Django源码分析之执行入口

魔法门

一般我们启动django，最简单的方法是进入project 目录，这时目录结构是这样的

然后我们执行python manage.py runserver，程序就开始执行了。

那django是如何从一个命令就启动整个server，启动的流程是如何的？

踏门而入

打开目录下的manage.py，内容是这样的：

#!/usr/bin/env pythonimport osimport sysif __name__ == "__main__":    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "django_learning.settings")    from django.core.management import execute_from_command_line    execute_from_command_line(sys.argv)

看来manage.py只是把命令行参数传给django.core.management模块中的execute_from_command_line 函数。

查看execute_from_command_line函数，可以发现实际执行的是ManagementUtility类的excute方法：

def execute(self):        """        Given the command-line arguments, this figures out which subcommand is        being run, creates a parser appropriate to that command, and runs it.        """        try:            subcommand = self.argv[1]        except IndexError:            subcommand = 'help'  # Display help if no arguments were given.        # Preprocess options to extract --settings and --pythonpath.        # These options could affect the commands that are available, so they        # must be processed early.        parser = CommandParser(None, usage="%(prog)s subcommand [options] [args]", add_help=False)        parser.add_argument('--settings')        parser.add_argument('--pythonpath')        parser.add_argument('args', nargs='*')  # catch-all        try:            options, args = parser.parse_known_args(self.argv[2:])            handle_default_options(options)        except CommandError:            pass  # Ignore any option errors at this point.        no_settings_commands = [            'help', 'version', '--help', '--version', '-h',            'compilemessages', 'makemessages',            'startapp', 'startproject',        ]        try:            settings.INSTALLED_APPS        except ImproperlyConfigured as exc:            self.settings_exception = exc            # A handful of built-in management commands work without settings.            # Load the default settings -- where INSTALLED_APPS is empty.            if subcommand in no_settings_commands:                settings.configure()        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            # In all other cases, django.setup() is required to succeed.            else:                django.setup()        self.autocomplete()        if subcommand == 'help':            if '--commands' in args:                sys.stdout.write(self.main_help_text(commands_only=True) + '\n')            elif len(options.args) < 1:                sys.stdout.write(self.main_help_text() + '\n')            else:                self.fetch_command(options.args[0]).print_help(self.prog_name, options.args[0])        # Special-cases: We want 'django-admin --version' and        # 'django-admin --help' to work, for backwards compatibility.        elif subcommand == 'version' or self.argv[1:] == ['--version']:            sys.stdout.write(django.get_version() + '\n')        elif self.argv[1:] in (['--help'], ['-h']):            sys.stdout.write(self.main_help_text() + '\n')        else:            self.fetch_command(subcommand).run_from_argv(self.argv)

其中

parser = CommandParser(None, usage="%(prog)s subcommand [options] [args]", add_help=False)        parser.add_argument('--settings')        parser.add_argument('--pythonpath')        parser.add_argument('args', nargs='*')  # catch-all        try:            options, args = parser.parse_known_args(self.argv[2:])            handle_default_options(options)        except CommandError:            pass  # Ignore any option errors at this point.

CommandParser其实类似于Argparse的一个解析命令行参数的类，从代码里可以看出我们可以直接在命令行指定settings文件和pythonpath。

no_settings_commands = [            'help', 'version', '--help', '--version', '-h',            'compilemessages', 'makemessages',            'startapp', 'startproject',        ]try:            settings.INSTALLED_APPSexcept ImproperlyConfigured as exc:            self.settings_exception = exc            # A handful of built-in management commands work without settings.            # Load the default settings -- where INSTALLED_APPS is empty.            if subcommand in no_settings_commands:                settings.configure()

这块代码就可以解释我们执行python manage.py start project 时django在背后会调用settings.configure方法，这里的settings是指django.conf.LazySettings的一个实例，configure方法其实就是使用django.conf.global_settings.py中的默认设置创建一份新的配置文件，作为我们新创建的project的settings.py

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            # In all other cases, django.setup() is required to succeed.            else:                django.setup()

autoreload.check_errors(django.setup)()其实也是调用django.setup方法，而django.setup方法

def setup():    """    Configure the settings (this happens as a side effect of accessing the    first setting), configure logging and populate the app registry.    """    from django.apps import apps    from django.conf import settings    from django.utils.log import configure_logging    configure_logging(settings.LOGGING_CONFIG, settings.LOGGING)    apps.populate(settings.INSTALLED_APPS)

负责初始化日志模块以及所有应用.

抽丝剥茧

剩下的代码最重要的就是这一句：

self.fetch_command(subcommand).run_from_argv(self.argv)

fetch_command会根据subcommand（这是我们执行python manage.py rumserver时传入的第二个参数：runserver），去django.core.management.commands中查找对应的command类，然后把所有命令行参数传给run_from_argv方法并执行，在runserver这个示例中，最终会调用django.utils.autoreload中的python_reloader或者jython_reloader新开一个线程：

def python_reloader(main_func, args, kwargs):    if os.environ.get("RUN_MAIN") == "true":        thread.start_new_thread(main_func, args, kwargs)        try:            reloader_thread()        except KeyboardInterrupt:            pass    else:        try:            exit_code = restart_with_reloader()            if exit_code < 0:                os.kill(os.getpid(), -exit_code)            else:                sys.exit(exit_code)        except KeyboardInterrupt:            pass

这里的main_func是commands/runserver.py中的inner_run方法：

def inner_run(self, *args, **options):        # If an exception was silenced in ManagementUtility.execute in order        # to be raised in the child process, raise it now.        autoreload.raise_last_exception()        threading = options.get('use_threading')        shutdown_message = options.get('shutdown_message', '')        quit_command = 'CTRL-BREAK' if sys.platform == 'win32' else 'CONTROL-C'        self.stdout.write("Performing system checks...\n\n")        self.check(display_num_errors=True)        self.check_migrations()        now = datetime.now().strftime('%B %d, %Y - %X')        if six.PY2:            now = now.decode(get_system_encoding())        self.stdout.write(now)        self.stdout.write((            "Django version %(version)s, using settings %(settings)r\n"            "Starting development server at http://%(addr)s:%(port)s/\n"            "Quit the server with %(quit_command)s.\n"        ) % {            "version": self.get_version(),            "settings": settings.SETTINGS_MODULE,            "addr": '[%s]' % self.addr if self._raw_ipv6 else self.addr,            "port": self.port,            "quit_command": quit_command,        })        try:            handler = self.get_handler(*args, **options)            run(self.addr, int(self.port), handler,                ipv6=self.use_ipv6, threading=threading)        except socket.error as e:            # Use helpful error messages instead of ugly tracebacks.            ERRORS = {                errno.EACCES: "You don't have permission to access that port.",                errno.EADDRINUSE: "That port is already in use.",                errno.EADDRNOTAVAIL: "That IP address can't be assigned to.",            }            try:                error_text = ERRORS[e.errno]            except KeyError:                error_text = force_text(e)            self.stderr.write("Error: %s" % error_text)            # Need to use an OS exit because sys.exit doesn't work in a thread            os._exit(1)        except KeyboardInterrupt:            if shutdown_message:                self.stdout.write(shutdown_message)            sys.exit(0)

最关键的是这两条语句：

handler = self.get_handler(*args, **options)run(self.addr, int(self.port), handler,ipv6=self.use_ipv6, threading=threading)

get_handler会返回django.core.servers.basehttp中定义的一个application（其实就是我们project下的wigs.py中定义的application）

这是run函数的内容

def run(addr, port, wsgi_handler, ipv6=False, threading=False):    server_address = (addr, port)    if threading:        httpd_cls = type(str('WSGIServer'), (socketserver.ThreadingMixIn, WSGIServer), {})    else:        httpd_cls = WSGIServer    httpd = httpd_cls(server_address, WSGIRequestHandler, ipv6=ipv6)    if threading:        # ThreadingMixIn.daemon_threads indicates how threads will behave on an        # abrupt shutdown; like quitting the server by the user or restarting        # by the auto-reloader. True means the server will not wait for thread        # termination before it quits. This will make auto-reloader faster        # and will prevent the need to kill the server manually if a thread        # isn't terminating correctly.        httpd.daemon_threads = True    httpd.set_app(wsgi_handler)    httpd.serve_forever()

可以看出run函数其实就是启动一个WSGIServer实例（WSGIServer继承python内置类simple_server.WSGIServer），并把handler设置为前面get_handler的返回值

水落石出

这样，一条python manage.py runserver命令的执行生命周期就一览无余了。
接下来，server就开始接收请求了。