摘要: 基于Django version 1.11.0 . alpha版本

鑒于筆者水平有限沪编，文中不免出現(xiàn)一些錯(cuò)誤寸爆，還請多多指教苍凛！

好了龄寞，下邊是正文....
首先大概看一下Django 項(xiàng)目的主要目錄壁拉，初步建立一下Django源碼的世界觀谬俄。

├── django          //工程代碼存放路徑
├── docs            //文檔
├── extras          
├── js_tests        //測試
├── scripts         //腳本
└── tests           //單元測試

Django核心代碼主要在django目錄下邊

django/
├── apps（app模塊）
├── bin（可執(zhí)行命令）
├── conf（配置）
├── contrib（其他開發(fā)者貢獻(xiàn)代碼）
├── core（核心組件）
├── db（ORM模塊）
├── dispatch
├── forms（表單模塊）
├── http
├── middleware（中間件）
├── template（模板）
├── templatetags
├── test（測試代碼）
├── urls（url路由模塊）
├── utils（工具代碼）
└── views（視圖模塊）

在django中我們常用的命令主要有兩個(gè)，一個(gè)是django-admin弃理，一個(gè)是xxxx,我們先看一下django-admin
1溃论、命令位置

lion@localhost:~/django/django$ whereis django-admin
django-admin: /usr/local/bin/django-admin /usr/local/bin/django-admin.py /usr/local/bin/django-admin.pyc

2、命令內(nèi)容

lion@localhost:~/django/django$ cat /usr/local/bin/django-admin
#!/usr/bin/python

# -*- coding: utf-8 -*-
import re
import sys

from django.core.management import execute_from_command_line

if __name__ == '__main__':
    sys.argv[0] = re.sub(r'(-script\.pyw?|\.exe)?$', '', sys.argv[0])
    sys.exit(execute_from_command_line())

其實(shí)對比不難發(fā)現(xiàn)案铺，django-admin命令其實(shí)對應(yīng)的是django源碼中的.django/bin/django-admin.py這個(gè)文件蔬芥。
django-admin.py 引用了django.core中的management，并調(diào)用了其execute_from_command_line函數(shù)控汉。
注：在最新版中django-admin和manage.py中調(diào)用的都是execute_from_command_line函數(shù)了笔诵，較舊版本的django中可能不同。
所以要分析django的命令系統(tǒng)姑子，就要從execute_from_command_line函數(shù)入手乎婿。
execute_from_command_line函數(shù)定義：

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

函數(shù)初始化ManagementUtility類，傳入argv（也就是命令行參數(shù)）參數(shù)街佑，并執(zhí)行execute方法
execute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',
        'startapp', 'startproject', 'compilemessages',
    ]

    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)

此方法主要解析命令行參數(shù)谢翎，加載settings配置捍靠，如果setting配置成功則執(zhí)行django.setup函數(shù)（此函數(shù)主要是加載App），最后一步調(diào)用的核心命令為fetch_command命令森逮，并執(zhí)行run_from_argv函數(shù)
先看一下fetch_command函數(shù)

def fetch_command(self, subcommand):
    """
    Tries to fetch the given subcommand, printing a message with the
    appropriate command called from the command line (usually
    "django-admin" or "manage.py") if it can't be found.
    """
    # Get commands outside of try block to prevent swallowing exceptions
    commands = get_commands()
    try:
        app_name = commands[subcommand]
    except KeyError:
        if os.environ.get('DJANGO_SETTINGS_MODULE'):
            # If `subcommand` is missing due to misconfigured settings, the
            # following line will retrigger an ImproperlyConfigured exception
            # (get_commands() swallows the original one) so the user is
            # informed about it.
            settings.INSTALLED_APPS
        else:
            sys.stderr.write("No Django settings specified.\n")
        sys.stderr.write(
            "Unknown command: %r\nType '%s help' for usage.\n"
            % (subcommand, self.prog_name)
        )
        sys.exit(1)
    if isinstance(app_name, BaseCommand):
        # If the command is already loaded, use it directly.
        klass = app_name
    else:
        klass = load_command_class(app_name, subcommand)
    return klass

這個(gè)fetch_command函數(shù)類似一個(gè)工廠函數(shù)榨婆，由get_commands函數(shù)掃描出所有的子命令，包括managemen中的子命令和app下的managemen中commands的子命令（自定義）褒侧，然后根據(jù)傳入的subcommand初始化Command類良风。
如果子命令不在commands字典內(nèi)的話，會(huì)拋出一個(gè)“Unknown command”的提示闷供，如果子命令存在則返回初始化的Command類烟央。
接著視角在返回到execute函數(shù)中，接著

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

將會(huì)調(diào)用fetch_command(subcommand)初始化Command類的run_from_argv方法歪脏。run_from_argv由各個(gè)Command的基類BaseCommand定義疑俭，最終將會(huì)調(diào)用各個(gè)子類實(shí)現(xiàn)的handle方法。從而執(zhí)行子命令的業(yè)務(wù)邏輯婿失。
至此钞艇，命令調(diào)用的邏輯基本完成。

筆者隨筆：
通過閱讀這一部分的代碼移怯，其中最值得學(xué)習(xí)的地方在于fetch_commands函數(shù)香璃，這是一個(gè)運(yùn)用工廠方法的最佳實(shí)踐，這樣不但最大程度的解耦了代碼實(shí)現(xiàn)舟误，同時(shí)使得命令系統(tǒng)更易于擴(kuò)展（App 自定義子命令就是一個(gè)很好的說明）
再有一點(diǎn)就是Command基類的定義葡秒，對于各種子命令的定義，基類完整的抽象出了command業(yè)務(wù)的工作邏輯嵌溢，提供了統(tǒng)一的命令調(diào)用接口使得命令系統(tǒng)更易于擴(kuò)展眯牧。