Setting a Plotly Dash dcc.dropdown value dynamically

Question

Im building a plotly Dash dcc.dropdown list dynamically using :

def BuildOptions(DataFrameSeries, AddAll):  
    OptionList = [{'label': i, 'value': i} for i in DataFrameSeries.unique()]
    if AddAll == 1:       
        OptionList.insert(0,{'label': 'All', 'value': 'All'})          
    return OptionList

it uses unique values in a df series, and inserts 'All' into the options list. I now want to Set the (default) value to 'All' if it exists or the [0] item in the list of K/V pairs.

    html.Div([
            dcc.Dropdown(
                id='Prov_DD',                    
                options=BuildOptions(data.TASKPROVINCE,1),
               # value=data.TASKPROVINCE[0],
               # value=dcc.Dropdown.value[0],
                 value='All'  # this works for those list that have 'All' 
                              # but I want [0] item   
                multi=True
            )],className='two columns'
            ),

Any way of setting the dcc drop down to a certain item in the options list of key value pairs by index?

Answer 1

You should define an options_array. Then use that array to populate the dropdown options and value parameters. Once the array is defined you can select a default value by index from the pre-defined array (options_array).

options_array = BuildOptions(data.TASKPROVINCE,1)
html.Div([
    dcc.Dropdown(
    id='Prov_DD',                    
    options=options_array,
    value=options_array[0],
    multi=True)
  ],
  className='two columns'
),

You won't be able to grab a value from dcc.Dropdown(options=...), because it's not actually instantiated yet.

Plus, at this time I think the only other way to pragmatically update dcc.Dropdown is by using callbacks.

Just define the array first.