Online ArcGIS resource help provides a list of layer properties which can be used as needed. I am not able to find a way to collapse a layer using ArcPy in it. Please suggest if there is a way to collapse a layer in current data frame.
It may not be a great solution but if you have saved layer files with them collapsed then RemoveLayer followed by InsertLayer sounds like it could do it:
However, my suggestion is that you vote for this ArcGIS Idea.