Replace all suitable lists with generators

[tlelson]

Particularly with the Dynamo load stuff. These could be very large.

The reason its not easy is because of testing the output. Need to work out how.

Perhaps, side_effect perhaps CopyMock

https://stackoverflow.com/questions/40248075/how-do-we-ensure-that-the-calls-in-the-mock-call-args-list-contain-calls-with-ar