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.
We’ll assume you have Django installed already. You can tell Django is installed by running the Python interactive interpreter and typing import django. If that command runs successfully, with no errors, Django is installed.
Where to get help:
Creating a project¶
If this is your first time using Django, you’ll have to take care of some initial setup. Namely, you’ll need to auto-generate some code that establishes a Django project – a collection of settings for an instance of Django, including database configuration, Django-specific options and application-specific settings.
From the command line, cd into a directory where you’d like to store your code, then run the command django-admin.py startproject mysite. This will create a mysite directory in your current directory.
Script name may differ in distribution packages
If you installed Django using a Linux distribution’s package manager (e.g. apt-get or yum) django-admin.py may have been renamed to django-admin. You may continue through this documentation by omitting .py from each command.
Mac OS X permissions
If you’re using Mac OS X, you may see the message “permission denied” when you try to run django-admin.py startproject. This is because, on Unix-based systems like OS X, a file must be marked as “executable” before it can be run as a program. To do this, open Terminal.app and navigate (using the cd command) to the directory where django-admin.py is installed, then run the command chmod +x django-admin.py.
You’ll need to avoid naming projects after built-in Python or Django components. In particular, this means you should avoid using names like django (which will conflict with Django itself) or test (which conflicts with a built-in Python package).
django-admin.py should be on your system path if you installed Django via python setup.py. If it’s not on your path, you can find it in site-packages/django/bin, where `site-packages` is a directory within your Python installation. Consider symlinking to django-admin.py from some place on your path, such as /usr/local/bin.
Where should this code live?
If your background is in PHP, you’re probably used to putting code under the Web server’s document root (in a place such as /var/www). With Django, you don’t do that. It’s not a good idea to put any of this Python code within your Web server’s document root, because it risks the possibility that people may be able to view your code over the Web. That’s not good for security.
Put your code in some directory outside of the document root, such as /home/mycode.
Let’s look at what startproject created:
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. Change into the mysite directory, 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. We've included this with Django so you can develop things rapidly, without having to deal with configuring a production server -- such as Apache -- until you're ready for production.
Now's a good time to note: DON'T use this server in anything resembling a production environment. It's intended only for use while developing. (We're in the business of making Web frameworks, not Web servers.)
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!
Changing the port
By default, the runserver command starts the development server on the internal IP at port 8000.
If you want to change the server's port, pass it as a command-line argument. For instance, this command starts the server on port 8080:
python manage.py runserver 8080
If you want to change the server's IP, pass it along with the port. So to listen on all public IPs (useful if you want to show off your work on other computers), use:
python manage.py runserver 0.0.0.0:8000
Full docs for the development server can be found in the runserver reference.
Now, edit settings.py. It's a normal Python module with module-level variables representing Django settings. Change the following keys in the DATABASES 'default' item to match your databases connection settings.
NAME -- The name of your database. If you're using SQLite, the database will be a file on your computer; in that case, NAME should be the full absolute path, including filename, of that file. If the file doesn't exist, it will automatically be created when you synchronize the database for the first time (see below).
When specifying the path, always use forward slashes, even on Windows (e.g. C:/homes/user/mysite/sqlite3.db).
USER -- Your database username (not used for SQLite).
PASSWORD -- Your database password (not used for SQLite).
HOST -- The host your database is on. Leave this as an empty string if your database server is on the same physical machine (not used for SQLite).
If you're new to databases, we recommend simply using SQLite (by setting ENGINE to 'django.db.backends.sqlite3'). SQLite is included as part of Python 2.5 and later, so you won't need to install anything else.
If you're using PostgreSQL or MySQL, make sure you've created a database by this point. Do that with "CREATE DATABASE database_name;" within your database's interactive prompt.
If you're using SQLite, you don't need to create anything beforehand - the database file will be created automatically when it is needed.
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 for the common case.
Each of these applications makes use of at least one database table, though, 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.
If you're interested, run the command-line client for your database and type \dt (PostgreSQL), SHOW TABLES; (MySQL), or .schema (SQLite) to display the tables Django created.
For the minimalists
Like we said above, the default applications are included for the common case, but not everybody needs them. If you don't need any or all of them, feel free to comment-out or delete the appropriate line(s) from INSTALLED_APPS before running syncdb. The syncdb command will only create tables for apps in INSTALLED_APPS.
Now that your environment -- a "project" -- is set up, you're set to start doing work.
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
What's the difference between a project and an app? An app is a Web application that does something -- e.g., a Weblog system, a database of public records or a simple poll app. A project is a collection of configuration and apps for a particular Web site. A project can contain multiple apps. An app can be in multiple projects.
Your apps can live anywhere on your Python path. In this tutorial, we'll create our poll app in the mysite directory for simplicity.
To create your app, make sure you're in the 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.
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 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.
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()
The code is straightforward. Each model is 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.
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.
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.
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.
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.
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. # No polls are in the system yet. >>> Poll.objects.all()  # Create a new Poll. >>> import datetime >>> p = Poll(question="What's up?", pub_date=datetime.datetime.now()) # Save the object into the database. You have to call save() explicitly. >>> p.save() # Now it has an ID. Note that this might say "1L" instead of "1", depending # on which database you're using. That's no biggie; it just means your # database backend prefers to return integers as Python long integer # objects. >>> p.id 1 # Access database columns via Python attributes. >>> p.question "What's up?" >>> p.pub_date datetime.datetime(2007, 7, 15, 12, 00, 53) # Change values by changing the attributes, then calling save(). >>> p.pub_date = datetime.datetime(2007, 4, 1, 0, 0) >>> p.save() # objects.all() displays all the polls in the database. >>> Poll.objects.all() [<Poll: Poll object>]
Wait a minute. <Poll: Poll object> 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.
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 # Make sure our __unicode__() addition worked. >>> Poll.objects.all() [<Poll: What's up?>] # Django provides a rich database lookup API that's entirely driven by # keyword arguments. >>> Poll.objects.filter(id=1) [<Poll: What's up?>] >>> Poll.objects.filter(question__startswith='What') [<Poll: What's up?>] # Get the poll whose year is 2007. >>> Poll.objects.get(pub_date__year=2007) <Poll: What's up?> >>> Poll.objects.get(id=2) Traceback (most recent call last): ... DoesNotExist: Poll matching query does not exist. # Lookup by a primary key is the most common case, so Django provides a # shortcut for primary-key exact lookups. # The following is identical to Poll.objects.get(id=1). >>> Poll.objects.get(pk=1) <Poll: What's up?> # Make sure our custom method worked. >>> p = Poll.objects.get(pk=1) >>> p.was_published_today() False # Give the Poll a couple of Choices. The create call constructs a new # choice object, does the INSERT statement, adds the choice to the set # of available choices and returns the new Choice object. Django creates # a set to hold the "other side" of a ForeignKey relation # (e.g. a poll's choices) which can be accessed via the API. >>> p = Poll.objects.get(pk=1) # Display any choices from the related object set -- none so far. >>> p.choice_set.all()  # 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) # Choice objects have API access to their related Poll objects. >>> c.poll <Poll: What's up?> # And vice versa: Poll objects get access to Choice objects. >>> p.choice_set.all() [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>] >>> p.choice_set.count() 3 # The API automatically follows relationships as far as you need. # Use double underscores to separate relationships. # This works as many levels deep as you want; there's no limit. # Find all Choices for any poll whose pub_date is in 2007. >>> Choice.objects.filter(poll__pub_date__year=2007) [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>] # Let's delete one of the choices. Use delete() for that. >>> c = p.choice_set.filter(choice__startswith='Just hacking') >>> c.delete()
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.
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