https://github.com/googleapis/google-api-python-client/blob/master/docs/oauth-server.md makes reference to SERVICE_ACCOUNT_FILE but doesn't explain where it comes from or what it contains.

Digging deeper leads me to https://google-auth.readthedocs.io/en/latest/reference/google.oauth2.service_account.html#google.oauth2.service_account.Credentials which says:

"To create credentials using a Google service account private key JSON file"

but that doesn't explain how or where to get the file from either.

I did end up discovering https://stackoverflow.com/questions/46287267/how-can-i-get-the-file-service-account-json-for-google-translate-api which also confirms that if I haven't got the JSON file, I've got to create a new service account :(.

It would help if the documentation made the steps to setting up the service account clearer ...