Dave: what seems to happen here is that mock correctly reporting to you that the object you passed to the mocked func is currently `{}`. What you probably expected it to be is to be equal to what it was at exact time when it was passed, i.e. before it was modified inside the method.

That's a reasonable expectation but it means it would have to be deepcopied, which could be expensive for large structures.

Do you think that noting this nuance in the docs for call_list_args would have helped you debug it in this case?