Boston Python Workshop/Saturday/Web app project

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[edit] 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.

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.

This page should say what you should actually expect to know. It is okay that you don't understand everything you are typing in. After a lot more learning, you will be able to. The first time, though, it's okay if you don't. Will and Katie have feedback for this page.

[edit] Writing your first Django app, part 1

Let’s learn by example.

This tutorial walks 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.

[edit] 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

[edit] Look at the files

Let’s look at files in the project:

workshop_mysite/
   public/
   README.mediawiki
   __init__.py
   manage.py
   settings.py
   urls.py

These files are:

  • README.mediawiki: Many projects come with README files that, well, you should read. This one does, too.
  • public/: This directory contains files the instructors put together so you can easily deploy your web app to Alwaysdata.com.
  • __init__.py: An empty file that tells Python that this directory should be considered a Python module. Because of the __init__.py file, you can use import to import workshop_mysite.
  • 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.

[edit] The development server

Let's verify this worked. Run the command:

python manage.py runserver

You'll see the following output on the command line:

Validating models...
0 errors found.

Django version 1.2, 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!

Exit the server by pressing CONTROL-C on your keyboard.

[edit] 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.

[edit] Database setup

Keep looking at settings.py: The DATABASES variable is 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.

[edit] Part 1.5: Creating polls

[edit] 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.

[edit] 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:

  • 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.

[edit] Django 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.

(If you've used SQL before, you might be interested to know that each Django model corresponds to a SQL table.)

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 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()

class Choice(models.Model):
    poll = models.ForeignKey(Poll)
    choice = models.CharField(max_length=200)
    votes = models.IntegerField()

Save the models.py file.

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.

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.

[edit] 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.

[edit] Django 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',
)

Save the settings.py file.

Now Django knows to include the polls app.

If you care about SQL, you can try the following command:

  • python manage.py sql polls

For now, let's just Django's syncdb tool to create the database tables for Poll objects:

python manage.py syncdb

The syncdb looks for apps that have not yet been set up. To set them up, it runs the necessary SQL commands against 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.

[edit] 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:

  1. Making sure polls is on the right path to be imported.
  2. Setting the DJANGO_SETTINGS_MODULE environment variable, which gives Django the path to your settings.py file.

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

Let's import the model classes we just wrote:

>>> from polls.models import Poll, Choice

To list all the current Polls:

>>> Poll.objects.all()
[]

It is an empty list because there are no polls. Let's add one!

>>> import datetime
>>> p = Poll(question="What's up?", pub_date=datetime.datetime.now())

Then we'll save the object into the database. You have to call save() explicitly.

>>> p.save()

Great. Now, because it's been saved, it has an ID in the database. You can see that by typing this into the Python shell:

>>> p.id
1

You can also access the database columns (Fields, in Django parlance) as Python attributes:

>>> p.question
"What's up?"
>>> p.pub_date
datetime.datetime(2007, 7, 15, 12, 00, 53)

We can time travel back in time! Or at least, we can send the Poll back in time:

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

Finally, we can also ask Django to show a list of all the Poll objects available:

>>> Poll.objects.all()
[<Poll: Poll object>]

Wait a minute. <Poll: Poll object> is an utterly unhelpful representation of this object. Let's fix that by editing the polls model Use your text editor to open 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.

(If you're using to Python programming from a time in the past, you might have seen __str__(). Django prefers you use __unicode__() instead.)

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. FIXME: add explanation of why we did this

Save these changes to the models.py file, and then start a new Python interactive shell by running python manage.py shell again:

>>> from polls.models import Poll, Choice

Check it out: our __unicode__() addition worked:

>>> Poll.objects.all()
[<Poll: What's up?>]

If you want to search your database, you can do it using the filter method on the objects attribute of Poll. For example:

>>> polls = Poll.objects.filter(question="What's up?")
>>> polls
[<Poll: What's up?>]
>>> polls[0].id
1

If you try to search for a poll that does not exist, filter will give you the empty list. The get method will always return one hit, or raise an exception.

>>> Poll.objects.filter(question="What time is it?")
[]
>>> Poll.objects.get(id=1)
<Poll: What's up?>
>>> Poll.objects.get(id=2)
Traceback (most recent call last):
    ...
DoesNotExist: Poll matching query does not exist.

[edit] Adding choices

Right now, we have a Poll in the database, but it has no Choices. See:

>>> p = Poll.objects.get(id=1)
>>> p.choice_set.all()
[]

So let's create three choices:

>>> p.choice_set.create(choice='Not much', votes=0)
<Choice: Not much>
>>> p.choice_set.create(choice='The sky', votes=0)
<Choice: The sky>
>>> c = p.choice_set.create(choice='Just hacking again', votes=0)
>>> c
<Choice: Just hacking again>

Every Choice can find the Poll that it belongs to:

>>> c.poll
<Poll: What's up?>

We just used this, but now I'll explain it: Because a Poll can have more than one Choice, Django creates the choice_set attribute on each Poll. You can use that to look at the list of available Choices, or to create them.

>>> p.choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
>>> p.choice_set.count()
3

[edit] Visualize the database in SQLite Manager

When you call .save() on a model instance, Django saves that to the database. (Remember, Django is a web programming framework built around the idea of saving data in a SQL database.)

Where is that database? Take a look at settings.py in your text editor. You can see that database.db is the filename. In settings.py Python calculates the path to the current file.

So now:

  • Open up Firefox
  • Find SQLite Manager in Tools->SQLite Manager
  • In the SQLite Manager menus, choose: Database->Connect Database
  • Find the workshop_mysite/database.db file.

Browse your tables! This is another way of looking at the data you just created.

Note: In order to find the database.db file, you might need to ask SQLite Manager to show you all files, not just the *.sqlite files.

I (the author of this tutorial) think it's really important that you be able to find this database file. So go ahead and do this step. Browse around! Hooray.

When you're satisfied with your Poll data, you can close it.

[edit] Save and share our work

We've done something! Let's share it with the world.

We'll do that with git and Github. On your own computer, get to a Terminal or a GitBash.

Use cd to get into the workshop_mysite directory. If it's a fresh Terminal, this is what you'll do:

cd Desktop
cd django_projects
cd workshop_mysite

Use git add to add the content of your files to git:

git add polls/*.py

And use git commit to commit those files:

git commit -m "I made these files and this is a message describing them"

Finally, use git push to push those up to your Github repository:

git push

Go to your Github account. Find the workshop_mysite repository. Do you see your files?

If so, proceed!

[edit] Enough databases for now

In the next section of the tutorial, you'll write views that let other people look at your polls.

[edit] Part 2: Letting the world see your polls, with views

We have all these polls in our database. However, no one can see them, because we never made any web pages that render the polls into HTML.

Let's change that with Django views.

[edit] Philosophy

A view is a “type” of Web page in your Django application that generally serves a specific function and has a specific template. For example, in a Weblog application, you might have the following views:

  • Blog homepage – displays the latest few entries.
  • Entry “detail” page – permalink page for a single entry.
  • Year-based archive page – displays all months with entries in the given year.
  • Month-based archive page – displays all days with entries in the given month.
  • Day-based archive page – displays all entries in the given day.
  • Comment action – handles posting comments to a given entry.

In our poll application, we’ll have the following four views:

  • Poll “index” page – displays the latest few polls.
  • Poll “detail” page – displays a poll question, with no results but with a form to vote.
  • Poll “results” page – displays results for a particular poll.
  • Vote action – handles voting for a particular choice in a particular poll.

In Django, each view is represented by a simple Python function.

[edit] Design your URLs

The first step of writing views is to design your URL structure. You do this by creating a Python module, called a URLconf. URLconfs are how Django associates a given URL with given Python code.

When a user requests a Django-powered page, the system looks at the ROOT_URLCONF setting, which contains a string in Python dotted syntax. Django loads that module and looks for a module-level variable called urlpatterns, which is a sequence of tuples in the following format:

(regular expression, Python callback function [, optional dictionary])

Django starts at the first regular expression and makes its way down the list, comparing the requested URL against each regular expression until it finds one that matches.

You might ask, "What's a regular expression?" Regular expressions are patterns for matching text. In this case, we're matching the URLs people go to, and using regular expressions to categorize them into different kinds of

(If (like me) you think regular expressions are intriguing and thrilling, you can read the Dive into Python guide to regular expressions sometime. Or you can look at this comic.)

In addition to matching text, regular expressions can capture text: regexps use parentheses to wrap the parts they're capturing.

For Django, when a regular expression matches the URL that a web surfer requests, Django extracts the captured values and passes them to a function of your choosing. This is the role of the callback function above.

[edit] Adding URLs to urls.py

When we ran django-admin.py startproject workshop_mysite to create the project, Django created a default URLconf. Take a look at settings.py for this line:

ROOT_URLCONF = 'workshop_mysite.urls'

That means that the default URLconf is workshop_mysite/urls.py.

Time for an example. Edit the file workshop_mysite/urls.py so it looks like this:

 from django.conf.urls.defaults import *

 urlpatterns = patterns('',
     (r'^polls/$', 'polls.views.index'),
     (r'^polls/(\d+)/$', 'polls.views.detail'),
     (r'^polls/(\d+)/results/$', 'polls.views.results'),
     (r'^polls/(\d+)/vote/$', 'polls.views.vote'),
 )

This is worth a review. When somebody requests a page from your Web site -- say, "/polls/23/", Django will load the urls.py Python module, because it's pointed to by the ROOT_URLCONF setting. It finds the variable named urlpatterns and traverses the regular expressions in order. When it finds a regular expression that matches -- r'^polls/(\d+)/$' -- it loads the function detail() from polls/views.py. Finally, it calls that detail() function like so:

detail(request=<HttpRequest object>, '23')

The '23' part comes from (\d+). Using parentheses around a pattern "captures" the text matched by that pattern and sends it as an argument to the view function; the \d+ is a regular expression to match a sequence of digits (i.e., a number).

(In Django, you have total control over the way your URLs look. People on the web won't see cruft like .py or .php at the end of your URLs.)

[edit] Finally: Write your first view

Well, we haven't created any views yet -- we just have the URLconf. But let's make sure Django is following the URLconf properly.

Fire up the Django development Web server:

python manage.py runserver

Now go to "http://localhost:8000/polls/" in your Web browser. You should get a pleasantly-colored error page with the following message:

ViewDoesNotExist at /polls/
Tried index in module polls.views. Error was: 'module'
object has no attribute 'index'
This error happened because you haven't written a function index() in the module polls/views.py.

Try "/polls/23/", "/polls/23/results/" and "/polls/23/vote/". The error messages tell you which view Django tried (and failed to find, because you haven't written any views yet).

Time to write the first view. Open the file polls/views.py and put the following Python code in it:

from django.http import HttpResponse

def index(request):
    return HttpResponse("Hello, world. You're at the poll index.")

This is the simplest view possible. Save the views.py file, then go to "/polls/" in your browse