Russell Owen: do you mind reviewing this small change? It adds a handful of tests of exception handling to DataIdContainer. There's a larger block of code that I'm not testing here because I don't understand how castDataIds is supposed to behave (somehow self.idList needs to get set to something ahead of time). A small improvement is better than nothing, anyway.

Jenkins run: https://ci.lsst.codes/blue/organizations/jenkins/stack-os-matrix/detail/stack-os-matrix/28229/pipeline

I'll have a look. For the record: dataIdList is a list of butler data IDs, which are simply dicts of key=value pairs. All values in the dicts start out as strings, because that is how they are read from the command line. Casting is used to convert keys to other types as needed (int is fairly common, for example "visit" should have an integer value). Clearly the documentation could use improvement.

Looks good. Thank you for doing this. I filed DM-15162 to improve the docs.

I had one minor request on github and will add another here: please use unittest.mock (which is in the standard library) instead of mock (a 3rd party package).

I'm confused: I'm using `unittest.mock`.

I was confused. You're right. Sorry.