def process_file(self, filepath, only_if_updated=True, safe_mode=True):
        """
        
        """
        from airflow.models.dag import DAG  # Avoid circular import

        found_dags = []

        # if the source file no longer exists in the DB or in the filesystem,
        # return an empty list
        # todo: raise exception?
        if filepath is None or not os.path.isfile(filepath):
            return found_dags

        try:
            # This failed before in what may have been a git sync
            # race condition
            file_last_changed_on_disk = datetime.fromtimestamp(os.path.getmtime(filepath))
            if only_if_updated \
                    and filepath in self.file_last_changed \
                    and file_last_changed_on_disk == self.file_last_changed[filepath]:
                return found_dags

        except Exception as e:
            self.log.exception(e)
            return found_dags

        mods = []
        is_zipfile = zipfile.is_zipfile(filepath)
        if not is_zipfile:
            if safe_mode:
                with open(filepath, 'rb') as f:
                    content = f.read()
                    if not all([s in content for s in (b'DAG', b'airflow')]):
                        self.file_last_changed[filepath] = file_last_changed_on_disk
                        # Don't want to spam user with skip messages
                        if not self.has_logged:
                            self.has_logged = True
                            self.log.info(
                                "File %s assumed to contain no DAGs. Skipping.",
                                filepath)
                        return found_dags

            self.log.debug("Importing %s", filepath)
            org_mod_name, _ = os.path.splitext(os.path.split(filepath)[-1])
            mod_name = ('unusual_prefix_' +
                        hashlib.sha1(filepath.encode('utf-8')).hexdigest() +
                        '_' + org_mod_name)

            if mod_name in sys.modules:
                del sys.modules[mod_name]

            with timeout(configuration.conf.getint('core', "DAGBAG_IMPORT_TIMEOUT")):
                try:
                    m = imp.load_source(mod_name, filepath)
                    mods.append(m)
                except Exception as e:
                    self.log.exception("Failed to import: %s", filepath)
                    self.import_errors[filepath] = str(e)
                    self.file_last_changed[filepath] = file_last_changed_on_disk

        else:
            zip_file = zipfile.ZipFile(filepath)
            for mod in zip_file.infolist():
                head, _ = os.path.split(mod.filename)
                mod_name, ext = os.path.splitext(mod.filename)
                if not head and (ext == '.py' or ext == '.pyc'):
                    if mod_name == '__init__':
                        self.log.warning("Found __init__.%s at root of %s", ext, filepath)
                    if safe_mode:
                        with zip_file.open(mod.filename) as zf:
                            self.log.debug("Reading %s from %s", mod.filename, filepath)
                            content = zf.read()
                            if not all([s in content for s in (b'DAG', b'airflow')]):
                                self.file_last_changed[filepath] = (
                                    file_last_changed_on_disk)
                                # todo: create ignore list
                                # Don't want to spam user with skip messages
                                if not self.has_logged:
                                    self.has_logged = True
                                    self.log.info(
                                        "File %s assumed to contain no DAGs. Skipping.",
                                        filepath)

                    if mod_name in sys.modules:
                        del sys.modules[mod_name]

                    try:
                        sys.path.insert(0, filepath)
                        m = importlib.import_module(mod_name)
                        mods.append(m)
                    except Exception as e:
                        self.log.exception("Failed to import: %s", filepath)
                        self.import_errors[filepath] = str(e)
                        self.file_last_changed[filepath] = file_last_changed_on_disk

        for m in mods:
            for dag in list(m.__dict__.values()):
                if isinstance(dag, DAG):
                    if not dag.full_filepath:
                        dag.full_filepath = filepath
                        if dag.fileloc != filepath and not is_zipfile:
                            dag.fileloc = filepath
                    try:
                        dag.is_subdag = False
                        self.bag_dag(dag, parent_dag=dag, root_dag=dag)
                        if isinstance(dag._schedule_interval, six.string_types):
                            croniter(dag._schedule_interval)
                        found_dags.append(dag)
                        found_dags += dag.subdags
                    except (CroniterBadCronError,
                            CroniterBadDateError,
                            CroniterNotAlphaError) as cron_e:
                        self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
                        self.import_errors[dag.full_filepath] = \
                            "Invalid Cron expression: " + str(cron_e)
                        self.file_last_changed[dag.full_filepath] = \
                            file_last_changed_on_disk
                    except AirflowDagCycleException as cycle_exception:
                        self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
                        self.import_errors[dag.full_filepath] = str(cycle_exception)
                        self.file_last_changed[dag.full_filepath] = \
                            file_last_changed_on_disk

        self.file_last_changed[filepath] = file_last_changed_on_disk
        return found_dags