Writing your first Django app, part 1
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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 and which version by running the following command:
$ python -c "import django; print(django.get_version())"
If Django is installed, you should see the version of your installation. If it isn’t, you’ll get an error telling “No module named django
”.
This tutorial is written for Django 1.9 and Python 3.4 or later.
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 following command:
$ django-admin startproject mysite
This will create a mysite directory in your current directory.
Note
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).Where should this code live?
If your background is in plain old PHP (with no use of modern frameworks), 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/ manage.py mysite/ __init__.py settings.py urls.py wsgi.py
These files are:
- The outer
mysite/
root directory is just a container for your project. Its name doesn’t matter to Django; you can rename it to anything you like. manage.py
: A command-line utility that lets you interact with this Django project in various ways.- The inner
mysite/
directory is the actual Python package for your project. Its name is the Python package name you’ll need to use to import anything inside it (e.g.mysite.urls
). mysite/__init__.py
: An empty file that tells Python that this directory should be considered a Python package.mysite/settings.py
: Settings/configuration for this Django project. -mysite/urls.py
: The URL declarations for this Django project; a “table of contents” of your Django-powered site.mysite/wsgi.py
: An entry-point for WSGI-compatible web servers to serve your project.
The development server
Let’s verify your Django project works. Change into the outer mysite
directory, if you haven’t already, and run the following commands:
$ python manage.py runserver
You’ll see the following output on the command line:
Performing system checks...System check identified no issues (0 silenced).You have unapplied migrations; your app may not work properly until they are applied.Run 'python manage.py migrate' to apply them.December 12, 2015 - 15:50:53Django version 1.9, using settings 'mysite.settings'Starting development server 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 on your network), use:
$ python manage.py runserver 0.0.0.0:8000
Automatic reloading of runserver
The development server automatically reloads Python code for each request as needed. You don’t need to restart the server for code changes to take effect. However, some actions like adding files don’t trigger a restart, so you’ll have to restart the server in these cases.
修改文件不用重启服务器,但是添加了新的文件需要重启服务器
Creating the Polls app
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 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 website. 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 admin.py apps.py migrations/ __init__.py models.py tests.py views.py
This directory structure will house the poll application.
Write your first view
Let’s write the first view. Open the file polls/views.py
and put the following Python code in it:
polls/views.pyfrom django.http import HttpResponsedef index(request): return HttpResponse("Hello, world. You're at the polls index.")
This is the simplest view possible in Django. To call the view, we need to map it to a URL - and for this we need a URLconf
.
To create a URLconf
in the polls directory, create a file called urls.py
. Your app directory should now look like:
polls/ __init__.py admin.py apps.py migrations/ __init__.py models.py tests.py urls.py views.py
In the polls/urls.py
file include the following code:
polls/urls.pyfrom django.conf.urls import urlfrom . import viewsurlpatterns = [ url(r'^$', views.index, name='index'),]
The next step is to point the root URLconf
at the polls.urls
module. In mysite/urls.py
, add an import for django.conf.urls.include
and insert an include()
in the urlpatterns
list, so you have:
mysite/urls.pyfrom django.conf.urls import include, urlfrom django.contrib import adminurlpatterns = [ url(r'^polls/', include('polls.urls')), url(r'^admin/', admin.site.urls),]
Doesn’t match what you see?
If you’re seeing include(admin.site.urls)
instead of just admin.site.urls
, you’re probably using a version of Django that doesn’t match this tutorial version. You’ll want to either switch to the older tutorial or the newer Django version.
You have now wired an index view into the URLconf
. Lets verify it’s working, run the following command:
$ python manage.py runserver
Go to http://localhost:8000/polls/
in your browser, and you should see the text “Hello, world. You’re at the polls index.”, which you defined in the index view.
The url()
function is passed four arguments, two required: regex and view, and two optional: kwargs, and name. At this point, it’s worth reviewing what these arguments are for.
url() argument: regex
The term “regex” is a commonly used short form meaning “regular expression”, which is a syntax for matching patterns in strings, or in this case, url patterns. Django starts at the first regular expression and makes its way down the list, comparing the requested URL against each regular expression until it finds one that matches.
Note that these regular expressions do not search GET and POST parameters, or the domain name. For example, in a request to https://www.example.com/myapp/, the URLconf will look for myapp/. In a request to https://www.example.com/myapp/?page=3
, the URLconf will also look for myapp/
.
Finally, a performance note: these regular expressions are compiled the first time the URLconf
module is loaded. They’re super fast (as long as the lookups aren’t too complex).
url()
argument: view
When Django finds a regular expression match, Django calls the specified view function, with an HttpRequest
object as the first argument and any “captured” values from the regular expression as other arguments. If the regex uses simple captures, values are passed as positional arguments; if it uses named captures, values are passed as keyword arguments. We’ll give an example of this in a bit.
url()
argument: kwargs
Arbitrary keyword arguments can be passed in a dictionary to the target view.
url()
argument: name
Naming your URL lets you refer to it unambiguously from elsewhere in Django especially templates. This powerful feature allows you to make global changes to the url patterns of your project while only touching a single file.
When you’re comfortable with the basic request and response flow, read part 2 of this tutorial to start working with the database.
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