django源码分析 python manage.py runserver

django是一个快速开发web应用的框架， 笔者也在django框架上开发不少web应用，闲来无事，就想探究一下django底层到底是如何实现的，本文记录了笔者对django源码的分析过程

I believe to become a better developer you MUST get a better understanding of the underlying software systems you use on a daily basis and that includes programming languages, compilers and interpreters, databases and operating systems, web servers and web frameworks. And, to get a better and deeper understanding of those systems you MUST re-build them from scratch, brick by brick, wall by wall.

笔者摘抄了一段话，送给阅读本文的读者

正文

如何分析django源码，笔者选择从django项目的启动方式开始 python manage.py runserver，本文主要分析了django项目的启动流程

#!/usr/bin/env python
import os
import sys

if __name__ == "__main__":
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "order.settings")
    try:
        from django.core.management import execute_from_command_line
    except ImportError:
        # The above import may fail for some other reason. Ensure that the
        # issue is really that Django is missing to avoid masking other
        # exceptions on Python 2.
        try:
            import django
        except ImportError:
            raise ImportError(
                "Couldn't import Django. Are you sure it's installed and "
                "available on your PYTHONPATH environment variable? Did you "
                "forget to activate a virtual environment?"
            )
        raise
    execute_from_command_line(sys.argv)

在manage.py文件中，我们看到启动文件的入口是 excute_from_command_line(sys.argv)

def execute_from_command_line(argv=None):
    """
    A simple method that runs a ManagementUtility.
    """
    utility = ManagementUtility(argv)
    utility.execute()

这个函数是将命令行参数传递给了ManagementUtility类，这个类的execute方法负责执行，这个方法主要是一些django的初始化参数的检查，以及通过sys.argv获取命令，得到相应的命令后，执行命令。

execute方法中的部分代码
...
if settings.configured:
    # Start the auto-reloading dev server even if the code is broken.
    # The hardcoded condition is a code smell but we can't rely on a
    # flag on the command class because we haven't located it yet.
    if subcommand == 'runserver' and '--noreload' not in self.argv:
        try:
            autoreload.check_errors(django.setup)()
        except Exception:
            # The exception will be raised later in the child process
            # started by the autoreloader. Pretend it didn't happen by
            # loading an empty list of applications.
            apps.all_models = defaultdict(OrderedDict)
            apps.app_configs = OrderedDict()
            apps.apps_ready = apps.models_ready = apps.ready = True
...

execute方法中有一段代码autoreload.check_errors(django.setup)()，会对django项目进行一些必要的初始化，并检查初始化的错误 django.setup()方法会注册项目app和配置日志文件，注册app即对settings.INSTALLED_APPS中的app进行导入，并执行一些初始化方法

进行完所有初始化动作，继续执行代码

execute方法中的部分代码
...
elif self.argv[1:] in (['--help'], ['-h']):
    sys.stdout.write(self.main_help_text() + '
')
else:
    self.fetch_command(subcommand).run_from_argv(self.argv)
...

self.fetch_command(subcommand)会返回一个BaseCommand类，主要是分析subcommand参数（subcommand是sys.argv里面获取到的)，导入相应的命令类，最后返回类

我们通过分析，runserver参数最终获取到的命令类是django/contrib/staticfiles/management/command/runserver.py 里的Command类

这是Command类的继承关系图。Command类通过run_from_argv(self.argv)执行命令

BaseCommand类中run_from_argv方法的部分代码
...
try:
    self.execute(*args, **cmd_options)
except Exception as e:
    if options.traceback or not isinstance(e, CommandError):
        raise
...

run_from_argv(self.argv)方法中主要通过execute()来继续执行，excute中会对django项目进行检查，然后通过self.handle()继续执行

RunserverCommand类里面的handle方法部分代码

def handle(self, *args, **options):
　　...
　　if not self.addr:
        self.addr = '::1' if self.use_ipv6 else '127.0.0.1'
        self._raw_ipv6 = self.use_ipv6
    self.run(**options)

handle()方法里面也进行了一些检查，然后继续执行self.run()来启动服务器

RunserverCommand中的部分代码
def run(self, **options):
    """
    Runs the server, using the autoreloader if needed
    """
    use_reloader = options['use_reloader']

    if use_reloader:
        autoreload.main(self.inner_run, None, options)
    else:
        self.inner_run(None, **options)

def inner_run(self, *args, **options):
    ...

    try:
        handler = self.get_handler(*args, **options)
        run(self.addr, int(self.port), handler,
            ipv6=self.use_ipv6, threading=threading, server_cls=self.server_cls)
    except socket.error as e:
    
    ...

run方法中选择了启动的解释器，最后都是通过inner_run中的run方法来执行，会启动一个WSGIServer， WSGIServer需要一个回调函数handler(或者application)，来执行django视图里面代码。

至此，django项目服务器启动流程完毕，启动了一个简单的WSGIServer，开始接受请求，解析请求参数，将请求参数传递给回调函数handler(或者application，django框架的核心内容），handler根据参数执行相应的代码，返回数据给WSGIServer，WSGIServer最终将数据返回给浏览器。

关于wsgi可以参考这篇文章，理解Python WSGI

总结：

我认为django启动流程中对于我们开发者最重要的一步在于django.setup()，里面做了很多初始化的工作，包括导入各个app的models，运行各个app的run函数，配置日志文件。我们如果想要在项目的启动的时候做一些我们自己的初始化动作，可以选择在这个地方下手。