The libary currently has www.googleapis.com/batch hardcoded as the endpoint for batch API requests unless caller manually sets batch_uri. See:

https://github.com/google/google-api-python-client/blob/master/googleapiclient/http.py#L1089

per Google Developers blog post this universal endpoint is being turned down in favor of per-API endpoints.

library needs to:

1. ensure all members of the batch are homogenous (same API) and throw an error if they're not.
2. utilize the batchPath from the individual API rather than the generic one, this would be something like:

service._rootDesc['rootUrl'] + service._rootDesc['batchPath']