当唔异常时，else方法也执行
try:
# If import_module succeeds, entry is a path to an app module,
# which may specify an app config class with default_app_config.
# Otherwise, entry is a path to an app config class or an error.
module = import_module(entry)
except ImportError:
# Track that importing as an app module failed. If importing as an
# app config class fails too, we'll trigger the ImportError again.
module = None
mod_path, _, cls_name = entry.rpartition('.')
# Raise the original exception when entry cannot be a path to an
# app config class.
if not mod_path:
raise
else:
try:
# If this works, the app module specifies an app config class.
entry = module.default_app_config
except AttributeError:
# Otherwise, it simply uses the default app config class.
return cls(entry, module)
else:
mod_path, _, cls_name = entry.rpartition('.')
