boards/abstracts/attachment_alias.py

from boards.abstracts.settingsmanager import SessionSettingsManager
from boards.models import Attachment


class AttachmentAlias:
    def get_image(self, alias):
        pass


class SessionAttachmentAlias(AttachmentAlias):
    def __init__(self, session):
        self.session = session

    def get_image(self, alias):
        settings_manager = SessionSettingsManager(self.session)
        return settings_manager.get_attachment_by_alias(alias)


class ModelAttachmentAlias(AttachmentAlias):
    def get_image(self, alias):
        return Attachment.objects.get_by_alias(alias)


def get_image_by_alias(alias, session):
    image = SessionAttachmentAlias(session).get_image(alias)\
            or ModelAttachmentAlias().get_image(alias)

    if image is not None:
        return image