Get Workflow with UUIDv4.

    :param workflow_uuid: UUIDv4 of a Workflow.
    :type workflow_uuid: String representing a valid UUIDv4.

    :rtype: reana-db.models.Workflow

def _get_workflow_by_uuid(workflow_uuid):
    """Get Workflow with UUIDv4.

    :param workflow_uuid: UUIDv4 of a Workflow.
    :type workflow_uuid: String representing a valid UUIDv4.

    :rtype: reana-db.models.Workflow
    """
    from reana_db.models import Workflow
    workflow = Workflow.query.filter(Workflow.id_ ==
                                     workflow_uuid).first()
    if not workflow:
        raise ValueError(
            'REANA_WORKON is set to {0}, but '
            'that workflow does not exist. '
            'Please set your REANA_WORKON environment '
            'variable appropriately.'.
            format(workflow_uuid))
    return workflow