def _init_flow_exceptions():
global _flow_exceptions<EOL>_flow_exceptions = ()<EOL>add_flow_exception(datastore_errors.Rollback)<EOL>try:<EOL><INDENT>from webob import exc<EOL><DEDENT>except ImportError:<EOL><INDENT>pass<EOL><DEDENT>else:<EOL><INDENT>add_flow_exception(exc.HTTPException)<EOL><DEDENT>
Internal helper to initialize _flow_exceptions.

    This automatically adds webob.exc.HTTPException, if it can be imported.