Python的Django REST框架中的序列化及请求和返回

from django.http import HttpResponse
from django.views.decorators.csrf import csrf_exempt
from rest_framework.renderers import JSONRenderer
from rest_framework.parsers import JSONParser
from snippets.models import Snippet
from snippets.serializers import SnippetSerializer

class JSONResponse(HttpResponse):
  """
  An HttpResponse that renders it's content into JSON.
  """
  def __init__(self, data, **kwargs):
    content = JSONRenderer().render(data)
    kwargs['content_type'] = 'application/json'
    super(JSONResponse, self).__init__(content, **kwargs)

我们API的目的是，可以通过view来列举全部的Snippet的内容，或者创建一个新的snippet

@csrf_exempt
def snippet_list(request):
  """
  List all code snippets, or create a new snippet.
  """
  if request.method == 'GET':
    snippets = Snippet.objects.all()
    serializer = SnippetSerializer(snippets)
    return JSONResponse(serializer.data)

  elif request.method == 'POST':
    data = JSONParser().parse(request)
    serializer = SnippetSerializer(data=data)
    if serializer.is_valid():
      serializer.save()
      return JSONResponse(serializer.data, status=201)
    else:
      return JSONResponse(serializer.errors, status=400)

注意，因为我们要通过client向该view post一个请求，所以我们要将该view 标注为csrf_exempt, 以说明不是一个CSRF事件。
Note that because we want to be able to POST to this view from clients that won't have a CSRF token we need to mark the view as csrf_exempt. This isn't something that you'd normally want to do, and REST framework views actually use more sensible behavior than this, but it'll do for our purposes right now.
我们也需要一个view来操作一个单独的Snippet，以便能更新／删除该对象。

@csrf_exempt
def snippet_detail(request, pk):
  """
  Retrieve, update or delete a code snippet.
  """
  try:
    snippet = Snippet.objects.get(pk=pk)
  except Snippet.DoesNotExist:
    return HttpResponse(status=404)

  if request.method == 'GET':
    serializer = SnippetSerializer(snippet)
    return JSONResponse(serializer.data)

  elif request.method == 'PUT':
    data = JSONParser().parse(request)
    serializer = SnippetSerializer(snippet, data=data)
    if serializer.is_valid():
      serializer.save()
      return JSONResponse(serializer.data)
    else:
      return JSONResponse(serializer.errors, status=400)

  elif request.method == 'DELETE':
    snippet.delete()
    return HttpResponse(status=204)

将views.py保存，在Snippets目录下面创建urls.py,添加以下内容：

urlpatterns = patterns('snippets.views',
  url(r'^snippets/$', 'snippet_list'),
  url(r'^snippets/(?P<pk>[0-9]+)/$', 'snippet_detail'),
)

注意我们有些边缘事件没有处理，服务器可能会抛出500异常。
8. 测试

现在我们启动server来测试我们的Snippet。

在python mange.py shell终端下执行（如果前面进入还没有退出）

>>quit()

执行下面的命令， 运行我们的server：

python manage.py runserver

Validating models...

0 errors found
Django version 1.4.3, using settings 'tutorial.settings'
Development server is running at http://127.0.0.1:8000/
Quit the server with CONTROL-C.

新开一个terminal来测试我们的server

序列化：

url http://127.0.0.1:8000/snippets/

[{"id": 1, "title": "", "code": "print \"hello, world\"\n", "linenos": false, "language": "python", "style": "friendly"}]
 url http://127.0.0.1:8000/snippets/1/

{"id": 1, "title": "", "code": "print \"hello, world\"\n", "linenos": false, "language": "python", "style": "friendly"}

Request and Response
1. Request Object  ――Request对象

rest framework 引入了一个继承自HttpRequest的Request对象，该对象提供了对请求的更灵活解析。request对象的核心部分是request.data属性，类似于request.post, 但在使用WEB API时，request.data更有效。

（1）request.POST  # Only handles form data.  Only works for 'POST' method.
（2）request.DATA  # Handles arbitrary data.  Works any HTTP request with content.
2. Response Object ――Response对象
rest framework引入了一个Response 对象，它继承自TemplateResponse对象。它获得未渲染的内容并通过内容协商content negotiation 来决定正确的content type返回给client。

return Response(data)  # Renders to content type as requested by the client.
3. Status Codes
在views当中使用数字化的HTTP状态码，会使你的代码不宜阅读，且不容易发现代码中的错误。rest framework为每个状态码提供了更明确的标识。例如HTTP_400_BAD_REQUEST。相比于使用数字，在整个views中使用这类标识符将更好。
4. 封装API views

在编写API views时，REST Framework提供了两种wrappers：

1). @api_viwe 装饰器 ――函数级别

2). APIView 类――类级别

这两种封装器提供了许多功能，例如，确保在view当中能够接收到Request实例；往Response中增加内容以便内容协商content negotiation 机制能够执行。

封装器也提供一些行为，例如在适当的时候返回405 Methord Not Allowed响应；在访问多类型的输入request.DATA时，处理任何的ParseError异常。
5. 汇总

我们开始用这些新的组件来写一些views。

我们不在需要JESONResponse 类（在前一篇中创建），将它删除。删除后我们开始稍微重构下我们的view

from rest_framework import status
from rest_framework.decorators import api_view
from rest_framework.response import Response
from snippets.models import Snippet
from snippets.serializers import SnippetSerializer

@api_view(['GET', 'POST'])
def snippet_list(request):
  """
  List all snippets, or create a new snippet.
  """
  if request.method == 'GET':
    snippets = Snippet.objects.all()
    serializer = SnippetSerializer(snippets)
    return Response(serializer.data)

  elif request.method == 'POST':
    serializer = SnippetSerializer(data=request.DATA)
    if serializer.is_valid():
      serializer.save()
      return Response(serializer.data, status=status.HTTP_201_CREATED)
    else:
      return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

上面的代码是对我们之前代码的改进。看上去更简洁，也更类似于django的forms api形式。我们也采用了状态码，使返回值更加明确。
下面是对单个snippet操作的view更新：

@api_view(['GET', 'PUT', 'DELETE'])
def snippet_detail(request, pk):
  """
  Retrieve, update or delete a snippet instance.
  """       
  try:
    snippet = Snippet.objects.get(pk=pk)
  except Snippet.DoesNotExist:
    return Response(status=status.HTTP_404_NOT_FOUND)

  if request.method == 'GET':
    serializer = SnippetSerializer(snippet)
    return Response(serializer.data)

  elif request.method == 'PUT':
    serializer = SnippetSerializer(snippet, data=request.DATA)
    if serializer.is_valid():
      serializer.save()
      return Response(serializer.data)
    else:
      return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

  elif request.method == 'DELETE':
    snippet.delete()
    return Response(status=status.HTTP_204_NO_CONTENT)

注意，我们并没有明确的要求requests或者responses给出content type。request.DATA可以处理输入的json请求，也可以输入yaml和其他格式。类似的在response返回数据时，REST Framework返回正确的content type给client。

6. 给URLs增加可选的格式后缀

利用在response时不需要指定content type这一事实，我们在API端增加格式的后缀。使用格式后缀，可以明确的指出使用某种格式，意味着我们的API可以处理类似http://example.com/api/items/4.json.的URL。

增加format参数在views中，如：

def snippet_list(request, format=None):
and

def snippet_detail(request, pk, format=None):

现在稍微改动urls.py文件，在现有的URLs中添加一个格式后缀pattterns (format_suffix_patterns):

from django.conf.urls import patterns, url
from rest_framework.urlpatterns import format_suffix_patterns

urlpatterns = patterns('snippets.views',
  url(r'^snippets/$', 'snippet_list'),
  url(r'^snippets/(?P<pk>[0-9]+)$', 'snippet_detail'),
)

urlpatterns = format_suffix_patterns(urlpatterns)

这些额外的url patterns并不是必须的。