Web Programming in Django Introduction to its Model-View-Controller - - PowerPoint PPT Presentation

Web Programming in Django

Introduction 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

CS 370, Günay (Emory) Django MVC intro Spring 2014 2 / 5

Agenda

Warm-up project: Good proposals! Tentatively due Feb 13th Today: Intro to Django & its Model-View-Controller framework Running Django on your laptops

CS 370, Günay (Emory) Django MVC intro Spring 2014 2 / 5

Agenda

Warm-up project: Good proposals! Tentatively due Feb 13th Today: Intro to Django & its Model-View-Controller framework Running Django on your laptops Surveys first. . .

CS 370, Günay (Emory) Django MVC intro Spring 2014 2 / 5

Entry/Exit Surveys

Exit survey: Web Programming

Tell me the steps between the server and client during an HTML web form display, submission, and response. To maintain user sessions, what pieces of persistent information must be stored on the client and the server side?

Entry survey: Django & MVC

In your opinion, what is the most useful feature of the Model-View-Controller approach? What parts of web programming are most tedious to you that you would like it handled by a high-level web framework?

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• high-level Python Web framework

that 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-generated

database-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 automatically

CREATE 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's automatically-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 mysite directory 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

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Good Design: Including other URLConfs

• Your urlpatterns can "include" other URLconf

modules.

– 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
• ne 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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