Boston Python Workshop/Saturday/Web app project

Overview
On Saturday, you can write and deploy a web application. It's an online poll where visitors can view choices (a bit of text, plus an image) and vote the option up and down.

My notes about this

 * Based on http://www.wiki.devchix.com/index.php?title=Rails_3_Curriculum and http://docs.djangoproject.com/en/dev/intro/tutorial01/

Overview
Note: This is one long page. It will take most of the afternoon to go through it.

If you stick with it, you will have deployed a web application to the world, where other people can play with it, and where you can modify it.

This is based heavily on the official tutorial for the Django web programming framework.

Writing your first Django app, part 1
Let’s learn by example.

Throughout this tutorial, we’ll walk you through the creation of a basic poll application.

It’ll consist of two parts:


 * A public site that lets people view polls and vote in them.
 * An admin site that lets you add, change and delete polls.

Switch to the right directory

 * In a terminal (or GitBash), get into the django_projects directory we created in the Friday setup portion of the tutorial. You can do that by typing this into your terminal:

cd Desktop cd django_projects

In the Friday setup portion of the workshop, you already saw how to use the django-admin.py command to start a project. The workshop coordinators already created a project, and you already forked it on Github. So now, you'll clone that to your computer.


 * Go to http://github.com/
 * Find your clone of workshop_mysite. Find the SSH URL for it, and copy that to the clipboard.
 * In the terminal, type: git clone followed by the URL for your personal fork of the workshop_mysite repository.
 * Make sure you can "cd" into it:

cd workshop_mysite

Look at the files
Let’s look at files are in the project:

workshop_mysite/ __init__.py   manage.py    settings.py    urls.py

These files are:


 * __init__.py: An empty file that tells Python that this directory should be considered a Python package. (Read more about packages in the official Python docs if you're a Python beginner.)
 * manage.py: A command-line utility that lets you interact with this Django project in various ways. You can read all the details about manage.py in django-admin.py and manage.py.
 * settings.py: Settings/configuration for this Django project. Django settings will tell you all about how settings work.
 * urls.py: The URL declarations for this Django project; a "table of contents" of your Django-powered site. You can read more about URLs in URL dispatcher.

The development server
Let's verify this worked. If you haven't already, and run the command python manage.py runserver. You'll see the following output on the command line:

Validating models... 0 errors found.

Django version 1.0, using settings 'mysite.settings' Development server is running at http://127.0.0.1:8000/ Quit the server with CONTROL-C.

You've started the Django development server, a lightweight Web server written purely in Python. The Django maintainers include this web server, but on a "deployment" like alwaysdata.com, you typically tie Django into an existing server like Apache.

Now that the server's running, visit http://127.0.0.1:8000/ with your Web browser. You'll see a "Welcome to Django" page, in pleasant, light-blue pastel. It worked!

Fixing security settings
Right now, everyone in the workshop has the same SECRET_KEY. According to the Django documentation, that is bad. So open up settings.py in your editor (for example, Komodo Edit).

settings.py is a Python script that only contains variable definitions. (Django looks at the values of these variables when it runs your web app.)

Find the variable named SECRET_KEY and set it to whatever string you want. Go on, we'll wait.

Database setup
Keep looking at settings.py: it has a dictionary with one key: default.

The value is itself another dictionary with information about the site's default database. You can see from the NAME that the Django project uses a file called database.db to store information.

Pop quiz: Does database.db exist right now?

While you're editing settings.py, take note of the INSTALLED_APPS setting towards the bottom of the file. That variable holds the names of all Django applications that are activated in this Django instance. Apps can be used in multiple projects, and you can package and distribute them for use by others in their projects.

By default, INSTALLED_APPS contains the following apps, all of which come with Django:


 * django.contrib.auth -- An authentication system.
 * django.contrib.contenttypes -- A framework for content types.
 * django.contrib.sessions -- A session framework.
 * django.contrib.sites -- A framework for managing multiple sites with one Django installation.
 * django.contrib.messages -- A messaging framework.

These applications are included by default as a convenience.

Each of these applications makes use of at least one database table, so we need to create the tables in the database before we can use them. To do that, run the following command:

python manage.py syncdb

The syncdb command looks at the INSTALLED_APPS setting and creates any necessary database tables according to the database settings in your settings.py file. You'll see a message for each database table it creates, and you'll get a prompt asking you if you'd like to create a superuser account for the authentication system. Go ahead and do that.

Creating models
Now that your environment -- a "project" -- is set up, you're set to start building the poll.

Each application you write in Django consists of a Python package, somewhere on your Python path, that follows a certain convention. Django comes with a utility that automatically generates the basic directory structure of an app, so you can focus on writing code rather than creating directories.

Projects vs. apps
We've talked a little about Django apps and projects. You might be wondering what the difference is.

Here are the things to know:


 * A project contains one more apps.
 * An app is component of a website that does something. For example, the Django administration app is something you'll see later in this tutorial
 * A project corresponds to a website: it contains a settings.py file, so it has a corresponding database.

Django apps can live anywhere on the "Python path." That just means that you have to be able to import them when your Django project runs.

In this tutorial, we'll create our poll app in the workshop_mysite directory for simplicity. In the future, when you decide that the world needs to be able to use your poll app and plug it into their own projects, you can publish that directory separately.

To create your app, make sure you're in the workshop_mysite directory and type this command:

python manage.py startapp polls

That'll create a directory polls, which is laid out like this:

polls/ __init__.py   models.py    tests.py    views.py

This directory structure will house the poll application.

The first step in writing a database Web app in Django is to define your models -- essentially, your database layout, with additional metadata.

Philosophy
A model is the single, definitive source of data about your data. It contains the essential fields and behaviors of the data you're storing. Django follows the DRY ("Don't Repeat Yourself") Principle. The goal is to define your data model in one place and automatically derive things from it.

In our simple poll app, we'll create two models: polls and choices. A poll has a question and a publication date. A choice has two fields: the text of the choice and a vote tally. Each choice is associated with a poll. (FIXME: Add image to Choice.)

These concepts are represented by simple Python classes. Edit the polls/models.py file so it looks like this:

from django.db import models class Poll(models.Model): question = models.CharField(max_length=200) pub_date = models.DateTimeField('date published') class Choice(models.Model): poll = models.ForeignKey(Poll) choice = models.CharField(max_length=200) votes = models.IntegerField

All models in Django code are represented by a class that subclasses django.db.models.Model. Each model has a number of class variables, each of which represents a database field in the model.

Each field is represented by an instance of a Field class -- e.g., CharField for character fields and DateTimeField for datetimes. This tells Django what type of data each field holds.

The name of each Field instance (e.g. question or pub_date ) is the field's name, in machine-friendly format. You'll use this value in your Python code, and your database will use it as the column name.

You can use an optional first positional argument to a Field to designate a human-readable name. That's used in a couple of introspective parts of Django, and it doubles as documentation. If this field isn't provided, Django will use the machine-readable name. In this example, we've only defined a human-readable name for Poll.pub_date. For all other fields in this model, the field's machine-readable name will suffice as its human-readable name.

Some Field classes have required elements. CharField, for example, requires that you give it a max_length. That's used not only in the database schema, but in validation, as we'll soon see.

Finally, note a relationship is defined, using ForeignKey. That tells Django each Choice is related to a single Poll. Django supports all the common database relationships: many-to-ones, many-to-manys and one-to-ones. Activating models

That small bit of model code gives Django a lot of information. With it, Django is able to:

Create a database schema (CREATE TABLE statements) for this app. Create a Python database-access API for accessing Poll and Choice objects.

But first we need to tell our project that the polls app is installed.

Philosophy

Django apps are "pluggable": You can use an app in multiple projects, and you can distribute apps, because they don't have to be tied to a given Django installation.

Edit the settings.py file again, and change the INSTALLED_APPS setting to include the string 'polls'. So it'll look like this:

INSTALLED_APPS = (   'django.contrib.auth',    'django.contrib.contenttypes',    'django.contrib.sessions',    'django.contrib.sites',    'polls' )

Now Django knows to include the polls app. Let's run another command:

python manage.py sql polls

You should see something similar to the following (the CREATE TABLE SQL statements for the polls app):

BEGIN; CREATE TABLE "polls_poll" (   "id" serial NOT NULL PRIMARY KEY,    "question" varchar(200) NOT NULL,    "pub_date" timestamp with time zone NOT NULL ); CREATE TABLE "polls_choice" (   "id" serial NOT NULL PRIMARY KEY,    "poll_id" integer NOT NULL REFERENCES "polls_poll" ("id"),    "choice" varchar(200) NOT NULL,    "votes" integer NOT NULL ); COMMIT;

Note the following:

The exact output will vary depending on the database you are using. Table names are automatically generated by combining the name of the app (polls) and the lowercase name of the model -- poll and choice. (You can override this behavior.) Primary keys (IDs) are added automatically. (You can override this, too.) By convention, Django appends "_id" to the foreign key field name. Yes, you can override this, as well. The foreign key relationship is made explicit by a REFERENCES statement. It's tailored to the database you're using, so database-specific field types such as auto_increment (MySQL), serial (PostgreSQL), or integer primary key (SQLite) are handled for you automatically. Same goes for quoting of field names -- e.g., using double quotes or single quotes. The author of this tutorial runs PostgreSQL, so the example output is in PostgreSQL syntax. The sql command doesn't actually run the SQL in your database - it just prints it to the screen so that you can see what SQL Django thinks is required. If you wanted to, you could copy and paste this SQL into your database prompt. However, as we will see shortly, Django provides an easier way of committing the SQL to the database.

If you're interested, also run the following commands:

python manage.py validate -- Checks for any errors in the construction of your models. python manage.py sqlcustom polls -- Outputs any custom SQL statements (such as table modifications or constraints) that are defined for the application. python manage.py sqlclear polls -- Outputs the necessary DROP TABLE statements for this app, according to which tables already exist in your database (if any). python manage.py sqlindexes polls -- Outputs the CREATE INDEX statements for this app. python manage.py sqlall polls -- A combination of all the SQL from the sql, sqlcustom, and sqlindexes commands.

Looking at the output of those commands can help you understand what's actually happening under the hood.

Now, run syncdb again to create those model tables in your database:

python manage.py syncdb

The syncdb command runs the sql from 'sqlall' on your database for all apps in INSTALLED_APPS that don't already exist in your database. This creates all the tables, initial data and indexes for any apps you have added to your project since the last time you ran syncdb. syncdb can be called as often as you like, and it will only ever create the tables that don't exist.

Read the django-admin.py documentation for full information on what the manage.py utility can do. Playing with the API

Now, let's hop into the interactive Python shell and play around with the free API Django gives you. To invoke the Python shell, use this command:

python manage.py shell

We're using this instead of simply typing "python", because manage.py sets up the project's environment for you. "Setting up the environment" involves two things:

Putting polls on sys.path. For flexibility, several pieces of Django refer to projects in Python dotted-path notation (e.g. 'polls.models'). In order for this to work, the polls package has to be on sys.path.

We've already seen one example of this: the INSTALLED_APPS setting is a list of packages in dotted-path notation.

Setting the DJANGO_SETTINGS_MODULE environment variable, which gives Django the path to your settings.py file.

Bypassing manage.py

If you'd rather not use manage.py, no problem. Just make sure mysite and polls are at the root level on the Python path (i.e., import mysite and import polls work) and set the DJANGO_SETTINGS_MODULE environment variable to mysite.settings.

For more information on all of this, see the django-admin.py documentation.

Once you're in the shell, explore the database API:

>>> from polls.models import Poll, Choice # Import the model classes we just wrote.

>>> Poll.objects.all []
 * 1) No polls are in the system yet.

>>> import datetime >>> p = Poll(question="What's up?", pub_date=datetime.datetime.now)
 * 1) Create a new Poll.

>>> p.save
 * 1) Save the object into the database. You have to call save explicitly.

>>> p.id 1
 * 1) Now it has an ID. Note that this might say "1L" instead of "1", depending
 * 2) on which database you're using. That's no biggie; it just means your
 * 3) database backend prefers to return integers as Python long integer
 * 4) objects.

>>> p.question "What's up?" >>> p.pub_date datetime.datetime(2007, 7, 15, 12, 00, 53)
 * 1) Access database columns via Python attributes.

>>> p.pub_date = datetime.datetime(2007, 4, 1, 0, 0) >>> p.save
 * 1) Change values by changing the attributes, then calling save.

>>> Poll.objects.all []
 * 1) objects.all displays all the polls in the database.

Wait a minute.  is, utterly, an unhelpful representation of this object. Let's fix that by editing the polls model (in the polls/models.py file) and adding a __unicode__ method to both Poll and Choice:

class Poll(models.Model): # ...   def __unicode__(self): return self.question

class Choice(models.Model): # ...   def __unicode__(self): return self.choice

It's important to add __unicode__ methods to your models, not only for your own sanity when dealing with the interactive prompt, but also because objects' representations are used throughout Django's automatically-generated admin.

Why __unicode__ and not __str__?

If you're familiar with Python, you might be in the habit of adding __str__ methods to your classes, not __unicode__ methods. We use __unicode__ here because Django models deal with Unicode by default. All data stored in your database is converted to Unicode when it's returned.

Django models have a default __str__ method that calls __unicode__ and converts the result to a UTF-8 bytestring. This means that unicode(p) will return a Unicode string, and str(p) will return a normal string, with characters encoded as UTF-8.

If all of this is jibberish to you, just remember to add __unicode__ methods to your models. With any luck, things should Just Work for you.

Note these are normal Python methods. Let's add a custom method, just for demonstration:

import datetime class Poll(models.Model): # ...   def was_published_today(self): return self.pub_date.date == datetime.date.today

Note the addition of import datetime to reference Python's standard datetime module.

Save these changes and start a new Python interactive shell by running python manage.py shell again:

>>> from polls.models import Poll, Choice

>>> Poll.objects.all []
 * 1) Make sure our __unicode__ addition worked.

>>> Poll.objects.filter(id=1) [] >>> Poll.objects.filter(question__startswith='What') []
 * 1) Django provides a rich database lookup API that's entirely driven by
 * 2) keyword arguments.

>>> Poll.objects.get(pub_date__year=2007) 
 * 1) Get the poll whose year is 2007.

>>> Poll.objects.get(id=2) Traceback (most recent call last): ... DoesNotExist: Poll matching query does not exist.

>>> Poll.objects.get(pk=1) 
 * 1) Lookup by a primary key is the most common case, so Django provides a
 * 2) shortcut for primary-key exact lookups.
 * 3) The following is identical to Poll.objects.get(id=1).

>>> p = Poll.objects.get(pk=1) >>> p.was_published_today False
 * 1) Make sure our custom method worked.

>>> p = Poll.objects.get(pk=1)
 * 1) Give the Poll a couple of Choices. The create call constructs a new
 * 2) choice object, does the INSERT statement, adds the choice to the set
 * 3) of available choices and returns the new Choice object. Django creates
 * 4) a set to hold the "other side" of a ForeignKey relation
 * 5) (e.g. a poll's choices) which can be accessed via the API.

>>> p.choice_set.all []
 * 1) Display any choices from the related object set -- none so far.

>>> p.choice_set.create(choice='Not much', votes=0)  >>> p.choice_set.create(choice='The sky', votes=0)  >>> c = p.choice_set.create(choice='Just hacking again', votes=0)
 * 1) Create three choices.

>>> c.poll 
 * 1) Choice objects have API access to their related Poll objects.

>>> p.choice_set.all [, , <Choice: Just hacking again>] >>> p.choice_set.count 3
 * 1) And vice versa: Poll objects get access to Choice objects.

>>> Choice.objects.filter(poll__pub_date__year=2007) [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
 * 1) The API automatically follows relationships as far as you need.
 * 2) Use double underscores to separate relationships.
 * 3) This works as many levels deep as you want; there's no limit.
 * 4) Find all Choices for any poll whose pub_date is in 2007.

>>> c = p.choice_set.filter(choice__startswith='Just hacking') >>> c.delete
 * 1) Let's delete one of the choices. Use delete for that.

For more information on model relations, see Accessing related objects. For more on how to use double underscores to perform field lookups via the API, see Field lookups. For full details on the database API, see our Database API reference.

When you're comfortable with the API, read part 2 of this tutorial to get Django's automatic admin working.