Difference between revisions of "Django for Designers"

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<!-- Instructor note: Note that your 'git status' really ought to be empty. Is it? If not, fix the instructions by adding a git commit etc. in the above. Then, git push origin HEAD:pre-part-5 -->
 
<!-- Instructor note: Note that your 'git status' really ought to be empty. Is it? If not, fix the instructions by adding a git commit etc. in the above. Then, git push origin HEAD:pre-part-5 -->
 
=== Part 6: Sharing with others ===
 
 
Now that you've built a web app, you might want to enable other people to see it.
 
 
==== With runserver ====
 
 
The first, easiest way to let other people see it is to let other people connect to your computer. (Note that this might not work if your computer has a firewall enabled.)
 
 
You've already been using the Django ''runserver'' command, but you have been the only one able to connect to it. We'll change that with two steps.
 
 
First, we'll ask runserver to listen to requests from the whole world, which is not the default. Stop your current runserver with ctrl-c, and then run this command:
 
 
<source lang="bash">
 
# in django-for-designers/myproject
 
$ python manage.py runserver 0.0.0.0:8000</source>
 
 
Second, we need to find out what IP ''address'' your computer is using on the wifi network. To find that out, visit a temporary website on the instructor's computer. Visit this link:
 
 
http://192.168.1.1:8100/
 
 
You will see a number like ''192.168.3.11'' within the "Your IP" section of the page. That is your computer's IP address, and you can ask a neighbor to visit your web app by going to your IP address in their browser. For example, if your IP is 192.168.3.11, they should visit:
 
 
http://192.168.3.11:8000/
 
 
(If you are wondering exactly who can connect to your app, it is typically only people on the same wifi/wired network as you, rather than the whole Internet. This is due to the a technique in wide use called "network address translation", and it is not considered perfect security.)
 
 
When people visit your app, they are interacting with the database stored on your laptop.
 
 
(Side note: The instructor is using [https://github.com/paulproteus/address-shower this app] to run the service where you can see your IP address.)
 
 
==== Deployment on Heroku ====
 
 
This method of running the code requires that you use your laptop like a server, and keep it running all the time. This isn't how you want to generally share your work with the world.
 
 
A more common way to ''deploy'' Django code is to copy your code to a hosting service and ask them to run it for you. In this tutorial, we'll do that with Heroku.
 
 
Note that when you access a remotely-hosted version of your code, it will typically use a separate database. Any information you store there won't be reflected in the code you run on your own computer.
 
 
Make sure you have a Heroku account. Visit https://api.heroku.com/signup and either sign up or login.
 
 
Make sure you have the Heroku ''toolbelt'' installed on your own computer. You can test if you have them by running, within a terminal or command prompt:
 
 
<source lang="bash">
 
# in django-for-designers/myproject
 
$ heroku</source>
 
 
If you get a command not found error, visit https://toolbelt.heroku.com/ and install their toolbelt. You may need to quit and re-launch your terminal program for the program to become available.
 
 
Once you have the heroku tools, make sure you are logged in to Heroku with the following command:
 
 
<source lang="bash">
 
# in django-for-designers/myproject
 
$ heroku login</source>
 
 
To prepare the app to ''run'' on Heroku, we need to make a few changes. First, we must instruct Heroku on how to launch your app. Second, we must make your app work with their Postgres database, rather than sqlite like you are using now.
 
 
Heroku reads a file called ''Procfile'' that tells them how to run your app's ''runserver'' command. So, create a file called Procfile with the following contents:
 
 
<source lang="bash">
 
web: python manage.py runserver 0.0.0.0:$PORT --noreload
 
</source>
 
 
Make sure to put it at the ''top'' of the project directory, namely in ''django-for-designers'', not ''django-for-designers/myproject''.
 
 
To configure the database, we will employ a trick at the bottom of the settings file: if we detect we are running on Heroku, we change the configuration to the one that Heroku provides.
 
 
To do that, you need to open ''settings.py'', go to the ''end'', and add the following lines:
 
 
<source lang="python">
 
import os
 
if 'DATABASE_URL' in os.environ:
 
import dj_database_url
 
DATABASES['default'] = dj_database_url.config()
 
</source>
 
 
Since Heroku indicates the configuration through an environment variable called DATABASE_URL, this configuration block will only apply on Heroku. Therefore, it is reasonable to commit this. So we'll do that:
 
 
<source lang="bash">
 
# in django-for-designers
 
$ git commit -a -m "Adding Heroku database config"</source>
 
 
Now you'll need to actually deploy it! To do that, make sure you are logged into Heroku on your computer by running:
 
 
<source lang="bash">
 
# in django-for-designers
 
$ heroku login
 
</source>
 
 
Then create a Heroku URL for your app:
 
 
<source lang="bash">
 
# in django-for-designers
 
$ heroku create
 
</source>
 
 
Take the step of copying your code to Heroku's servers via git:
 
 
<source lang="bash">
 
# in django-for-designers
 
$ git push heroku HEAD:master
 
</source>
 
 
Now Heroku has a copy of your code, and is configured to run it when people visit the URL you saw when you ran ''heroku create''. But there is one more thing to do, which is to configure the database with the ''syncdb'' command.
 
 
<source lang="bash">
 
# in django-for-designers
 
$ heroku run 'cd myproject ; python manage.py syncdb --migrate'
 
</source>
 
 
The heroku shell will prompt you for questions that will seem earlier familiar.
 
 
After all that -- you are done! Congratulations, and enjoy your new live web app!
 
 
==== Brief discussion of other deployment options ====
 
 
Heroku is not the only way to deploy Python and Django apps to the web. There are a few other ways:
 
 
* If you rent "VPS" or dedicated server space, or connect a machine to the Internet and refuse to turn it off, you can install serve your app out through Apache and mod_wsgi, or any number of ways.
 
* If you want other hosting similar to Heroku, we recommend taking a look at OpenShift or CloudFoundry, or any number of other 'Platform as a service' systems.
 
* If you want something in the middle, you can use a service like WebFaction.
 
 
==== Sharing on Github ====
 
 
So far, people have been able to interact with your app, but they haven't been able to view its source code. In this section, you will share your app's code publicly on the web.
 
 
You'll also be able to browse your app's code yourself on Github!
 
 
(One important note: if the security of your app mattered, you would need to hide the ''SECRET_KEY'' variable from onlookers. For now, we will carry on without worry about that.)
 
 
You've been using git all along, but with Github, you can publish your work too.
 
 
First things first, you will need an account on Github. Visit https://github.com/ and create an account, or sign in if you already have one.
 
 
Then, click the button to ''create a new repo''. It is in the top-right, and should take you to https://github.com/new .
 
 
For consistency, make the repository name ''django-for-designers''. Be sure ''not'' to check the box asking to initialize the repo. Then click ''Create repository''. Wait for your new empty Github project page to load in your web browser.
 
 
Once you have done that, you will need to reconfigure the git repository on your own computer so it can easily copy data to Github. To do that, we will inform it about a new ''remote'' repository. For this to work, you have to visit your Github project page and find the "HTTP" link specific to django-for-designers within your Github account. Copy that to the clipboard, and then type:
 
 
<source lang="bash">
 
# in django-for-designers
 
$ git remote add mygithub __PASTE_HERE__
 
</source>
 
 
Once you have added it, you can push there:
 
 
<source lang="bash">
 
# in django-for-designers
 
$ git push mygithub --all
 
</source>
 
 
Now just reload that Github page, and you should see the files you have been working on! You'll also see that since we passed ''--all'' to git, all the branches are available on Github.
 
   
 
=== Part 7: Exercises for the reader ===
 
=== Part 7: Exercises for the reader ===

Revision as of 04:39, 12 March 2013

Introduction

In this tutorial, we will explain topics and provide commands for you to run on your own computer. You will leave with a working social bookmarking web app!

This is a tutorial on web programming, so we will go beyond just Django and discuss third-party Django apps and other real-world web development tools. We'll also be emphasizing areas of Django that particularly affect designers, such as static files, template inheritance, and AJAX.

Things you should know already

  • HTML familiarity
  • Basic Python proficiency
  • Basic or better Javascript proficiency
  • Pre-requisites that we will help with

We expect you to have git, Python, and a few other elements ready on your laptop before the tutorial. We have published a laptop setup guide that steps you through:

  • Installing Python, git, pip, virtualenv, and a reasonable text editor
  • Setting up your env with Django, South, and django-debug-toolbar
  • Basic command line knowledge (cd, ls, etc)
  • Basic git knowledge
  • Setting up your git repo for the tutorial

Things you do not need to know already:

  • Django :)
  • What an ORM is
  • Anything database related

Curriculum

Part 3: Models, our database, and making it visible

Remember to make a new branch for section 3 based off the official branch!

# in django-for-designers/myproject
$ git branch my-branch-3 origin/pre-part-3
$ git checkout my-branch-3

Introduction to databases and the ORM, or: finally, something we couldn't've done with plain HTML/CSS/JS!

Okay, being able to design your URLs however you want, without them having to correspond to your actual file structure, is pretty neat. But besides that, we haven't done anything yet that you couldn't do with just HTML, CSS, and JavaScript. Time to fix that.

One key thing that separates web apps from web pages is that apps typically store data somewhere. The code that runs the app chooses what data seems most important to show at the moment. When writing an app using Django, we configure the storage through Django models.

Every Django app comes with a models.py in which you list each kind of data you want to store. These are configured through Python classes (to name the kind of data) with a sequence of attributes (which control the pieces of data that make up the model). If you're familiar with, or interested in, SQL, these correspond to tables and columns respectively.

The models that you define here are built on top of Django's object-relational mapper, or ORM. Because you define your models as Python objects, you can effectively write queries against a relational database by instead coding in Python with reference to your objects. Django then maps your Python code into relational database queries in SQL.

By configuring your data access in Python, Django makes it easy to take advantage of your data layout in other places. For example, you'll see later how the ORM makes it easy to automatically generate forms that ask for exactly the information you need. It is also generally convenient to write your entire app in one language, rather than being required to switch to SQL to do queries. The ORM also handles "escaping," which makes your SQL queries handle Unicode and other strange characters properly. These conveniences let you avoid mojibake-style data corruption and SQL injection attacks effortlessly.

TODO (models.py --> ORM --> DB chart)

Creating a basic model

Let's make it so we can store some real live data in our application! In our simple bookmarking app, we’ll create two models: Bookmarks and Tags. What information do we need to store about these objects?

A bookmark has:

  • A URL
  • A title (optionally)
  • A timestamp for when the bookmark was made

A tag has:

  • The tag slug/name

A tag also needs to know which bookmarks it applies to.

How do we represent this information in a Django model?

Open up bookmarks/models.py in your editor.

from django.db import models


class Bookmark(models.Model):
    url = models.URLField()
    timestamp = models.DateTimeField(auto_now_add=True)
    title = models.CharField(max_length=200, blank=True, default="")

What's going on here?

  • We've made a Bookmark class that inherits from Django's model class.
  • We gave it a url, which is a URLField (a field that expects a string that parses as a valid URL).
  • We gave it a timestamp, which is a DateTimeField (a field that expects a Python datetime object). auto_now_add means that the field will automatically set itself to the time when the model is created.
  • Finally, we gave it a title, which is a CharField (a field that expects a string of some length or less, in this case less than 200 characters). "blank=True" means that Django will permit this field to be blank. We also set a default of the empty string, so that if someone fails to specify a title we permit that.

Now let's add a tag model! In the same file, below the Bookmark model:

class Tag(models.Model):
    bookmark = models.ManyToManyField(Bookmark)
    slug = models.CharField(max_length=50, unique=True)

The tag slug is stored in another CharField, which we've seen before. But what's this ManyToManyField? Well, it's one type of field for denoting a relationship between two models.

In relational databases, there's two basic types of relationships. In a ForeignKey relationship, one model relates to one and only one other model. You could imagine this like a Car model and a Wheel model. Each Wheel instance belongs to one and only one Car, so it would have a ForeignKey field for its car. The car, of course, can have multiple wheels.

With a ManyToManyField, on the other hand, the relationship isn't exclusive for either of the models involved. For instance, you could imagine having a Pizza model and a Topping model. Each Pizza can have multiple Toppings, and each Topping can be on multiple Pizzas.

(There are other variants of these fields, but this is the basic concept. See https://docs.djangoproject.com/en/dev/ref/models/fields/#module-django.db.models.fields.related for more details for how these relationship fields work in Django.)

In this case, each Bookmark can have multiple Tags, and each Tag can apply to multiple Bookmarks. So we use a ManyToManyField to store that information.

Creating database tables for your app (carefully, with South)

When you first create your models, you might not always know exactly what fields your models will need in advance. Maybe someday your bookmarks app will have preview thumbnail images! Then you would want to add another field to the model to store that information. Maybe someday you'll decide that tracking all the timestamps is silly, and want to delete that field.

Unfortunately, Django (and most database-using software) can’t figure out how to handle model changes very well on its own. Fortunately, a Django app called South that we installed earlier can handle these database changes--called ‘migrations’--for us.

Now that we’ve made our first version of our models file, let’s set up our bookmarks app to work with South so that we can make migrations with it in the future!

On the command line, write:

# in django-for-designers/myproject
$ python manage.py schemamigration bookmarks --initial

As you can see, that’s created a migrations directory for us, and automatically made a new migration file inside it.

# in django-for-designers/myproject
$ ls bookmarks/migrations/
0001_initial.py  __init__.py

All we need to do now is apply our new migration:

# in django-for-designers/myproject
$ python manage.py migrate bookmarks

Great! Now our database file knows about bookmarks and its new models, and if we need to change our models, South is set up to handle those changes. We’ll come back to South later.

IMPORTANT: You can't migrate an app if it's already been synced in the database using

python manage.py syncdb

. But you do need to run syncdb at least once before you use south (since south itself uses syncdb to give itself space in your database). That's why it's super important that when you run syncdb, south should be listed under INSTALLED_APPS, but none of your own apps should be, and after you add your app to INSTALLED_APPS, you must not run syncdb again until after you've already set up migrations with that app.

Add and commit all your work, including the migrations folder that South generated for you!

Add some bookmarks via the command line

Now, let’s hop into the interactive Python shell and play around with the free API ("Application programming interface" -- APIs aren't just data doodads that hip web startups provide for you, they're an important concept in software architecture.) that Django gives you. To invoke the Python shell, use this command:

# in django-for-designers/myproject
$ python manage.py shell

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

  • Making sure bookmarks (and any other apps you might have) are on the right path to be imported.
  • Setting the DJANGO_SETTINGS_MODULE environment variable, which gives Django the path to your settings.py file.

Once you’re in the shell, let's explore the database API. Let's import the model classes we just wrote:

>>> from bookmarks.models import Bookmark, Tag

List all the current Bookmarks:

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

How many bookmarks is this? It's an empty list, so zero!

Let's add a bookmark:

>>> b = Bookmark(url="http://www.bringinthecats.com/", title="Funny link")

We could specify the value for timestamp as well here, but we don't have to since when we wrote its model definition, we specified that timestamp has a default value (of when the bookmark is created). We did specify a title for the bookmark, even though we didn't have to since it's an optional field.

Try getting the ID number of our new bookmark 'b' by typing:

>>> b.id

Python has given you back your prompt because the value of b.id is None. That is because we have not yet saved the object; to the database, it does not exist.

Save your bookmark to the database. In Django, you have to call save() explicitly.

>>> b.save()

Now, try again to get the id of the Bookmark instance. Because it’s been saved, it has an ID in the database now! Even though we didn't specify one in our models.py, every saved model instance automatically has an id field.

>>> b.id
1

Access the database columns (Fields, in Django parlance) as Python attributes:

>>> b.title
"Funny link"
>>> b.timestamp
datetime.datetime(2011, 12, 1, 3, 3, 55, 841929)

We can change the bookmark title by changing its title attribute, then calling save().

>>> b.title = "WHEEEE"
>>> b.save()
>>> b.title
"WHEEEE"

If we now ask Django to show a list of all the Bookmark objects available, we can see it's no longer an empty list!

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

Fix the hideous default model representation

Wait a minute! <Bookmark: Bookmark object> is an utterly unhelpful representation of this object. Let’s fix that by editing the Bookmark model. Use your text editor to open the bookmarks/models.py file and adding a __unicode__() method to both Bookmark and Tag:

class Bookmark(models.Model):
    url = models.URLField()
    timestamp = models.DateTimeField(auto_now_add=True)
    title = models.CharField(max_length=200, blank=True, default="")

    def __unicode__(self):
        return self.url


class Tag(models.Model):
    bookmark = models.ManyToManyField(Bookmark)
    slug = models.CharField(max_length=50, unique=True)

    def __unicode__(self):
        return self.slug

Let's see our new shiny __unicode__ methods in action. Start a new Python interactive shell by running:

# in django-for-designers/myproject
$ python manage.py shell
>>> from bookmarks.models import Bookmark, Tag
>>> Bookmark.objects.all()
[<Bookmark: http://www.bringinthecats.com/>]

Save and commit your changes.

Adding more data via the shell

>>> b = Bookmark(url="http://www.google.com")
>>> b.save()
>>> b = Bookmark(url="https://us.pycon.org/2013/", title="PyCon US website")
>>> b.save()
>>> b = Bookmark(url="https://www.djangoproject.com")
>>> b.save()

We've created a bunch of bookmarks, but no tags! Let's change that.

>>> b = Bookmark.objects.all()[0]
>>> b
<Bookmark: http://www.bringinthecats.com/>
>>> b.tag_set.create(slug="cats")
<Tag: cats>
>>> b.tag_set.create(slug="music")
<Tag: music>
>>> b.tag_set.create(slug="funny")
<Tag: funny>
>>> b.save()

Slicing and dicing Django data

There are many methods for searching across your Django models.

We can filter our bookmarks, for instance for ones with a particular URL:

>>> foo = Bookmark.objects.filter(url="http://www.bringinthecats.com/")
>>> foo
[<Bookmark: http://www.bringinthecats.com/>]

Or for bookmarks which have titles:

>>> titled_bookmarks = Bookmark.objects.exclude(title="")
>>> titled_bookmarks
[<Bookmark: http://www.bringinthecats.com/>, <Bookmark: https://us.pycon.org/2013/>]

If you try to use filter to search for a question that does not exist, filter will give you the empty list.

>>> Bookmark.objects.filter(title="Who framed Roger Rabbit?")
[]

The get method, on the other hand, returns exactly one hit. If it finds zero matches, or more than one match, it will raise an exception.

>>> Bookmark.objects.get(id=1)
<Bookmark: http://www.bringinthecats.com/>
>>> Bookmark.objects.get(id=4)
Traceback (most recent call last):
    ...
DoesNotExist: Bookmark matching query does not exist.

More information on making queries with Django's ORM can be found in the Django docs at https://docs.djangoproject.com/en/dev/topics/db/queries/#retrieving-objects.

Part 3.5: Changing our mind and adding users

D'oh! You know what every social bookmarking app has, that ours doesn't have? Users!

I don't mean like the number of people using it--I mean a way to store different users' accounts and keep track of who owns which bookmarks. So let's change our app to include this feature.

Lucky for us, Django comes with an app for user accounts and authentication from the get-go! In fact, it's already installed. If you look back at your settings.py file, you'll see that in INSTALLED_APPS, there is an entry for 'django.contrib.auth'.

INSTALLED_APPS = (
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.sites',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    # Uncomment the next line to enable the admin:
    # 'django.contrib.admin',
    # Uncomment the next line to enable admin documentation:
    # 'django.contrib.admindocs',
    'south',
    'bookmarks',
)

That's our authentication app!

Let's open the Django shell and play with this app a bit.

# in django-for-designers/myproject
$ python manage.py shell
>>> from django.contrib.auth.models import User
>>> User.objects.all()
[<User: karen>]

Whaaaaat?? There's already a User here. How can that be?

You might recall making a 'superuser' account when you first set up your Django project. That superuser was, in fact, created using Django's built-in auth app.

What is our user account's id number?

>>> me = User.objects.all()[0]
>>> me.id
1

Neato!

Add user field to bookmark

Now we need to create a relationship between the built-in User model and our Bookmark model.

First, in our models.py file, we need to import django.contrib.auth's built-in User model so that we can refer to it in our models.

from django.db import models
from django.contrib.auth.models import User

Now we need to think--what kind of relationship do users and bookmarks have?

Well, a user can have multiple bookmarks. But (right now, anyway) a bookmark should only have one user. So that means that we should use a ForeignKey field to add the user to our Bookmark model.

class Bookmark(models.Model):
    author = models.ForeignKey(User)
    title = models.CharField(max_length=200, blank=True, default="")
    url = models.URLField()
    timestamp = models.DateTimeField(auto_now_add=True)

    def __unicode__(self):
        return self.url

While we're at it, let's update the __unicode__ method too to let us know to whom a bookmark belongs to.

class Bookmark(models.Model):
    author = models.ForeignKey(User)
    title = models.CharField(max_length=200, blank=True, default="")
    url = models.URLField()
    timestamp = models.DateTimeField(auto_now_add=True)

    def __unicode__(self):
        return "%s by %s" % (self.url, self.author.username)

Make a migration in South

Now that we've added a field to our model, we are going to need to create a database migration. South can help us do this!

To create our migration, run

# in django-for-designers/myproject
$ python manage.py schemamigration bookmarks --auto

Note that we're now using --auto instead of --initial (which we used back when we first wrote our models and set up our app to use South).

Eep! Before it will make our migration file, South wants some information from us:

 ? The field 'Bookmark.author' does not have a default specified, yet is NOT NULL.
 ? Since you are adding this field, you MUST specify a default
 ? value to use for existing rows. Would you like to:
 ?  1. Quit now, and add a default to the field in models.py
 ?  2. Specify a one-off value to use for existing columns now
 ? Please select a choice: 2

We could in theory modify our models to specify a default value for author, or make it optional. But neither of those sound like good options--we *want* future bookmarks to be forced to have an author! So we'll choose 2, to set up a default value for our new author field just for the purposes of this migration.

 ? Please enter Python code for your one-off default value.
 ? The datetime module is available, so you can do e.g. datetime.date.today()
 >>>

We then need to come up with a default value. Well, right now there's only one User in our system who the sample bookmarks we'd entered so far could belong to--our superuser account, which (if you don't remember) had an ID number of 1.

So let's enter 1 for our default.

 ? Please enter Python code for your one-off default value.
 ? The datetime module is available, so you can do e.g. datetime.date.today()
 >>> 1
 + Added field author on bookmarks.Bookmark
Created 0002_auto__add_field_bookmark_author.py. You can now apply this migration with: ./manage.py migrate bookmarks

Remember, the first step creates the migration, but doesn't run it. So let's do what South says and run a command to apply our migration!

# in django-for-designers/myproject
$ python manage.py migrate bookmarks
Running migrations for bookmarks:
 - Migrating forwards to 0002_auto__add_field_bookmark_author.
 > bookmarks:0002_auto__add_field_bookmark_author
 - Loading initial data for bookmarks.
Installed 0 object(s) from 0 fixture(s)

Save and commit your work to add users to your bookmarks app!

Templates and links for login/logout/etc

Django's auth app comes with built-in views, which we can use to handle login and logout functionality for our users. Once we point some URLs at them, that is.

First, let's edit urls.py:

urlpatterns = patterns('',
    url(r'^$', 'bookmarks.views.index', name='home'),
    url(r'^bookmarks/$', 'bookmarks.views.index', name='bookmarks_view'),
    url(r'^tags/(\w+)/$', 'bookmarks.views.tag'),
    url(r'^login/$', 'django.contrib.auth.views.login'),
)

Now if anyone goes to localhost:8000/login/, the built-in login view will get triggered.

Run your dev server and try that. What error do you see?

While there is a built-in login view, there is no built-in login template. We need to build one for it. While we could put it anywhere and tell login explicitly where to look, by default the login view expects the template to reside at templates/registration/login.html. So we may as well put it there.

# in django-for-designers/myproject
$ cd bookmarks/templates/
# in django-for-designers/myproject/bookmarks/templates
$ mkdir registration

Create a new file within the new registration directory called login.html, and put this inside:

{% extends "base.html" %}

{% block subheader %}Login{% endblock %}

{% block content %}
{% if form.errors %}
<p>Your username and password didn't match. Please try again.</p>
{% endif %}

<form method="post" action="{% url 'django.contrib.auth.views.login' %}">
{% csrf_token %}
<table>
<tr>
    <td>{{ form.username.label_tag }}</td>
    <td>{{ form.username }}</td>
</tr>
<tr>
    <td>{{ form.password.label_tag }}</td>
    <td>{{ form.password }}</td>
</tr>
</table>

<input type="submit" value="login" />
<input type="hidden" name="next" value="/" />
</form>
{% endblock %}

Note the hidden input with the name "next". This input tells the login view what URL to send the user to after they successfully log in. We have it set to '/', so it'll just take them back to the home page.

We need to add a logout view too! Let's do that. Back in urls.py:

url(r'^login/$', 'django.contrib.auth.views.login'),
url(r'^logout/$', 'django.contrib.auth.views.logout', {'next_page': '/'})

The dictionary after the logout URL sends some extra arguments to the logout view. Specifically it tells the logout view where to send the user after they log out. We could make a special goodbye splash page or something, but nah, let's just send them back to the home page again.

Now we need to add a login/logout link to our site so people can actually use these views! We want this link to be at the top of every page, so we'll edit our base template:

<body>
    <div id="container">
        <div id="header">
            {% block bookmark_widget %}
            {% endblock %}
            <div id="authentication">
            {% if user.is_authenticated %}
                Hi {{user}}! <a href="{% url 'django.contrib.auth.views.logout' %}">Logout</a>
            {% else %}
                <a href="{% url 'django.contrib.auth.views.login' %}">Login</a>
            {% endif %}
            </div>
            <h1><a href="/">My bookmarking app</a></h1>
        </div>
        [ ... ]

Our template checks to see if there's a logged-in user, and if so, shows a hello message and a logout link. If the user isn't logged in, it shows a login link instead.

Check http://localhost:8000 and try logging in with the superadmin username and password you created before! It should work. :)

You might be wondering--where did the 'user' variable in the template come from? If you look at views.py, you'll notice we never added a user variable to our context dictionary. So how did this happen?

The answer is in the function we are using to render our templates, render(). render() automatically uses the request we sent it to create a Django RequestContext, which contains a bunch of extra context variables that get sent along to every view that uses a RequestContext. The Django auth app adds the current user to the RequestContext automatically. This is handy, since you don't want to have to look up the current user to every single view you ever write separately, just so the nav section on your website will work everywhere!

There are other functions that return an HTMLResponse, like render(), but don't include a RequestContext. render_to_response() is one common shortcut function that doesn't include it by default; another was the HttpResponse() function we used earlier! Just something to remember--if you're trying to send a piece of data to almost every page in your web app, 1.) you probably want to use render() in your views, and 2.) you want to find a way to add your data to your app's RequestContext. (https://docs.djangoproject.com/en/dev/ref/templates/api/#django.template.RequestContext says more about this!)

Save and commit your changes in adding login/logout functionality.

Modify views and templates to use model data

Let's edit our views and templates so they use real bookmark data from our database!

Add more data

First, let's add a bit more bookmark and tag data to our database. We'll use a Python loop to add a lot of bookmarks at once.

# in django-for-designers/myproject
$ python myproject/manage.py shell

That will bring us to the Django management shell. Within that special Python prompt, type the following:

(Remember, any time you see ">>>", you don't have to type those characters.)

>>> import django.contrib.auth.models
>>> me = django.contrib.auth.models.User.objects.all()[0]

This tells Python we'll be accessing the user model, and then grabs the first user, storing it in a variable called me.

Keeping that Python prompt open, run the following commands:

>>> import bookmarks.models
>>> import random
>>> tag_names = ['kids', 'read_on_plane', 'send_to_mom']
>>> # Create the Tag objects
>>> for tag_name in tag_names:
...     tag = bookmarks.models.Tag(slug=tag_name)
...     tag.save()
... 
>>> kids_urls = ['http://pbskids.org/', 'http://www.clubpenguin.com/', 'http://www.aplusmath.com/hh/index.html', 'http://kids.nationalgeographic.com/kids/activities/recipes/lucky-smoothie/', 'http://www.handwritingforkids.com/', 'http://pinterest.com/catfrilda/origami-for-kids/', 'http://richkidsofinstagram.tumblr.com/', 'http://www.dorkly.com/picture/50768/', 'http://www.whyzz.com/what-is-paint-made-of', 'http://yahooligans.com/']
>>> for kids_url in kids_urls:
...    b = bookmarks.models.Bookmark(author=me, url=kids_url)
...    b.save()
...    how_many_tags = random.randrange(len(tag_names))
...    for tag_name in random.sample(tag_names, how_many_tags):
...        b.tag_set.add(bookmarks.models.Tag.objects.get(slug=tag_name))
...
>>>

This creates a number of bookmarks, tagged appropriately! You can exit the shell by typing:

>>> exit()
Get views.py to talk to our models

Then, we'll need to have our views.py file import the bookmark model and send data to the index view.

We'll add an import statement:

from bookmarks.models import Bookmark

And we'll edit index(request):

def index(request):
    bookmarks = Bookmark.objects.all()
    context = {
        'bookmarks': bookmarks
    }
    return render(request, 'index.html', context)

Wait! We don't actually want to load every bookmark in our database when we go to the front page. If we have lots of bookmarks, that will get slow and unwieldy quickly.

Instead, let's show the 10 most recent bookmarks:

def index(request):
    bookmarks = Bookmark.objects.all().order_by('-timestamp')[:10]
    context = {
        'bookmarks': bookmarks
    }
    return render(request, 'index.html', context)

We also want our tag view to show real bookmarks. We only want to show bookmarks that have been tagged with the given tag. So we'll edit the import statement we just added:

from bookmarks.models import Bookmark, Tag

And alter the tag() definition as well:

def tag(request, tag_name):
    tag = Tag.objects.get(slug=tag_name)
    bookmarks = tag.bookmarks.all()
    context = {
        'tag': tag,
        'bookmarks': bookmarks,
    }
    return render(request, 'tag.html', context)
Change templates to handle bookmark data

Now, let's modify our templates to use this data.

In index.html, update just block content per the following. Make sure to keep the extends directive and the block subheader directive.

{% block content %}
<ul class="bookmarks">
    {% for bookmark in bookmarks %}
    <li>
        <a class="bookmark-link" href="">{{ bookmark }}</a>
        <div class="metadata"><span class="author">Posted by Jane Smith</span> | <span class="timestamp">2012-2-29</span> | <span class="tags"><a href="">funny</a> <a href="">haha</a></span></div>
    </li>
    {% endfor %}
</ul>
{% endblock %}

Instead of writing out each bookmark list element individually ahead of time, we are using a Django template language for loop to create a list element for each bookmark. Thus, we only have to specify the HTML formatting of our list elements once!

Start your development server and look at http://localhost:8000 now. You'll see that instead of the fake HTML we had before, each link's text is the unicode representation of each bookmark in your database. Cool, huh?

That's okay, but the unicode representation isn't really what we want to go there. Let's fix that:

    <a class="bookmark-link" href="{{ bookmark.url }}">
    {% if bookmark.title %}{{ bookmark.title }}{% else %}{{ bookmark.url }}{% endif %}
     </a>

If the bookmark has a title, we'll make the title the link. Otherwise we'll show the URL. We also fill in the URL on the <a> tag, to make the link work.

We also want to fill in the other metadata, like tags and author info. To do that. (FIXME: Where do people make this change?)

<div class="metadata"><span class="author">Posted by {{ bookmark.author }}</span> | <span class="timestamp">{{ bookmark.timestamp }}</span>
        {% if bookmark.tag_set.all %}| <span class="tags">
            {% for tag in bookmark.tag_set.all %}
                <a href="{% url 'bookmarks.views.tag' tag.slug %}">{{ tag.slug }}</a></span>
            {% endfor %}
        {% endif %}
</div>

We check if there are any tags for the bookmark, and if so loop over the tags to put each of them in.

Additionally, we use Django's {% url %} tag to generate the URL for our tag's link. The URL tag takes first an argument which is the path of a particular view function (bookmarks.views.tag), then additional arguments for any input variables that the function expects to glean from the URL. (As you may recall, the tag view takes an argument tag_name, which is the name of the tag in question).

Why use {% url %} instead of just writing "/tags/{⁠{ tag.slug }}"? Django principle of DRY--Don't Repeat Yourself--means that we want to avoid duplicating work as much as possible. If later down the line we decided to change our URL structure so that tag pages would appear at "/bookmarks/by_tag/<tag_name>" instead, we'd have to go in and fix all these hard-coded URL patterns by hand. Using the {% url %} tag makes Django generate our URL for us, based on our urls.py file, so any changes we make automatically get propagated outward!

Run

# in django-for-designers/myproject
$ python manage.py runserver

and confirm that your tag links working and our other changes are visible. It's easy to typo or make a mistake with {% url %} tags, so we want to make sure everything is working.

The default timestamp formatting that Django gave us is pretty neat, but in our original mockup we just used the date--not this long timestamp. Fortunately, Django's built-in filters make it easy to format a date or time any way we want! Since we just want to show the date, we'll use the date filter:

<span class="timestamp">{{ bookmark.timestamp|date:"Y-m-d" }}</span>

We tell Django that we're using a filter via the |filter_name syntax. The arguments that come after the colon are a standard Python code for describing different ways of formatting dates. You can read more about the date filter and all the different formatting codes at https://docs.djangoproject.com/en/dev/ref/templates/builtins/#date. For our purposes, Y outputs the full four-digit year, while m and d outputs the month and the day as two digit numbers. The hyphens we put between them are included in the formatting, too -- if we wanted the date to use slashes instead, we'd simply write |date:"Y/m/d".

Spin up your dev server, if you haven't got it running already, and check out your changes! Then save and commit your precious work.

Dealing with errors

Let's update our tag.html template to use the same formatting as index.html. Since we're using the same formatting for the list of bookmarks, and there aren't any other content block differences between index.html and tag.html, we can simply modify our tag template to inherit from index.html. Then the only block we need to overwrite is the subheader:

{% extends 'index.html' %}

{% block subheader %}Bookmarks tagged {{ tag }}{% endblock %}

Now we can go to http://localhost:8000/tags/funny and see a nicely styled list of bookmarks tagged with that tag!

What happens if we go to http://localhost:8000/tags/asdfjkl?

Eek, a DoesNotExist error! That's not so great. Our application is erroring on one of our view lines:

tag = Tag.objects.get(slug=tag_name)

Fortunately, Django has a shortcut function that can help us -- get_object_or_404(). This function will attempt to get a Django model based on the parameters you give it, and if it fails, automatically throw an HTML 404 error.

from django.shortcuts import render, get_object_or_404
from bookmarks.models import Bookmark, Tag


[...]


def tag(request, tag_name):
    tag = get_object_or_404(Tag, slug=tag_name)
    bookmarks = tag.bookmarks.all()
    context = {
        'tag': tag,
        'bookmarks': bookmarks,
    }
    return render(request, 'tag.html', context)

Now http://localhost:8000/tags/asdfjkl will throw a nicer 404 error. We could even make a pretty 404.html template to handle such errors! There are also shortcuts in Django for 500 and 403 (Forbidden) errors, if you want to handle and style those as well.

Save and commit your error-catching work.

Let me show off

So far, we've all only been able to access our own Django-powered sites. In this section, you will see how to access my (the instructor's) super cool bookmarks site! (We'll talk later about how you can share your code the same way. It might require firewall configuration changes on your computer, so we save that complexity for later.)

Assuming that you and the instructor are on the same wifi network, you can visit a temporary website on the instructor's computer. Visit this link:

http://192.168.1.1:8000/

(If you are wondering exactly who can connect to the instructor's app, it is typically only people on the same wifi/wired network as her, rather than the whole Internet. This is due to the a technique in wide use called "network address translation", and it is not considered perfect security.)

When you visit the instructor's app, you are interacting with the database stored on her laptop. So all the changes made by other people in the room are reflected in what you see! So try not to be too obscene.


Part 7: Exercises for the reader

Updating and deleting bookmarks (the last two parts of CRUD)

Write your own styles / JS frontend behavior

Handling user-uploaded media

So far, our website has only included visual effects created by the site admin. It's time to change that by letting your users upload custom images for tags.

A note about dependencies: For Django's image support to work properly, your system must have the Python Imaging Library. This is available either as "PIL", as "Image", or as "pillow".

Because that introduces a lot of complexity, this tutorial glosses over it. If you are especially excited about this, ask a TA to work with you on it.

Here are the basic steps:

First, you need to install PIL. Follow the instructions here to do so.

Second, you need to modify your virtualenv to permit system site packages to work. To do that, do:

$ virtualenv --system-site-packages .

Now you should be able to import Image in your python prompt. If not, talk to a TA.

We will need to enhance the tag model to accept an uploaded file. Open up bookmarks/model.py in your favorite editor. Change the Tag class definition so it looks like this:

class Tag(models.Model):
    bookmark = models.ManyToManyField(Bookmark)
    slug = models.CharField(max_length=50, unique=True)
    image = models.ImageField()

You then need to create and execute a schema migration. Then, commit the changed models files and the the new migration files to git.

When that is done, you should add a new form to the tag.html view that lets users upload a new image. You will need to add a corresponding view that processes the form, and then redirects the user back to the tag view.

One final note: Because this section deals with file uploads, it will work inconsistently on Heroku. Heroku does not promise to keep any uploaded files. Your two options would be (1) reconfigure your app to use a different storage engine, for example uploading your images to a service like Amazon S3; or (2) you could use a different hosting service, such as OpenShift, that does not have this behavior.

Installing django-debug-toolbar and what it's useful for

Django's operation can be somewhat opaque. When a page does not show the information you were expecting, there can a great number of reasons: perhaps some data was not saved to the database, or perhaps the variable name the template was expecting did not match the name you provided in the context from the view.

Django debug toolbar is an open source Django app that gives you more insight into how Django is working. In this section, we will show you how to install it, demonstrate at least one thing it is useful for, and explain at least one gotcha that you should be aware of.

Now is a good time to make sure you are using a Django app that actually works! If your current branch is not something you're confident of, this is a good moment to create a new branch based on known-working code. To do that:

# in django-for-designers
$ git branch my-debug-toolbar-work origin/pre-part-5
$ git checkout my-debug-toolbar-work

First, we will need to install it. Typically, you would need to add it to requirements.txt for your own project. In the case of the tutorial, we already configured requirements.txt to have it, but we will show you how to add it as if we hadn't.

Open requirements.txt in your favorite text editor. Make sure the following line appears:

django-debug-toolbar

Once that is done, you would run the following command. (It is safe to run it now, even though it may not be needed).

# in django-for-designers
$ pip install -r requirements.txt

This reads requirements.txt and ensures your virtualenv has all the packages installed, downloading and installing them if necessary.

Now that it is available, we need to tell Django to enable it. django-debug-toolbar requires a few adjustments to your settings.py, which you can find in myproject/settings.py. First, look for MIDDLEWARE_CLASSES. Add the following string as the final indented line in the sequence, and make sure there is a comma at the end of the line before it:

    'debug_toolbar.middleware.DebugToolbarMiddleware',

Configure the list of IP addresses will be able to see the debug toolbar. To do that, add this to the end of myproject/settings.py:

INTERNAL_IPS=('127.0.0.1',)

Finally, make sure it appears in the list of INSTALLED_APPS. If not, added it to the end of that list as follows:

    'debug_toolbar',

Now that it is installed, stop and start your runserver. Then take a look at http://127.0.0.1:8000/ -- do you see the new toolbar in the top right corner?

If so, great!

We'll show you two neat tricks the debug toolbar can offer you. First, if you click on the SQL box, you will see the page expand into a list of all the SQL queries that your front page executed, and how long they took. If your pages are loading slowly, you may find that you can trim down the number or the complexity of these queries.

Second, if you click Templates, you can see which templates were rendered, and (excitingly) what data was passed to them. This is the simplest way to see what information was passed to the templates. Although the interface is somewhat complicated, it is more helpful than repeatedly reloading a page with different "print" statements in it!

There are two gotchas that one must be aware of when using django-debug-toolbar:

  1. By default, it only works when your DEBUG is True. Most of the time, this is a good fit; it means users out on the 'net can't view this advanced interface. (That's because when you "deploy" an app, you are supposed to set DEBUG to False.)
  2. It "absorbs" redirects. Any time that your code would generate a redirect in the browser, the debug toolbar "catches" that and lets you see what is going on. This can be great, but it can also get annoying.

You can read more about these features, how to change them, and what else the debug toolbar can do on its official website!

Writing your own tests

Automated testing is a way to run code that verifies your code. There are a few benefits to writing tests:

  • If you wrote the tests before you wrote the code, then you can vividly see that your code worked: the test failed before you wrote the code, and it passes after you wrote it.
  • Having a test suite gives you confidence that a change you are making does not break existing functionality. Since many, many bug fix accidentally break other functionality, this is of huge importance.
  • Having a test suite gives other people confidence that your app does the things it says it does. This is helpful when showing the code to another developer or when listing its features to another stakeholder.

Django has your back here: it comes with a built-in ability to run tests if you write them, and it comes with a test suite of its own.

First, let's take a look at how to run tests. The auth app that we have been using for user login provides a test suite. You can run that test suite as follows:

# in django-for-designers/myproject
$ python manage.py test auth

You'll see a lot of . characters print out, and finally a message like:

Ran 181 tests in 7.764s

OK
Destroying test database for alias 'default'...

Congratulations! You can now rest assured that the auth app works properly. (This is good, since you are relying on it!)

In the rest of this section, you'll see how to write your own tests and learn other helpful tips about testing.

Running and writing tests for bookmarks

We can run just the tests for our bookmarks app as follows:

# in django-for-designers/myproject
$ python manage.py test bookmarks

We'll see something like:

Creating test database for alias 'default'...
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

It may be surprising that there is even one test to run, given that so far, we have not paid any attention to tests! Take a look at bookmarks/tests.py; in there, you'll see some automatically created hints by Django about how to write new tests, and one simple test.

Let's deconstruct that test right now:

from django.test import TestCase


class SimpleTest(TestCase):
    def test_basic_addition(self):
        """
        Tests that 1 + 1 always equals 2.
        """
        self.assertEqual(1 + 1, 2)

This is a Python class named SimpleTest, inheriting from django's TestCase class. This inheritance is required for Django to discover the test when you run manage.py test. (You can read more about that here.)

It has one method; the fact that it starts with test_ is also essential to it being discovered. Within the method, there is a text description (known as a docstring), and finally, a one-line body.

This body is the heart of the test. It calculates a value, and then asserts that is equal to a stored, known-correct value.

To make this clearer, let's change the test so that it asserts that 1+1 is 3. To do that, replace the class with the following:

from django.test import TestCase


class SimpleTest(TestCase):
    def test_basic_addition(self):
        """
        Tests that 1 + 1 always equals 2.
        """
        self.assertEqual(1 + 1, 3)

Now run the bookmarks test suite:

# in django-for-designers/myproject
$ python manage.py test bookmarks

As the test runs, you see a F printed rather than pleasing, relaxing .. Additionally, once all the tests are over, Django's test runner prints the details of what failed:

======================================================================
FAIL: test_basic_addition (bookmarks.tests.SimpleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../bookmarks/tests.py", line 16, in test_basic_addition
    self.assertEqual(1 + 1, 3)
AssertionError: 2 != 3

Now that we have a basic understanding of tests, go into bookmarks/tests.py and remove the SimpleTest class entirely.

Adding a new test

Writing tests for a Django app is similar to interacting with the app in the manage.py shell. One key difference is that every test case starts out with a blank database, rather than using your app's database.

Let's add a test for your bookmarks app that verifies the index view: namely, that if there is a bookmark in the database, it gets passed to the template in a context variable called bookmarks.

To do that, open up bookmarks/tests.py and add the following to the end of the file.

from django.contrib.auth.models import User
from bookmarks.models import Bookmark

class BookmarkViewTest(TestCase):
    def test_bookmark_shows_up(self):
        # Create a user to own the bookmark
        me = User.objects.create(username='me')

        # Create a sample bookmark
        mark = Bookmark.objects.create(user=me, title='Title of the song', url='http://example.com/')

        # Visit the home page
        c = Client()
        response = c.get('/')

        # Make sure it has the bookmark data
        as_sent_to_template = response.context['bookmarks']
        self.assertTrue(mark in as_sent_to_template)

This class has just one method, which begins with test_ so that it can be picked-up by the Django test runner.

In the test database, which starts out as blank, there are no users, so we must create one. (You can change that through test fixtures.)

Then we create one bookmark, and ask a the Django test client to load the home page. The test client is like a special, Django-aware web browser. Instead of doing an HTTP request against the actual website, it calls code directly within the Django app. This gives it access to, for example, the template context.

Finally, we extract the bookmark list sent to the template, and we verify that the bookmark we created shows up in the list.

If we run this test, we will see the familiar (if terse) output from the test runner:

# in django-for-designers/myproject
$ python manage.py test bookmarks
Creating test database for alias 'default'...
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK
Destroying test database for alias 'default'...

Hooray! You have written your first test.

Notice that test only validates that we pass the correct data to the template. We do not yet validate that the template actually renders the bookmark! To do that, you would need to test response.body.

One other aspect of testing that can be very helpful is using tests to help you refactor your code. (Refactoring is the process of taking code that works and making it simpler to understand without breaking it.)

In particular, our index view contains some string manipulation to normalize tags. That code, so far, has been somewhat under-specified; we do not really have a statement anywhere of precisely how it should work. One very valid exercise would be to remove that code from index, move it into a function, and then write at few test cases to cover it with tests.

At some point, you may be wondering how you know if you have written enough tests. One answer is by measuring your code's coverage, which is the fraction of your web app that the test suite executes. You can be more confident that the code works properly if it is covered by your test suite. Django does not come with coverage tools built-in, but with the help of django-coverage you can take those measurements. Code coverage is not a perfect way to determine if you have written enough tests, but getting your code to 100$ covered is a solid first goal.

Providing your data as an API

APIs, short for application programming interfaces, are a way for your Django site to provide services to other bits of code.

One common use for APIs is making data available to your own Javascript code. Another common use is making them available for other people to use in their own Python scripts or in Javascript widgets. In this section, you will learn how to add a Django app named tastypie to your project and see how it can easily make your data available.

Deciding on the API we'll provide

Before going forward, we should decide what kind of information we will export, and how we will export it.

For this example, since all the bookmarks on the site are public, we will provide a list of all bookmarks. We will let users of the API filter the bookmarks by the user, if they like. To keep things simple at first, we won't provide the bookmarks' tags.

Adding Tastypie as a dependency

First, we will edit requirements.txt to add a dependency on django-tastypie. To do so, we simply add one line to the end of requirements.txt:

django-tastypie

Now that our project depends on it, you can simply run (so long as the virtualenv is activated):

# in django-for-designers
$ pip install -r requirements.txt

That will download and install all the dependencies for the app, which will bring in tastypie.

Finally, add it to INSTALLED_APPS, toward the end. Within that sequence, add this on a line on its own:

    'tastypie',

With all those changes made, let's commit:

# in django-for-designers
$ git commit -a -m 'Adding dependency on tastypie'
create blank api.py file and add it to urls.py

Within bookmarks, create a new file called api.py. Let the contents of that file be:

from tastypie.resources import ModelResource
from bookmark.models import Bookmark


class BookmarkResource(ModelResource):
    class Meta:
        queryset = Bookmark.objects.all()
        resource_name = 'bookmark'

For this file to be invoked in request processing, we have to tie it into a url. So let's edit urls.py. First, add a new import to the top of the file:

from bookmarks.api import BookmarkResource

Below the imports, add this on a line of its own:

bookmark_resource = BookmarkResource()

Within the urlpatterns sequence, add this on a line of its own:

    (r'^api/', include(bookmark_resource.urls)),

Now your API should be live and on the web! Visit http://127.0.0.1:8000/api/bookmark/?format=json and you should see a machine-readable list of all the bookmarks on your site!

With all that done and working, now is a great time to commit. Run:

# in django-for-designers
$ git status

and make sure you git add any new files. With that addressed, run:

# in django-for-designers
$ git commit -a -m 'Added an API with Tastypie'
Add our api method to api.py
  • Update Python code to export data via Tastypie
  • Demo that it works, in a browser
  • git commit
Add the ability to filter by user
  • Update Python code to export data via Tastypie
  • Demo that it works, in a browser
  • git commit
Further directions
  • Exporting tags

Regular expressions

Text patterns can be encoded as regular expressions. You've already seen them in urls.py. We won't go into great depth here, but we will recommend some resources so you can understand them well, visualize them, and know how to write Python code that takes advantage of them.

First, get to know how they work. To do that, I recommend visualizing them.

Visualizing

To start with that, open up http://www.regexper.com/ and enter in one of the regular expressions we used. For example, enter this:

^$

Then, click the Display button (or hit ENTER on your keyboard). Contrast that with these examples:

^bookmarks\/$

(This is similar to the URL for the bookmarks view. We had to escape the slash character (/) by putting a backslash (\) before it because the regexper.com tool uses JavaScript-esque regular expressions rather than Python regular expressions. Let's ignore that for now.)

You'll see that it matches the exact string of "bookmarks/"; it also constrains things so that the line ends after the word "bookmarks/". If you remove that "$" character, the string "bookmarks/ahoy" would also match.

Executing regular expressions

You can execute regular expressions in a few ways.

First, there is the Python API for that. You can see it in action here:

You can also use http://regexpal.com/ to execute regular expressions

More reading and resources
  • http://regexpal.com/ has a great Quick Reference that you can find by clicking on those words in the top right of the web page.


Learn about relational databases

Django uses a relational database to store your data. In this section, we will explain the very basics of relational databases, give you pointers to more information, and show you how to explore your data.

Within the tutorial, we have used Django's ORM to access our data. That has obscured how the data is actually stored... so now it is time to take a quick look.

SQL is a query language for creating, reading, and modifying databases. SQL queries look something like:

SELECT title FROM bookmarks

You can get access to a SQL shell from within Django by running:

# in django-for-designers/myproject
$ python manage.py dbshell

and you can now execute the above query.

These databases are considered relational because the normal way to use them involves using references between tables to avoid repetition of data. In our tutorial, for example, a Bookmark pointed to a User. This is not the only way to build the app; we could have embedded all of the user's information within each Bookmark. That would have a major downside: the various copies of the information could get out of date.

This tutorial used sqlite, a very popular relational database, but there are plethora of others, including:

  • MySQL
  • Postgres
  • Oracle
  • Microsoft SQL Server

Django can be used with any of these. sqlite is convenient because it requires no setup step; however, popular hosting services typically use MySQL or Postgres. sqlite has one major downside -- it cannot support multiple changes to the database made while each other are being made. That limitation is completely OK for an app running on your laptop.

Even though Django's ORM looks just like Python, it does complex work behind the scenes. This work deserves some respect! One common source of slowness in web applications is executing too many SQL queries.

Exploring your app's data

The simplest way to explore your app's data visually is by finding the database.db file and opening it in a graphical browser. On Linux, Mac, or Windows, install Firefox, and then install the https://addons.mozilla.org/en-us/firefox/addon/sqlite-manager/ add-on.

From there, you can open your database and browse around.

Try to run the above SQL query against your database within SQLite Manager.

Other tools
  • Use http://sqlzoo.net/ to learn more about SQL by trying interactively to write queries that solve problems.