Python Extension Development (Labs): View node with output table

Hi all,

using the Python Extension Development API I’m trying to develop a view node which displays a matplotlib plot and returns an output table at the same time.

Unluckily, during execution this results in the following error:

Execute failed: no view could be created for ArrowSourceTable[cols=28, rows=6, batches=1]

Please see the following MWE:

import knime.api as knapi
import knime.extension as knext  # type: ignore[import-untyped]
import matplotlib as mpl
import matplotlib.pyplot as plt
@knext.node(
name="Plot stuff and return data",
node_type=knext.NodeType.VISUALIZER,
category="/community/my-views/,
)
@knext.input_table(
name="some input", description="blah",
)
@knext.output_view(
name="my shiny plot", description="also blah"
)
@knext.output_table(
name="table to return", description="something"
)
class PlotAndReturnTable:
"""Some docstring."""
def configure(
self,
configure_context: knext.ConfigurationContext,
some_input_schema: knext.Schema,
) -> knext.Schema:  # <<<<<<<<<<<<<<<<<<<<<<<<<<<<< how to annotate the return types when there is a view?
"""
Configure the Python node.
Parameters
----------
configure_context
_description_
some_input_schema
_description_
Returns
-------
knext.Schema
_description_
"""
return some_input_schema
def execute(
self,
exec_context: knext.ExecutionContext,
some_input_data: knext.Table,
) -> tuple[knapi.views.NodeView, knext.Table]:
"""
Execute node.
Parameters
----------
exec_context
_description_
some_input_data
_description_
Returns
-------
tuple[knapi.views.NodeView, knext.Table]
_description_
"""
# Plot something:
line_hndls: list[mpl.lines.Line2D] = plt.plot([0, 1], [2, 3])
return knext.view_matplotlib(), some_input_data

Additional findings:

• NodeView is not available as part of knime.extensions, but required to correctly annotate types. We need to use knime.api.views.NodeView which is probably not very straightforward to find.

• Without the output table, the plot is shown correctly. But when I open the plot in a separate window and close it afterwards, the workflow pane turns white and all my nodes are “gone” - at least visually -, forcing me to restart Knime.

I’m using Knime AP 5.8.1 LTS on Linux.

Any idea how to implement/fix this?

Thanks in advance and best regards,

Johannes