一个发挥到极致的yield案例

def find_commands(management_dir):
    """
    Given a path to a management directory, returns a list of all the command
    names that are available.

    Returns an empty list if no commands are defined.
    """
    command_dir = os.path.join(management_dir, 'commands')
    # l = []
    # for _, name, is_pkg in pkgutil.iter_modules([npath(command_dir)]): #生成器走一回，返回对象含多个元素
    #     if not is_pkg and not name.startswith('_'):
    #         l.append(name)
    # l结果：['check', 'compilemessages', 'createcachetable', 'dbshell', 'diffsettings', 'dumpdata', 'flush', 'inspectdb', 'loaddata', 'makemessages',
    # 'makemigrations', 'migrate', 'runserver', 'sendtestemail', 'shell', 'showmigrations', 'sqlflush', 'sqlmigrate', 'sqlsequencereset', 'squashmigrations',
    # 'startapp', 'startproject', 'test', 'testserver']

    return [name for _, name, is_pkg in pkgutil.iter_modules([npath(command_dir)])
            if not is_pkg and not name.startswith('_')]

def iter_modules(path=None, prefix=''):
    """Yields (module_loader, name, ispkg) for all submodules on path,
    or, if path is None, all top-level modules on sys.path.

    'path' should be either None or a list of paths to look for
    modules in.

    'prefix' is a string to output on the front of every module name
    on output.
    """

    if path is None:
        importers = iter_importers()
    else:
        importers = map(get_importer, path)

    yielded = {}
    for i in importers: #生成器走一回，返回对象只有一个元素
        for name, ispkg in iter_importer_modules(i, prefix): #生成器走一回，返回对象含多个元素，数量与path路径的文件数一致
            if name not in yielded:
                yielded[name] = 1
                yield i, name, ispkg


@simplegeneric
def iter_importer_modules(importer, prefix=''):
    if not hasattr(importer, 'iter_modules'):
        return []
    return importer.iter_modules(prefix)

def get_importer(path_item):
    """Retrieve a PEP 302 importer for the given path item

    The returned importer is cached in sys.path_importer_cache
    if it was newly created by a path hook.

    The cache (or part of it) can be cleared manually if a
    rescan of sys.path_hooks is necessary.
    """
    try:
        importer = sys.path_importer_cache[path_item]
    except KeyError:
        for path_hook in sys.path_hooks:
            try:
                importer = path_hook(path_item)
                sys.path_importer_cache.setdefault(path_item, importer)
                break
            except ImportError:
                pass
        else:
            importer = None
    return importer