set/get FieldData with numpy_interface.dataset_adapter

Hello, I am doing experiments, trying to set and get FieldData on e.g. a PolyData, UnstructuredGrid, etc., with numpy_interface.dataset_adapter. I am particularity interested in string FieldData, used to store various attributed of a vtk object, but also in numeric arrays.

However if I try to use the same syntax as for Points, I always get errors.

import vtk
from vtk.numpy_interface import dataset_adapter as dsa
import numpy as np

my_polydata = vtk.vtkPolyData()
# add some points and cells to my_polydata here..

my_polydata_dsa = dsa.WrapDataObject(my_polydata)

# set string example
my_polydata_dsa.FieldData["key_1"] = "string_1"

>>> TypeError: 'DataSetAttributes' object does not support item assignment

# numeric example
my_polydata_dsa.FieldData["key_2"] = [1.0, 2.0, 3.0]

>>> TypeError: 'DataSetAttributes' object does not support item assignment

# get examples (do not work due to previous errors)
print(my_polydata_dsa.FieldData["key_1"])
print(my_polydata_dsa.FieldData["key_2"])

I also tried with .FieldData.keys(), .FieldData.values(), .FieldData.append(), but I cannot understand the right syntax.

Thanks very much!