django1/django/contrib/messages/storage/fallback.py

from django.contrib.messages.storage.base import BaseStorage
from django.contrib.messages.storage.cookie import CookieStorage
from django.contrib.messages.storage.session import SessionStorage


class FallbackStorage(BaseStorage):
    """
    Try to store all messages in the first backend. Store any unstored
    messages in each subsequent backend.
    """
    storage_classes = (CookieStorage, SessionStorage)

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.storages = [storage_class(*args, **kwargs)
                         for storage_class in self.storage_classes]
        self._used_storages = set()

    def _get(self, *args, **kwargs):
        """
        Get a single list of messages from all storage backends.
        """
        all_messages = []
        for storage in self.storages:
            messages, all_retrieved = storage._get()
            # If the backend hasn't been used, no more retrieval is necessary.
            if messages is None:
                break
            if messages:
                self._used_storages.add(storage)
            all_messages.extend(messages)
            # If this storage class contained all the messages, no further
            # retrieval is necessary
            if all_retrieved:
                break
        return all_messages, all_retrieved

    def _store(self, messages, response, *args, **kwargs):
        """
        Store the messages and return any unstored messages after trying all
        backends.

        For each storage backend, any messages not stored are passed on to the
        next backend.
        """
        for storage in self.storages:
            if messages:
                messages = storage._store(messages, response, remove_oldest=False)
            # Even if there are no more messages, continue iterating to ensure
            # storages which contained messages are flushed.
            elif storage in self._used_storages:
                storage._store([], response)
                self._used_storages.remove(storage)
        return messages