root/django/trunk/django/contrib/admin/__init__.py

from django.contrib.admin.options import ModelAdmin, HORIZONTAL, VERTICAL
from django.contrib.admin.options import StackedInline, TabularInline
from django.contrib.admin.sites import AdminSite, site

def autodiscover():
    """
    Auto-discover INSTALLED_APPS admin.py modules and fail silently when 
    not present. This forces an import on them to register any admin bits they
    may want.
    """
    import imp
    from django.conf import settings

    for app in settings.INSTALLED_APPS:
        # For each app, we need to look for an admin.py inside that app's
        # package. We can't use os.path here -- recall that modules may be
        # imported different ways (think zip files) -- so we need to get
        # the app's __path__ and look for admin.py on that path.

        # Step 1: find out the app's __path__ Import errors here will (and
        # should) bubble up, but a missing __path__ (which is legal, but weird)
        # fails silently -- apps that do weird things with __path__ might
        # need to roll their own admin registration.
        try:
            app_path = __import__(app, {}, {}, [app.split('.')[-1]]).__path__
        except AttributeError:
            continue

        # Step 2: use imp.find_module to find the app's admin.py. For some
        # reason imp.find_module raises ImportError if the app can't be found
        # but doesn't actually try to import the module. So skip this app if
        # its admin.py doesn't exist
        try:
            imp.find_module('admin', app_path)
        except ImportError:
            continue

        # Step 3: import the app's admin file. If this has errors we want them
        # to bubble up.
        __import__("%s.admin" % app)