_person ( "id" serial NOT NULL

PRIMARY KEY, "first_name" varchar(30) NOT NULL,

"last_name" varchar(30) NOT NULL );

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Example: Person (first_name, last_name): 2

• Each field should be an instance of the Field class– The database column type (e.g. INTEGER, VARCHAR)

– The widget to use in Django's admin interface, if you care to use it (e.g. <input type="text">, <select>).

– The minimal validation requirements, used in Django'sautomatically-generated forms.

• Field reference:https://docs.djangoproject.com/en/1.3/ref/models/fields/#model-field-types

• More details: https://docs.djangoproject.com/en/1.3/topics/db/models/

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Django Models

• https://docs.djangoproject.com/en/dev/ref/models/instances/?from=olddocs

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Example 2: Django blog site

• Step-by-step tutorial: http://www.webmonkey.com/2010/02/get_started_with_django/

• Try using: http://dutch.mathcs.emory.edu:8100/blog/

• Admin interface: http://dutch.mathcs.emory.edu:8100/admin/

• Code: /home/cs370000/src/Django-1.3.1/blog/

• Start: python manage.py runserver 0.0.0.0:8100

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Recap: Django project pieces

• django-admin.py startproject blog

– blog/

__init__.py: A file required for Python to treat the mysitedirectory as a package

manage.py: A command-line utility that lets you interact with this project. Try: python manage.py help

settings.py: Settings/configuration for this project.

urls.py: The URLs for this project (“table of contents”)

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Django Project vs. App

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Django architecture encourages this

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Example:

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URL dispatcher

• A clean, elegant URL scheme is an important detail in a high-quality Web application.

• Django lets you design URLs however you want

– There’s no .php or .cgi required

• Cool URIs don't change:

– http://www.w3.org/Provider/Style/URI

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How Django processes a request

1. Determine the root URLconf module to use. Ordinarily, this is the value of the ROOT_URLCONF setting, can be changed.

2. Load URLconf and looks for variable urlpatterns. This should be a Python list, in the format returned by the function django.conf.urls.patterns().

3. Iterate through each URL pattern, in order, and stop at the first one that matches the requested URL. (REGEX)

4. If one of the regexes matches, import and call the given view, which is a Python function. The view gets passed an HttpRequest as its first argument and any values captured in the regex as remaining arguments.

5. If no regex matches, or if an exception is raised during any point in this process, invoke an appropriate error-handling view.

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2-minute REGEX review

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Symbol Matches

. (dot) Any single character

\d Any single digit

[A-Z] Any character between A and Z (uppercase)

[a-z] Any character between a and z (lowercase)

[A-Za-z] Any character between a and z (case-insensitive)

+ One or more of the previous expression (e.g., \d+ matches one or more digits)

[^/]+ One or more characters until (and not including) a forward slash

? Zero or one of the previous expression (e.g., \d? matches zero or one digits)

*Zero or more of the previous expression (e.g., \d* matches zero, one or more than one digit)

{1,3}Between one and three (inclusive) of the previous expression (e.g., \d{1,3} matches one, two or three digits)

http://www.djangoproject.com/r/python/re-module/

Python Regexes, cont’d

• To capture a value from the URL, just put parenthesis around it.

• There's no need to add a leading slash, because every URL has that.

• For example, it's ^wiki, not ^/wiki. The 'r' is optional but recommended.

• Named regular-expressions: (?P<name>pattern)

– (?P<page_name>[^/]+)/$',

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Example URLconf

from django.conf.urls.defaults import *

#http://cs370.mathcs.emory.edu:8100/wiki/CSRF

urlpatterns = patterns('',

(r'^wiki/(?P<page_name>[^/]+)/$',

'blog.wiki.views.view_page'),

(r'^wiki/(?P<page_name>[^/]+)/edit/$',

'blog.wiki.views.edit_page'),

(r'^wiki/(?P<page_name>[^/]+)/save/$',

'blog.wiki.views.save_page'),

)

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Passing Parameters to Views


Good Design: Including other URLConfs

• Your urlpatterns can "include" other URLconfmodules.– Allows modular sites (good design)

• urlpatterns = patterns('',(r'^wiki/', include('blog.wiki.urls'))

)

– Note But: (wiki/urls.py):• (r'^(?P<page_name>[^/]+)/$', 'blog.wiki.views.view_page')

• More details:– https://docs.djangoproject.com/en/dev/topics/http/urls/

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Design Pattern: Include URLs

• There should be one urls.py at the project level, and one urls.py at each app level. The project level urls.py should include each of the urls.py under a prefix.:

• http://agiliq.com/books/djangodesignpatterns/urls.html

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Onwards and upwords…

• Django book:

– http://www.djangobook.com/en/2.0/

• Common mistakes/gotchas:

– https://code.djangoproject.com/wiki/NewbieMistakes

• To be continued…

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