让我们通过例子来学习.
通过这个教程, 我们将循序渐进地创建一个基本的投票应用程序.
它将会包含两部分：
我们将假定您 已经安装了 Django . 你可以通过在 Pyhton 解释器中输入 import django 来验证. 如果该命令运行成功, 且没 有错误, 那就表明 Django 已经安装好了.
从哪里得到帮助:
如果您在学习本教程中遇到问题, 请在 django-users 上发贴或在 #django on irc.freenode.net 与其他可能会帮助您的 Django 用户交 流.
如果你是第一次使用 Django , 那么必须进行一些初始设置. 可以通过自动生成 一些代码来建立一个项目 project – 一个 Django 项目是一个 Django 实例的设置的集合, 包括数据库配置, Django 的定义选项和程序定义 的设置.
通过命令行, 使用 cd 命令进入你要存放代码的目录, 然后运行下面的命令:
django-admin.py startproject mysite
它会在当前目录下创建一个 mysite 目录.
脚本的名称可能因发行的包不同而有所区别
如果你是使用的Linux包管理器安装的Django(例如：apt-get 或 yum) django-admin.py 可能被重命名为 django-admin . 你可能需要忽略 掉本教程中的每一条命令中的 .py .
Mac OS X 权限问题
如果你是使用的 Mac OS X 操作系统, 当你尝试运行 django-admin.py startproject 你可能会看到 “permission denied” 的提示. 这是因为在 Unix类型的操作系统比如 OS X 中, 一个文件必须被标记上 “可执行” 属性, 你才可以运行它. 要赋予权限, 你首先需要打开 Terminal.app 终端程序, (使用 cd 命令) 进入到 django-admin.py 所安装的目录, 然后执行 sudo chmod +x django-admin.py.
Note
你需要避免使用 python 或 Django 的保留字作为项目名称, 如 django （这是 Django 的保留字）或 test （与 Python 内建的包 名称冲突）.
如果你是通过 python setup.py 来安装的 Django 那么 django-admin.py 应该已经在你的系统路径当中了. 如果它还不在你的系统路径中, 你可以在 site-packages/django/bin 中找 到它, 这里的 site-packages 是你系统中 Python 安装路径下的一个目录. 同时建议你在你的系统路径中, 比如 /usr/local/bin 下建立一个 django-admin.py 的软连接.
我的代码应该放在哪里？
如果你有 PHP 语言编程背景的话, 你可能会将你的代码放在 Web 服务器的 文档根目录 （例如 /var/www ). 在 Django 中, 你不必这么做. 将任 何 Python 代码放在你的 Web 服务器文档根目录不是一个好主意. 因为这可 能会增加人们通过 Web 方式查看到你的代码的风险. 这样很不安全.
将你的代码放置在你的文档根目录 之外 例如 /home/mycode .
我们一起来看看 startproject 都创建了些什么:
mysite/
manage.py
mysite/
__init__.py
settings.py
urls.py
wsgi.py
和你看到的文件不一样么？
默认的项目布局最近刚刚改变过. 如果你看到的是一个”扁平”结构的目录布 局（不包含内部的 mysite/ 目录）, 你很可能正在使用一个和本教 程版本不一致的 Django 版本. 你最好去查看对应的旧版教程, 或者使用最 新的 Django 版本.
这些文件是:
Let’s verify this worked. Change into the outer 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 1.4, 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 mysite/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 database connection settings.
ENGINE – Either 'django.db.backends.postgresql_psycopg2', 'django.db.backends.mysql', 'django.db.backends.sqlite3' or 'django.db.backends.oracle'. Other backends are also available.
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' and NAME to the place where you’d like to store the database. SQLite is included as part of Python 2.5 and later, so you won’t need to install anything else to support your database.
Note
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, set TIME_ZONE to your time zone. The default value is the Central time zone in the U.S. (Chicago).
Also, note the INSTALLED_APPS setting toward the bottom of the file. That 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:
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 right next to your manage.py file so that it can be imported as its own top-level module, rather than a submodule of mysite.
To create your app, make sure you’re in the same directory as manage.py and type this command:
python manage.py startapp polls
That’ll create a directory polls, which is laid out like this:
polls/
__init__.py
models.py
tests.py
views.py
This directory structure will house the poll application.
The first step in writing a database Web app in Django is to define your models – essentially, your database layout, with additional metadata.
Philosophy
A model is the single, definitive source of data about your data. It contains the essential fields and behaviors of the data you’re storing. Django follows the DRY 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.
Each field is represented by an instance of a Field class – e.g., CharField for character fields and DateTimeField for datetimes. This tells Django what type of data each field holds.
The name of each Field instance (e.g. question or pub_date ) is the field’s name, in machine-friendly format. You’ll use this value in your Python code, and your database will use it as the column name.
You can use an optional first positional argument to a Field to designate a human-readable name. That’s used in a couple of introspective parts of Django, and it doubles as documentation. If this field isn’t provided, Django will use the machine-readable name. In this example, we’ve only defined a human-readable name for Poll.pub_date. For all other fields in this model, the field’s machine-readable name will suffice as its human-readable name.
Some Field classes have required elements. CharField, for example, requires that you give it a max_length. That’s used not only in the database schema, but in validation, as we’ll soon see.
Finally, note a relationship is defined, using ForeignKey. That tells Django each Choice is related to a single Poll. Django supports all the common database relationships: many-to-ones, many-to-manys and one-to-ones.
That small bit of model code gives Django a lot of information. With it, Django is able to:
But first we need to tell our project that the polls app is installed.
Philosophy
Django apps are “pluggable”: You can use an app in multiple projects, and you can distribute apps, because they don’t have to be tied to a given Django installation.
Edit the settings.py file again, and change the INSTALLED_APPS setting to include the string 'polls'. So it’ll look like this:
INSTALLED_APPS = (
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.sites',
'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',
'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") DEFERRABLE INITIALLY DEFERRED,
"choice" varchar(200) NOT NULL,
"votes" integer NOT NULL
);
COMMIT;
Note the following:
If you’re interested, also run the following 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’ve 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.
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 the DJANGO_SETTINGS_MODULE environment variable, which gives Django the Python import path to your settings.py file.
Bypassing manage.py
If you’d rather not use manage.py, no problem. Just set the DJANGO_SETTINGS_MODULE environment variable to mysite.settings and run python from the same directory manage.py is in (or ensure that directory is on the Python path, so that import mysite works).
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.
# Support for time zones is enabled in the default settings file, so
# Django expects a datetime with tzinfo for pub_date. Use timezone.now()
# instead of datetime.datetime.now() and it will do the right thing.
>>> from django.utils import timezone
>>> p = Poll(question="What's new?", pub_date=timezone.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 new?"
>>> p.pub_date
datetime.datetime(2012, 2, 26, 13, 0, 0, 775217, tzinfo=<UTC>)
# Change values by changing the attributes, then calling save().
>>> p.question = "What's up?"
>>> 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.
Why __unicode__() and not __str__()?
If you’re familiar with Python, you might be in the habit of adding __str__() methods to your classes, not __unicode__() methods. We use __unicode__() here because Django models deal with Unicode by default. All data stored in your database is converted to Unicode when it’s returned.
Django models have a default __str__() method that calls __unicode__() and converts the result to a UTF-8 bytestring. This means that unicode(p) will return a Unicode string, and str(p) will return a normal string, with characters encoded as UTF-8.
If all of this is gibberish 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
from django.utils import timezone
# ...
class Poll(models.Model):
# ...
def was_published_recently(self):
return self.pub_date >= timezone.now() - datetime.timedelta(days=1)
Note the addition of import datetime and from django.utils import timezone, to reference Python’s standard datetime module and Django’s time-zone-related utilities in django.utils.timezone, respectively. If you aren’t familiar with time zone handling in Python, you can learn more in the time zone support docs.
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 2012.
>>> Poll.objects.get(pub_date__year=2012)
<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_recently()
True
# 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 2012.
>>> Choice.objects.filter(poll__pub_date__year=2012)
[<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.
Dec 14, 2013