在 plotly/dash 中为数据表的数据源返回一个空字典

Question

My callback function reads a value selected by the user (site name) and then queries data for that site and returns 3 figures and one dictionary (df.to_dict('records') to supply the data for a datatable.

If the user selects a site for which there is no data, I return {} . That seems to break it. If I select a site, the data table fills in properly, switch to another site, same thing. Once I select a site with no data, the data table will no longer update, no matter which site I select.

Some relevant code:

The output is defined as:

Output('emission_table','data'),

The return from the callback is:

return time_series_figure,emissions_df.to_dict('records'),site_map,hotspot_figure

html.Div(style={'float':'left','padding':'5px','width':'49%'}, children = [
    dash_table.DataTable(id='emission_table', data=[],columns=[
    #    {'id': "site", 'name': "Site"},
        {'id': "dateformatted", 'name': "date"}, 
        {'id': "device", 'name': "device"}, 
        {'id': "emission", 'name': "Emission"},
        {'id': "methane", 'name': "CH4"},
        {'id': "wdir", 'name': "WDIR"},
        {'id': "wspd", 'name': "WSPD"},
        {'id': "wd_std", 'name': "WVAR"}],
    #    {'id': "url", 'name':'(Link for Google Maps)','presentation':'markdown'}],
    fixed_rows={'headers': True},
    row_selectable='multi',
    style_table={'height': '500px', 'overflowY': 'auto'},
    style_cell={'textAlign': 'left'})

]),

Any ideas what is happening? Is there a better way for the callback to return an empty data source for the datatable?

Thanks!

Answer 1

You haven't shared enough of your code (your callback specifically) to see what is happening exactly, however:

If the user selects a site for which there is no data, I return {}

is at least one reason why it doesn't work. The data property of a Dash Datatable needs to be a list and not a dictionary. You can however put dictionaries inside the list. Each dictionary inside the list corresponds to a row in the Data Table.

So to re-iterate and more directly answer your question:

Is there a better way for the callback to return an empty data source for the datatable?

Yes you can return a list with any number of dictionaries inside.