confusing code snippet when registers all standard API extensions

[hougangliu]

nova/api/openstack/extensions.py:def load_standard_extensions(

    for dirpath, dirnames, filenames in os.walk(our_dir):
        # Compute the relative package name from the dirpath
        relpath = os.path.relpath(dirpath, our_dir)
        if relpath == '.':
            relpkg = ''
        else:
            relpkg = '.%s' % '.'.join(relpath.split(os.sep))

        # Now, consider each file in turn, only considering .py files
        for fname in filenames:
            root, ext = os.path.splitext(fname)

            # Skip __init__ and anything that's not .py
            if ext != '.py' or root == '__init__':
                continue

            # Try loading it
            classname = "%s%s" % (root[0].upper(), root[1:])
            classpath = ("%s%s.%s.%s" %
                         (package, relpkg, root, classname))

            if ext_list is not None and classname not in ext_list:
                logger.debug("Skipping extension: %s" % classpath)
                continue

            try:
                ext_mgr.load_extension(classpath)
            except Exception as exc:
                logger.warn(_('Failed to load extension %(classpath)s: '
                              '%(exc)s'),
                            {'classpath': classpath, 'exc': exc})

        # Now, let's consider any subdirectories we may have...
        subdirs = []
        for dname in dirnames:
            # Skip it if it does not have __init__.py
            if not os.path.exists(os.path.join(dirpath, dname, '__init__.py')):
                continue

            # If it has extension(), delegate...
            ext_name = "%s%s.%s.extension" % (package, relpkg, dname)
            try:
                ext = importutils.import_class(ext_name)
            except ImportError:
                # extension() doesn't exist on it, so we'll explore
                # the directory for ourselves
                subdirs.append(dname)
            else:
                try:
                    ext(ext_mgr)
                except Exception as exc:
                    logger.warn(_('Failed to load extension %(ext_name)s:'
                                  '%(exc)s'),
                                {'ext_name': ext_name, 'exc': exc})

        # Update the list of directories we'll explore...
        dirnames[:] = subdirs >>>>this is unused, so we can remove subdirs

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/140028

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/140028
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=23eb5555ee1ddc60a6b6e22f8a9713314265b88c
Submitter: Jenkins
Branch: master

commit 23eb5555ee1ddc60a6b6e22f8a9713314265b88c
Author: Joe Gordon <email address hidden>
Date: Mon Dec 8 10:32:17 2014 -0500

    Clarify point of setting dirname in load_standard_extensions

    Changing dirname to alter how os.walk visits directories is a little
    known feature, so add a note inline explaining how it works.

    This came up because someone filed a bug saying this block of code was
    confusing.

    Closes-Bug: #1399546
    Change-Id: I4cab323fcc7a4e2aab2ca7c36d6b9505b6a05bcb