Django源码解析（1）：启动程序

python manage.py runserver 8008

if __name__ == "__main__":
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "middleware_demo.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)

def execute_from_command_line(argv=None):
    """Run a ManagementUtility."""
    utility = ManagementUtility(argv)
    utility.execute()

utility = ManagementUtility(argv)

utility.execute()

    def execute(self):
        """
        Given the command-line arguments, figure out which subcommand is being
        run, create a parser appropriate to that command, and run 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.

        try:
            settings.INSTALLED_APPS
        except ImproperlyConfigured as exc:
            self.settings_exception = exc

        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

                    # Remove options not compatible with the built-in runserver
                    # (e.g. options for the contrib.staticfiles‘ runserver).
                    # Changes here require manually testing as described in
                    # #27522.
                    _parser = self.fetch_command(‘runserver‘).create_parser(‘django‘, ‘runserver‘)
                    _options, _args = _parser.parse_known_args(self.argv[2:])
                    for _arg in _args:
                        self.argv.remove(_arg)

            # 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)