KeyError while using Altair with Panel

[labdmitriy]

When I try to use Altair with Panel to show geo map with default data transformer in jupyterlab, I have a KeyError that probably is similar to error in #422.
I've updated altair/vega/panel/jupyter packages to the latest versions before experiments.
The code is the following:

alt.data_transformers.enable('default', max_rows=None)

areas = ['Country', 'Sub-Region', 'Region']
area = pn.widgets.Select(name='Area type:', options=areas)

@pn.depends(area.param.value)
def plot_map(area):
    counter_name = area.lower() + '_count'
    countries = alt.topo_feature(data.world_110m.url, 'countries')
    
    country_map = alt.Chart(countries).mark_geoshape(
    ).encode(
        color=f'{counter_name}:Q',
        tooltip=[
            alt.Tooltip('region:N', title='Region'),
            alt.Tooltip('sub-region:N', title='Sub-Region'),
            alt.Tooltip('answer:N', title='Country'),
            alt.Tooltip(f'{counter_name}:Q', title=f'Questions Count by {area}')
        ]
    ).transform_lookup(
        lookup='id',
        from_=alt.LookupData(merged_stats_df, 'id', merged_stats_df.columns.tolist())
    ).project(
        type='mercator'
    ).properties(
        width=600,
        height=400
    )

    borders = alt.Chart(countries).mark_geoshape(
        fill='white',
        stroke='grey',
        strokeWidth=1
    ).transform_filter(
        datum.id!=10
    ).properties(
        width=600,
        height=400
    )
   
    return (borders + country_map)

pn.Row(
    pn.Column(area),
    plot_map
).servable()

Traceback:

KeyError                                  Traceback (most recent call last)
/opt/conda/lib/python3.6/site-packages/IPython/core/formatters.py in __call__(self, obj, include, exclude)
    968 
    969             if method is not None:
--> 970                 return method(include=include, exclude=exclude)
    971             return None
    972         else:

/opt/conda/lib/python3.6/site-packages/panel/viewable.py in _repr_mimebundle_(self, include, exclude)
    292         comm = state._comm_manager.get_server_comm()
    293         doc = _Document()
--> 294         model = self._render_model(doc, comm)
    295         if config.embed:
    296             return render_model(model)

/opt/conda/lib/python3.6/site-packages/panel/viewable.py in _render_model(self, doc, comm)
    261         if comm is None:
    262             comm = state._comm_manager.get_server_comm()
--> 263         model = self.get_root(doc, comm)
    264 
    265         if config.embed:

/opt/conda/lib/python3.6/site-packages/panel/viewable.py in get_root(self, doc, comm)
    416         """
    417         doc = doc or _curdoc()
--> 418         root = self._get_model(doc, comm=comm)
    419         self._preprocess(root)
    420         ref = root.ref['id']

/opt/conda/lib/python3.6/site-packages/panel/layout.py in _get_model(self, doc, root, parent, comm)
    113         if root is None:
    114             root = model
--> 115         objects = self._get_objects(model, [], doc, root, comm)
    116         props = dict(self._init_properties(), objects=objects)
    117         model.update(**self._process_param_change(props))

/opt/conda/lib/python3.6/site-packages/panel/layout.py in _get_objects(self, model, old_objects, doc, root, comm)
    105                 child, _ = pane._models[root.ref['id']]
    106             else:
--> 107                 child = pane._get_model(doc, root, model, comm)
    108             new_models.append(child)
    109         return new_models

/opt/conda/lib/python3.6/site-packages/panel/param.py in _get_model(self, doc, root, parent, comm)
    614         if ref in self._models:
    615             self._cleanup(root)
--> 616         model = self._inner_layout._get_model(doc, root, parent, comm)
    617         self._models[ref] = (model, parent)
    618         return model

/opt/conda/lib/python3.6/site-packages/panel/layout.py in _get_model(self, doc, root, parent, comm)
    113         if root is None:
    114             root = model
--> 115         objects = self._get_objects(model, [], doc, root, comm)
    116         props = dict(self._init_properties(), objects=objects)
    117         model.update(**self._process_param_change(props))

/opt/conda/lib/python3.6/site-packages/panel/layout.py in _get_objects(self, model, old_objects, doc, root, comm)
    105                 child, _ = pane._models[root.ref['id']]
    106             else:
--> 107                 child = pane._get_model(doc, root, model, comm)
    108             new_models.append(child)
    109         return new_models

/opt/conda/lib/python3.6/site-packages/panel/pane/vega.py in _get_model(self, doc, root, parent, comm)
    104         else:
    105             json = self._to_json(self.object)
--> 106             self._get_sources(json, sources)
    107         props = self._process_param_change(self._init_properties())
    108         model = VegaPlot(data=json, data_sources=sources, **props)

/opt/conda/lib/python3.6/site-packages/panel/pane/vega.py in _get_sources(self, json, sources)
     76                 import altair as alt
     77                 if (not isinstance(self.object.data, alt.Data) and
---> 78                     columns == set(self.object.data)):
     79                     data = ColumnDataSource.from_df(self.object.data)
     80                 else:

/opt/conda/lib/python3.6/site-packages/altair/utils/schemapi.py in __getitem__(self, item)
    233 
    234     def __getitem__(self, item):
--> 235         return self._kwds[item]
    236 
    237     def __setitem__(self, item, val):

KeyError: 0

Row
    [0] Column
        [0] Select(name='Area type:', options=['Country', 'Sub-Region', ...], value='Country')
    [1] ParamFunction(function)

plot_map() function is working correctly standalone and with ipywidgets interact() function, but here I have this error.
Corresponding to this answer (https://stackoverflow.com/questions/57713504/altair-plot-not-showing-when-using-panel), we need to use default data transformer.
Workaround from #422 is also not working in this case.

[philippjfr]

Thanks for the bug report. Could you update your example to ensure it is fully reproducible? I can guess the imports but don't know where to get data from.

[labdmitriy]

Sure,
merged_stats_df.zip

Full code is the following:

import pandas as pd

import altair as alt
from altair import datum
from vega_datasets import data

import panel as pn
pn.extension('vega')

merged_stats_df = pd.read_csv('merged_stats_df.csv')

alt.renderers.enable('default')
alt.data_transformers.enable('default', max_rows=None)

areas = ['Country', 'Sub-Region', 'Region']
area = pn.widgets.Select(name='Area type:', options=areas)

@pn.depends(area.param.value)
def plot_map(area):
    counter_name = area.lower() + '_count'
    countries = alt.topo_feature(data.world_110m.url, 'countries')
    
    country_map = alt.Chart(countries).mark_geoshape(
    ).encode(
        color=f'{counter_name}:Q',
        tooltip=[
            alt.Tooltip('region:N', title='Region'),
            alt.Tooltip('sub-region:N', title='Sub-Region'),
            alt.Tooltip('answer:N', title='Country'),
            alt.Tooltip(f'{counter_name}:Q', title=f'Questions Count by {area}')
        ]
    ).transform_lookup(
        lookup='id',
        from_=alt.LookupData(merged_stats_df, 'id', merged_stats_df.columns.tolist())
    ).project(
        type='mercator'
    ).properties(
        width=600,
        height=400
    )

    borders = alt.Chart(countries).mark_geoshape(
        fill='white',
        stroke='grey',
        strokeWidth=1
    ).transform_filter(
        datum.id!=10
    ).properties(
        width=600,
        height=400
    )
   
    return (borders + country_map)

pn.Row(
    pn.Column(area),
    plot_map
).servable()

You can try to call for example plot_map('Country') to see that the function itself is working correctly.