2019年12月2日，Django終於正式發佈了3.0版本。懷着無比的期待，我們來嘗試一下吧！
（附ASGI官方文檔地址：https://asgi.readthedocs.io/en/latest/extensions.html）
一、創建Django3工程
利用Pycharm的方便，直接通過virtualenv創建虛擬環境，並安裝Django3.0。
打開控制檯，看看都安裝了哪些庫：
(venv) D:\work\for_test\django3>pip list Package Version ------ asgiref 3.2.3 Django 3.0 pip 10.0.1 pytz 2019.3 setuptools 39.1.0 sqlparse 0.3.0
除了pytz和sqlparse，又自動安裝了asgiref。
asigref由Django軟件基金會開發和維護，是一個Django生態中的庫。
先啓動一下服務器，看看Django是否正常運行：
沒毛病，可以把服務器關了！
二、學習官方文檔
畢竟是非常重要和複雜的新特性，先到官網取取經，看看文檔怎麼寫的。
找了一圈，What？就這麼點東西？
先康康吧。
How to deploy with ASGI As well as WSGI, Django also supports deploying on ASGI, the emerging Python standard for asynchronous web servers and applications. Django's startproject management command sets up a default ASGI configuration for you, which you can tweak as needed for your project, and direct any ASGI-compliant application server to use. Django includes getting-started documentation for the following ASGI servers: How to use Django with Daphne How to use Django with Uvicorn The application object Like WSGI, ASGI has you supply an application callable which the application server uses to communicate with your code. It's commonly provided as an object named application in a Python module accessible to the server. The startproject command creates a file <project_name>/asgi.py that contains such an application callable. It's not used by the development server (runserver), but can be used by any ASGI server either in development or in production. ASGI servers usually take the path to the application callable as a string; for most Django projects, this will look like myproject.asgi:application. Warning While Django's default ASGI handler will run all your code in a synchronous thread, if you choose to run your own async handler you must be aware of async-safety. Do not call blocking synchronous functions or libraries in any async code. Django prevents you from doing this with the parts of Django that are not async-safe, but the same may not be true of third-party apps or Python libraries. Configuring the settings module When the ASGI server loads your application, Django needs to import the settings module — that's where your entire application is defined. Django uses the DJANGO_SETTINGS_MODULE environment variable to locate the appropriate settings module. It must contain the dotted path to the settings module. You can use a different value for development and production; it all depends on how you organize your settings. If this variable isn't set, the default asgi.py sets it to mysite.settings, where mysite is the name of your project. Applying ASGI middleware To apply ASGI middleware, or to embed Django in another ASGI application, you can wrap Django's application object in the asgi.py file. For example: from some_asgi_library import AmazingMiddleware application = AmazingMiddleware(application)
文檔短小無力，內容稀少！
總結就以下幾點：
- 不能用python manage.py runserver的方式啓動ASGI服務器，這隻會啓動傳統的、默認的WSGI服務器，也就是老版本的東西
- 要啓動ASGI服務器，你需要使用Daphne或者Uvicorn。推薦使用Daphne，這是Django軟件基金會開發的一個基於ASGI (HTTP/WebSocket)的服務器。
- 有一個myproject.asgi:application文件，是ASGI通信的接口，Django默認自帶
- 你可以配置DJANGO_SETTINGS_MODULE 環境，或者使用默認的your_project.settings
- 可以使用第三方ASGI中間件
再簡單粗暴點，全文的意思是，你需要安裝Daphne，然後調用Django的application來啓動ASGI服務器。
好吧，我們看看Daphne，點擊Django給的連接，跳轉到相關頁面，內容更少：
How to use Django with Daphne Daphne is a pure-Python ASGI server for UNIX, maintained by members of the Django project. It acts as the reference server for ASGI. Installing Daphne You can install Daphne with pip: python -m pip install daphne Running Django in Daphne When Daphne is installed, a daphne command is available which starts the Daphne server process. At its simplest, Daphne needs to be called with the location of a module containing an ASGI application object, followed by what the application is called (separated by a colon). For a typical Django project, invoking Daphne would look like: daphne myproject.asgi:application This will start one process listening on 127.0.0.1:8000. It requires that your project be on the Python path; to ensure that run this command from the same directory as your manage.py file.
翻譯過來就是：
- pip安裝daphne
- 執行daphne myproject.asgi:application命令，啓動ASGI服務器
- 瀏覽器訪問127.0.0.1:8000
咱們照做！
三、啓動ASGI服務器
通過pip install daphne即可安裝最新版本的daphne：
Collecting pycparser (from cffi!=1.11.3,>=1.8->cryptography>=2.7->autobahn>=0.18->daphne) Installing collected packages: idna, hyperlink, zope.interface, attrs, six, Automat, constantly, PyHamcrest, incremental, pycparser, cffi, cry ptography, pyopenssl, pyasn1, pyasn1-modules, service-identity, twisted, txaio, autobahn, daphne Successfully installed Automat-0.8.0 PyHamcrest-1.9.0 attrs-19.3.0 autobahn-19.11.1 cffi-1.13.2 constantly-15.1.0 cryptography-2.8 daphne-2.4. 0 hyperlink-19.0.0 idna-2.8 incremental-17.5.0 pyasn1-0.4.8 pyasn1-modules-0.2.7 pycparser-2.19 pyopenssl-19.1.0 service-identity-18.1.0 six-1 .13.0 twisted-19.10.0 txaio-18.8.1 zope.interface-4.7.1
爲了安裝daphne，需要額外安裝這麼多依賴包！我們再pip list看一下：
(venv) D:\work\for_test\django3>pip list Package Version ---------------- ------- asgiref 3.2.3 attrs 19.3.0 autobahn 19.11.1 Automat 0.8.0 cffi 1.13.2 constantly 15.1.0 cryptography 2.8 daphne 2.4.0 Django 3.0 hyperlink 19.0.0 idna 2.8 incremental 17.5.0 pip 10.0.1 pyasn1 0.4.8 pyasn1-modules 0.2.7 pycparser 2.19 PyHamcrest 1.9.0 pyOpenSSL 19.1.0 pytz 2019.3 service-identity 18.1.0 setuptools 39.1.0 six 1.13.0 sqlparse 0.3.0 Twisted 19.10.0 txaio 18.8.1 zope.interface 4.7.1
不管了，就當沒看見。
安裝成功後，我們會獲得一個daphne可執行命令，下面我們來啓動服務器吧。
執行daphne django3.asgi:application命令。（將其中的django3換成你的工程名字，在manage.py文件所在的路徑下執行）
(venv) D:\work\for_test\django3>daphne django3.asgi:application 2019-12-04 10:20:07,637 INFO Starting server at tcp:port=8000:interface=127.0.0.1 2019-12-04 10:20:07,637 INFO HTTP/2 support not enabled (install the http2 and tls Twisted extras) 2019-12-04 10:20:07,637 INFO Configuring endpoint tcp:port=8000:interface=127.0.0.1 2019-12-04 10:20:07,637 INFO Listening on TCP address 127.0.0.1:8000
當然，我們也是可以指定ip和port等參數的，詳細請學習daphne文檔https://pypi.org/project/daphne/
讀一下人家的啓動信息：
- 默認啓動地址是127.0.0.1:8000
- 沒有開啓HTTP/2的支持（需要安裝額外的包）
- 配置了端點，開始監聽端口
Nice，通過瀏覽器來訪問一下吧！
依然是熟悉的白底火箭圖！圖片我就不貼了，看起來沒問題。
可是，這是基於HTTP的同步通信，還不是異步請求！讓我們嘗試一下websocket吧！
四、嘗試Websocket
瀏覽器中按F12進入‘坦克模式'，再進入console控制檯，嘗試發送一個Websocket請求：
忽視前面的css問題，重點在於這個ws請求，居然無法建立連接！返回狀態碼500，也就是服務器錯誤！
再看看Pycharm後臺的信息：
127.0.0.1:5991 - - [04/Dec/2019:10:30:05] "WSCONNECTING /" - - 2019-12-04 10:30:05,246 ERROR Exception inside application: Django can only handle ASGI/HTTP connections, not websocket. File "d:\work\for_test\django3\venv\lib\site-packages\daphne\cli.py", line 30, in asgi await self.app(scope, receive, send) File "d:\work\for_test\django3\venv\lib\site-packages\django\core\handlers\asgi.py", line 146, in __call__ % scope['type'] Django can only handle ASGI/HTTP connections, not websocket. 127.0.0.1:5991 - - [04/Dec/2019:10:30:05] "WSDISCONNECT /" - -
重點在這句Django can only handle ASGI/HTTP connections, not websocket.
什麼？Django不支持Websocket？說好的ASGI，說好的全雙工異步通信呢？
難道是我哪裏搞錯了？不行，我得看看源碼去！
五、Django3.0源碼
一路點點點，翻了個遍，發現Django3.0與之前的2.2關於ASGI的區別就是多了下面兩個文件：
django.core.asgi很簡單，一看就懂，不多說：
import django from django.core.handlers.asgi import ASGIHandler def get_asgi_application(): django.setup(set_prefix=False) return ASGIHandler()
關鍵是django.core.handlers.asgi這個文件，不到300行，總共就2個類，代碼就不貼了：
- ASGIRequest：繼承了HttpRequest類，一看就知道是構造請求對象
- ASGIHandler：繼承了base.BaseHandler，這個就類似WSGIHandler，用於具體處理ASGI請求
讓我們看看它的其中一段代碼：
# Serve only HTTP connections. # FIXME: Allow to override this. if scope['type'] != 'http': raise ValueError( 'Django can only handle ASGI/HTTP connections, not %s.' % scope['type'] )
這就是我們前面在瀏覽器中發送ws請求，但是無法建立連接的原因！
如果scope的type屬性不是http，那麼彈出ValueError異常，並提示不支持該類連接！
再看看人家寫的註釋，明明白白的寫着Serve only HTTP connections. FIXME: Allow to override this.。翻譯過來就是隻支持HTTP連接，請自己重寫這部分內容修復這個缺陷。FIXME！FIXME！
好吧，我承認白高興了一場。
六、總結
與Django3.0關於異步通信的初體驗很不好，有下面幾點猜測：
- 可能我水平不行，不會用Django
- 可能不是通過Websocket，而是別的手段與ASGI通信
- Django真的目前只提供了個接口，內部實現還沒做
由於相關資料太少，多方查找，據說：
- Django會在後續的版本陸續實現完整的原生的異步通信能力，從同步機制切換到可兼容的異步機制
- 目前3.0階段，要與websocket通信依然得通過django-channel庫，還不是原生支持
## 此文爲轉載，作者不詳