How to fix Dash App Exception - Invalid number of output values Expected 2, got 1?

Question

I using plotly dash app framework and have callback functions with multiple output values.

When I attempt to deploy the app, it throws an Dash exception. Invalid number of output values... I have tried to use [] and remove them. However, still noticing the exception.

Here's what App Layout looks like:

app.layout = html.Div([

    # header
    html.Div([ 

              dcc.Link(
                 href=app.get_relative_path('/home'),
                 children='Home'
              ),

              dcc.Link(
                 href=app.get_relative_path('/history'),
                 children='History'
              ),

              # For pdf downloads
              dash_extensions.Download(id="download"),
              dcc.Location(id="url"),

              # Page content
              html.Div(id="page-content"),

              # Tab content
              html.Div(id="tab_content"),

      ])

Callback

# Update page content 

@app.callback([
               Output('page-content', 'children'),
               Output('download', 'data')
              ],
              [Input('url', 'pagename')]
             )
def display_content(page_name):

    # Pages and Links

    if not page_name:
        return [dash.no_update, dash.no_update]

    if page_name == 'history:
        return [history.layout(), dash.no_update]

    else:
        return ['404 Error', dash.no_update]

Traceback from the logs:

Traceback (most recent call last):
File "/app/.heroku/python/lib/python3.6/site-packages/flask/app.py", line 1950, in full_dispatch_request
....
_validate.validate_multi_return(output_spec, output_value, callback_id)
File "/app/.heroku/python/lib/python3.6/site-packages/dash/_validate.py", line 126, in validate_multi_return
callback_id, len(outputs_list), len(output_value)
172.17.0.19 - - [03/Sep/2020:06:03:32 +0000] "POST /dash-app/_dash-update-component HTTP/1.1" 500 69 "https://example.com/dash-app" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.83 Safari/537.36"

dash.exceptions.InvalidCallbackReturnValue: Invalid number of output values for ..page-content.children...download.data...

Expected the output type to be a list or tuple, but got 1

Answer 1

the method display_content expects you to return two values. Instead you return a single value, a list.

So instead write:

return history.layout(), dash.no_update

and

return '404 Error', dash.no_update