Get All Placed/Unplaced Views

views = FilteredElementCollector(doc).OfCategory(BuiltInCategory.OST_Views).WhereElementIsNotElementType().ToElements()
unplaced_views = [view for view in views if not view.get_Parameter(BuiltInParameter.VIEWPORT_SHEET_NUMBER).AsString()]
placed_views   = [view for view in views if view.get_Parameter(BuiltInParameter.VIEWPORT_SHEET_NUMBER).AsString()]