Introduction to Python and Django

Introduction to
(Python and)
the Django Framework
Prashant Punjabi
Solution Street
9/26/2014

Python
!
• Created by Guido van Rossum
in the late 1980s
!
• Named after ‘Monty Python’s
Flying Circus’
!
• Python 2.0 was released on 16
October 2000
!
• Python 3.0 a major,
backwards-incompatible
release, was released on 3
December 2008
!
• Guido is the BDFL

Python
• General Purpose, high-level programming language
• Supports multiple programming paradigms
• object-oriented, functional, structured, imperative(?)
• Dynamically typed
• Many implementations
• CPython (reference)
• Jython
• IronPython .. and many more

The Zen of Python
• “Core philosophy”
• Beautiful is better than ugly
• Explicit is better than implicit
• Simple is better than complex
• Complex is better than complicated
• Readability counts
• >>> import this

Data Types
• Numbers
• int, ﬂoat, long (and complex)
• Strings
• str, unicode
• Sequence Types
• str, unicode, lists, tuples (and bytearrays, buffers, xrange)
• strings, tuples are ‘immutable’
• lists are mutable
• Mapping Types
• dict

Functions
• Defined using the keyword.. def
• Followed by the function name and the parenthesized list of formal parameters.
• The statements that form the body of the function start at the next line,
• and must be indented (just like this line)
• The first statement of the function body can optionally be a string literal
• this string literal is the function’s documentation string, or docstring.
• Arguments are passed using call by value (where the value is always an object
reference, not the value of the object)
• Functions always return a value
• If not return is explicitly defined, the function returns None

Control Flow
• if
• if…elif…else
• for
• range
• break, continue, else
• while

Truthi-nessTM
• An empty list ([])
• An empty tuple (())
• An empty dictionary ({})
• An empty string ('')
• Zero (0)
• The special object None
• The object False (obviously)
• Custom objects that deﬁne their own Boolean context behavior (this is
advanced Python usage)

Modules and Packages
• A module is a file containing Python definitions and statements.
• The file name is the module name with the suffix .py appended.
• Within a module, the module’s name (as a string) is available as the value of the
global variable __name__
• Modules can be executed as a script
• python module.py [args]
• __name__ is set to __main__
• Packages are a way of structuring Python’s module namespace by using “dotted
module names”
• The __init__.py file is required to make Python treat a directory as containing
packages

Classes
• Python’s class mechanism adds classes with a minimum of new syntax and
semantics
• Python classes provide all the standard features of Object Oriented
Programming
• multiple base classes
• a derived class can override any methods of its base class or classes
• a method can call the method of a base class with the same name
• Objects can contain arbitrary amounts and kinds of data
• Class and Instance Variables
• Static Methods

Standard Library
• Operating System Interface
• File handling
• String pattern matching
• Regular expressions
• Mathematics
• Internet Access
• Dates and Times
• Collections
• Unit Tests

Django
• Django grew organically from real-world applications
• Born in the fall of 2003, in Lawrence, Kansas, USA
• Adrian Holovaty and Simon Willison - web
programmers at the Lawrence Journal-World
newspaper
• Released it in July 2005 and named it Django, after the
jazz guitarist Django Reinhard
• “For Perfectionists with Deadlines”

Getting Started
• Installation
• pip install Django
• ¯_(ツ)_/¯
• Creating a Django project
• django-admin.py startproject django_example
• Adding an ‘app’ to your Django project
• python manage.py startapp music

MVC - Separation of Concerns
• models.py
• description of the database table, represented by a Python class, called a model
• create, retrieve, update and delete records in your database using simple Python code
• views.py
• contains the business logic for the page
• contains functions, each of which are called a view functions or simply views
• urls.py
• ﬁle speciﬁes which view is called for a given URL pattern
• Templates
• describes the design of the page
• uses a template language with basic logic statements

models.py
• Each model is represented by a class that subclasses django.db.models.Model
• Class variables represents a database fields in the model
• A field is represented by an instance of a Field class eg CharField,
DateTimeField
• The name of each Field instance is used by the database as the column name
• Some Field classes have required arguments, others have optional arguments
• CharField, for example, requires that you give it a max_length
• default is an optional argument for many Field classes
• ForeignKey field is used to define relationships
• Django supports many-to-one, many-to-many and one-to-one

Queries
• Each model has at least one Manager, and it’s called objects by default.
• Managers are accessible only via model classes
• Enforce a separation between “table-level” operations and “record-
level” operations.
• A QuerySet represents a collection of objects from your database
• It can have zero, one or many ﬁlters
• A QuerySet equates to a SELECT statement, and a ﬁlter is a limiting
clause such as WHERE or LIMIT.
• Example

Migrations
• New in Django 1.7
• Previously accomplished by an external package called south
• Keeps the database in sync with the model objects
• Commands
• migrate
• makemigrations
• sqlmigrate
• squashmigrations
• Data Migrations
• python manage.py makemigrations --empty music

urls.py
• Django lets you design URLs however you want, with no
framework limitations.
• “Cool URIs don’t change”
• URL conﬁguration module (URLconf)
• simple mapping between URL patterns (regular expressions)
to Python functions (views)
• capture parts of URL as parameters to view function (named
groups)
• can be constructed dynamically

views.py
• A Python function that takes a Web request and returns a Web response
• HTML contents of a Web page,
• a redirect, or a 404 error,
• an XML document,
• an image
• . . . or anything
• The convention is to put views in a ﬁle called views.py
• but it can be pretty much anywhere on your python path
• ‘Django Shortcuts’
• redirect, reverse, render_to_response,

Templates
• Designed to strike a balance between power and ease
• A template is simply a text ﬁle. It can generate any text-based format (HTML, XML,
CSV, etc.).
• Variables - {{ variable }}
• Replaced with values when the template is evaluated
• Use a dot (.) to access attributes of a variable
• Dictionary lookup, attribute or method lookup or numeric index lookup
• {{ person.name }} or {{ person.get_full_name }} or {{ books.
1 }}
• Filters can be used to modify variables for display
• {{ name|lower }}

Templates
• Tags control the logic of the template
• {% tag %}
• Commonly used tags
• for
• if, elif and else
• block and extends - Template inheritance

Template Inheritance
• Most powerful part of Django’s template engine
• Build a base “skeleton” template that contains all the
common elements of your site and defines blocks that
child templates can override.
• {% extends %} must be the first template tag in that
template.
• More {% block %} tags in your base templates are
better
• Child templates don’t have to define all parent blocks

Forms
• Django handles three distinct parts of the work involved in forms
• preparing and restructuring data ready for rendering
• creating HTML forms for the data
• receiving and processing submitted forms and data from the client
• The Django Form class
• Form class’ ﬁelds map to HTML form <input> elements
• Fields manage form data and perform validation when a form is submitted
• Fields are represented to a user in the browser as HTML “widgets”
• Each ﬁeld type has an appropriate default Widget class, but these can be
overridden as required.

Batteries (still) included
• Authentication and Authorization
• Emails
• File Uploads
• Session
• Caching
• Transactions
• .. and so on

See also..
• The Django ‘admin’ app
• django-admin.py and manage.py
• ModelForms
• Generic Views or Class based views
• Static File deployment
• Settings
• Middleware
• Similar to Servlet Filters in Java

References
• Python - Wikipedia page (http://en.wikipedia.org/wiki/
Python_(programming_language))
• The Python Tutorial (https://docs.python.org/2/tutorial/
index.html)
• The Django Project (https://www.djangoproject.com/)
• The Django Book (http://www.djangobook.com/en/2.0/
index.html)
• The Django Documentation (https://
docs.djangoproject.com/en/1.7/)

Introduction to Python and Django

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