API(一)之Serialization

virtualenv is a tool to create isolated Python environments.

建立一个新的环境

Before we do anything else we'll create a new virtual environment, using virtualenv. This will make sure our package configuration is kept nicely isolated from any other projects we're working on.

#操作virtualenv

ubuntu@ubuntu:~/tutorial$ sudo pip3 install virtualenv
ubuntu@ubuntu:~/tutorial$ virtualenv env
Running virtualenv with interpreter /usr/bin/python2   #这里默认启动py2，要想启动py3需要切换到py3的目录
错误...

ubuntu@ubuntu:~/tutorial$ cd /usr/lib/python3/dist-packages
ubuntu@ubuntu:/usr/lib/python3/dist-packages$ ls env
bin  lib  local  pip-selfcheck.json  share
ubuntu@ubuntu:/usr/lib/python3/dist-packages$ cd env/bin
ubuntu@ubuntu:/usr/lib/python3/dist-packages/env/bin$ source activate   #激活virtualenv
(env) ubuntu@ubuntu:~$ deactivate                                       #释放virtualenv
ubuntu@ubuntu:~$

# 激活virtualenv

virtualenv env
source env/bin/activate

Now that we're inside a virtualenv environment, we can install our package requirements.

# 安装所需模块

pip3 install django
pip3 install djangorestframework
pip3 install pygments  # We'll be using this for the code highlighting

Note: To exit the virtualenv environment at any time, just 键入deactivate. For more information see the virtualenv documentation.

入门

摘要：入门部分创建了一个Django项目。

Okay, we're ready to get coding. To get started, let's create a new project to work with.

# Linux下创建一个项目

cd ~
django-admin.py startproject tutorial
cd tutorial

#结果：
ubuntu@ubuntu:~$ tree tutorial
tutorial
├── manage.py
└── tutorial
        ├── __init__.py
        ├── settings.py
        ├── urls.py
        └── wsgi.py

1 directory, 5 files

Once that's done we can create an app that we'll use to create a simple Web API.

# 创建一个app

python3 manage.py startapp snippets

#结果：
ubuntu@ubuntu:~$ tree tutorial
tutorial
├── manage.py
├── snippets
│   ├── admin.py
│   ├── apps.py
│   ├── __init__.py
│   ├── migrations
│   │   └── __init__.py
│   ├── models.py
│   ├── tests.py
│   └── views.py
└── tutorial
    ├── __init__.py
    ├── __pycache__
    │   ├── __init__.cpython-35.pyc
    │   └── settings.cpython-35.pyc
    ├── settings.py
    ├── urls.py
    └── wsgi.py

4 directories, 14 files

We'll need to add our new snippets app and the rest_framework app to INSTALLED_APPS. Let's edit the tutorial/settings.py file:

# 配置文件

INSTALLED_APPS = (
    ...
　　'snipppets',
    'rest_framework',
)

Please note that if you're using Django <1.9, you need to replace snippets.apps.SnippetsConfig with snippets.

Okay, we're ready to roll.

创建一个可以使用的model

For the purposes of this tutorial we're going to start by creating a simple Snippet model that is used to store code snippets. Go ahead and edit the snippets/models.py file. Note: Good programming practices include comments. Although you will find them in our repository version of this tutorial code, we have omitted them here to focus on the code itself.

# models.py

from django.db import models
from pygments.lexers import get_all_lexers
from pygments.styles import get_all_styles

LEXERS = [item for item in get_all_lexers() if item[1]]
LANGUAGE_CHOICES = sorted([(item[1][0], item[0]) for item in LEXERS])
STYLE_CHOICES = sorted((item, item) for item in get_all_styles())


class Snippet(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    title = models.CharField(max_length=100, blank=True, default='')
    code = models.TextField()
    linenos = models.BooleanField(default=False)
    language = models.CharField(choices=LANGUAGE_CHOICES, default='python', max_length=100)
    style = models.CharField(choices=STYLE_CHOICES, default='friendly', max_length=100)

    class Meta:
        ordering = ('created',)   #排序字段

We'll also need to create an initial migration for our snippet model, and sync the database for the first time.

# 初始化数据库

python3 manage.py makemigrations snippets
python3 manage.py migrate

（不用）创建一个Serializer类

The first thing we need to get started on our Web API is to provide a way of 序列化和反序列化 the snippet instances into representations such as json. We can do this by declaring serializers that work very similar to Django's forms. Create a file in the snippets directory named serializers.py and add the following.

#serializers.py

from rest_framework import serializers
from snippets.models import Snippet, LANGUAGE_CHOICES, STYLE_CHOICES


class SnippetSerializer(serializers.Serializer):
    id = serializers.IntegerField(read_only=True)
    title = serializers.CharField(required=False, allow_blank=True, max_length=100)
    code = serializers.CharField(style={'base_template': 'textarea.html'})
    linenos = serializers.BooleanField(required=False)
    language = serializers.ChoiceField(choices=LANGUAGE_CHOICES, default='python')
    style = serializers.ChoiceField(choices=STYLE_CHOICES, default='friendly')

    def create(self, validated_data):
        """
        Create and return a new `Snippet` instance, given the validated data.
        """
        return Snippet.objects.create(**validated_data)

    def update(self, instance, validated_data):
        """
        Update and return an existing `Snippet` instance, given the validated data.
        """
        instance.title = validated_data.get('title', instance.title)
        instance.code = validated_data.get('code', instance.code)
        instance.linenos = validated_data.get('linenos', instance.linenos)
        instance.language = validated_data.get('language', instance.language)
        instance.style = validated_data.get('style', instance.style)
        instance.save()
        return instance

the serializer class 的第一部分定义了序列化/反序列化的字段。The create() and update()methods define how fully fledged instances are created or modified when calling serializer.save()

serializer class 与 Django的 Form class 非常相似, 并且包含类似的验证标记, such as required, max_length and default.

在某些情况下，the field flags还可以控制serializer的显示方式，比如渲染html页面. 上面的 {'base_template': 'textarea.html'} 就相当于Django-Form 中的 widget=widgets.Textarea. 这对于控制可浏览API的显示方式特别有用，我们将在本教程的后面看到。

实际上，我们也可以使用 the ModelSerializer class 来节省一些时间 , 但是现在我们将保留这种serializer定义。

使用Serializers串行器

摘要：这一部分都是在python shell下操作的，进行序列化和反系列化操作。

在我们进一步了解之前，让我们熟悉一下Serializer类的使用。让我们进入Django shell。

# python shell下

python manage.py shell

好的，几个导入之后，让我们创建一些代码片段来处理。

from snippets.models import Snippet
from snippets.serializers import SnippetSerializer
from rest_framework.renderers import JSONRenderer
from rest_framework.parsers import JSONParser

snippet = Snippet(code='foo = "bar"
')   #实例化一个对象并填写code字段，然后保存。
snippet.save()

snippet = Snippet(code='print "hello, world"
')
snippet.save()

现在我们已经有了几个片段实例。我们来序列化其中一个实例。

serializer = SnippetSerializer(snippet)   #snippet是Snippet class的对象
serializer.data # {'id': 2, 'title': u'', 'code': u'print "hello, world"
', 'linenos': False, 'language': u'python', 'style': u'friendly'}

type(serializer.data)   #<class 'rest_framework.utils.serializer_helpers.ReturnDict'>，不是python原生数据类型。

此时，我们将model实例转换为Python原生数据类型。为了完成序列化过程，我们先将数据转化为json。

content = JSONRenderer().render(serializer.data)
content

# b'{"id": 2, "title": "", "code": "print \"hello, world\"\n", "linenos": false, "language": "python", "style": "friendly"}'

type(content)    #<class 'bytes'>

反序列化是类似的. 首先我们将一个流解析为Python原生数据类型...

from django.utils.six import BytesIO

stream = BytesIO(content)

type(stream)                           #<class '_io.BytesIO'>

data = JSONParser().parse(stream)

type(data)                             #<class 'dict'>，python原生数据类型。

小结：

实例化：snippet = Snippet(code='foo = "bar" ')
序列化：serializer = SnippetSerializer(snippet)

因为serializer.data不是python原生数据类型（<class 'rest_framework.utils.serializer_helpers.ReturnDict'>）
要想转为python原生数据类型需要两个中间步骤：
      转为json：content = JSONRenderer().render(serializer.data)
      转为BytesIO：stream = BytesIO(content)
转为原生数据类型：data = JSONParser().parse(stream)

...然后我们将这些原生数据类型恢复到 a fully populated object instance。

serializer = SnippetSerializer(data=data)   #data是python原生数据类型。
serializer.is_valid()
# True
serializer.validated_data
# OrderedDict([('title', ''), ('code', 'print "hello, world"
'), ('linenos', False), ('language', 'python'), ('style', 'friendly')])
serializer.save()
# <Snippet: Snippet object>

请注意，API 与 forms 很相似。当我们使用serializer编写视图函数时，相似性会更加明显。

我们也可以序列化 querysets 而不是 model instances. 为了做到这一点， 我们仅仅添加 a many=True flag to the serializer arguments.

serializer = SnippetSerializer(Snippet.objects.all(), many=True)
serializer.data
# [OrderedDict([('id', 1), ('title', u''), ('code', u'foo = "bar"
'), ('linenos', False), ('language', 'python'), ('style', 'friendly')]), OrderedDict([('id', 2), ('title', u''), ('code', u'print "hello, world"
'), ('linenos', False), ('language', 'python'), ('style', 'friendly')]), OrderedDict([('id', 3), ('title', u''), ('code', u'print "hello, world"'), ('linenos', False), ('language', 'python'), ('style', 'friendly')])]

（使用）使用ModelSerializers

SnippetSerializer复制了Snippet中的大量信息。如果可以保持代码更简洁，那将是很好的。

与Django提供Form类和ModelForm类的方式相同，REST框架包括Serializer类和ModelSerializer类。

让我们来看看使用ModelSerializer类重构我们的serializer。再次打开文件 snippets/serializers.py , and replace the SnippetSerializer class with the following.

from snippets import models
from rest_framework import serializers

class SnippetSerializer(serializers.ModelSerializer):   #之前继承的是serializers.Serializer
class Meta:
    model = models.Snippet
    fields = ('id', 'title', 'code', 'linenos', 'language', 'style')

serializers有一个很好的属性是你可以检查一个serializer instance的所有字段, by printing its representation. 打开Django shell python manage.py shell，然后尝试以下操作：

from snippets.serializers import SnippetSerializer
serializer = SnippetSerializer()
print(repr(serializer))
# SnippetSerializer():
#    id = IntegerField(label='ID', read_only=True)
#    title = CharField(allow_blank=True, max_length=100, required=False)
#    code = CharField(style={'base_template': 'textarea.html'})
#    linenos = BooleanField(required=False)
#    language = ChoiceField(choices=[('Clipper', 'FoxPro'), ('Cucumber', 'Gherkin'), ('RobotFramework', 'RobotFramework'), ('abap', 'ABAP'), ('ada', 'Ada')...
#    style = ChoiceField(choices=[('autumn', 'autumn'), ('borland', 'borland'), ('bw', 'bw'), ('colorful', 'colorful')...

记住，ModelSerializer类不会做任何特别神奇的事情，它们只是创建 serializer classes 快捷方式：

• 自动确定具有的字段。
• 默认实现create()和update()方法。

（不用，在第二篇加了format）使用Serializer编写常规的Django视图

让我们来看看该如何使用Serializer类来编写一些API视图。目前我们不会使用任何REST框架的其他功能，我们只编写一些常规的Django视图函数。

编辑 snippets/views.py文件，并添加以下内容。

from django.http import HttpResponse, JsonResponse
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

The root of our API 将是一个视图，它支持列出所有现有的片段，或创建一个新的片段。

@csrf_exempt
def snippet_list(request):
    """
    List all code snippets, or create a new snippet.
    """
    if request.method == 'GET':
        snippets = Snippet.objects.all()                      #querysets
        serializer = SnippetSerializer(snippets, many=True)   #序列化querysets，many=True
        return JsonResponse(serializer.data, safe=False)      #safe=False，是啥？？？

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

请注意，因为我们希望能够从不具有CSRF token的客户端以POST方法访问此视图函数，因此我们需要用csrf_exempt标记视图。这不是你通常想要做的事情，REST框架视图实际上使用比这更有明显的行为，但它现在将用于我们的目的。

我们还需要一个与 an individual snippet对应的视图，并可用于检索，更新或删除the snippet。

@csrf_exempt
def snippet_detail(request, pk):
    """
    Retrieve, update or delete a code snippet.
    """
    try:
        snippet = Snippet.objects.get(pk=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)
        return JsonResponse(serializer.errors, status=400)

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

最后我们需要把这些视图函数连接起来。创建snippets/urls.py文件： 

#app下面的urls.py

from django.conf.urls import url
from snippets import views

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

我们还需要在tutorial/urls.py文件中连接根urlconf ，以包含我们的片段应用程序的URL。

from django.conf.urls import url, include

urlpatterns = [
    url(r'^', include('snippets.urls')),
]

It's worth noting that there are a couple of edge cases we're not dealing with properly at the moment. If we send malformed json, or if a request is made with a method that the view doesn't handle, then we'll end up with a 500 "server error" response. Still, this'll do for now.

测试我们在Web API上的第一次尝试

现在我们可以启动一个运行我们的代码片段的示例服务器。

退出shell...

quit()

...and start up Django's development server.

python manage.py runserver 192.168.66.250:9000

Validating models...

0 errors found
Django version 1.11, using settings 'tutorial.settings'
Development server is running at http://127.0.0.1:8000/
Quit the server with CONTROL-C.

Notes：在settings.py中添加ALLOWED_HOSTS = {'192.168.66.200',}

在另一个终端窗口中，我们可以测试服务器。

我们可以使用curl或httpie来测试我们的API 。Httpie是用Python编写的用户友好的http客户端。我们来安装它。

您可以使用pip安装httpie：

#

pip install httpie

最后，我们可以得到所有片段的列表：

#

http http://127.0.0.1:8000/snippets/

HTTP/1.1 200 OK
...
[
  {
    "id": 1,
    "title": "",
    "code": "foo = "bar"
",
    "linenos": false,
    "language": "python",
    "style": "friendly"
  },
  {
    "id": 2,
    "title": "",
    "code": "print "hello, world"
",
    "linenos": false,
    "language": "python",
    "style": "friendly"
  }
]

或者我们可以通过引用其id来获取特定的代码段：

#

http http://127.0.0.1:8000/snippets/2/

HTTP/1.1 200 OK
...
{
  "id": 2,
  "title": "",
  "code": "print "hello, world"
",
  "linenos": false,
  "language": "python",
  "style": "friendly"
}

同样，您可以通过在网络浏览器中访问这些URL来显示相同​​的json。