dash.no_update is not serializable by json is raised if other, not serializiable type is passed as output

[smoky1337]

Hi everyone,

thanks for providing such a wonderful package. I had an issue and I have been cracking my head at this long enough to belief this is a bug with dash.no_update not being serializiable. While writing this issue, I found that dash.no_update is not the issue but obfusicates the real error.

Situation: Given a user picked date range, I gather data and display it. However, I want to cover the case when there simply is no data to present.
For this, I aim to not update the already presented data and instead show an alert, notifiying the user of the absence of data.

See this line (context below):

return no_update, no_update, True, df

df is an AssertionError. Apparently, those are not serializiable by json. However, see error log at bottom, dash reports that dash.no_update seems to be the culprit.

The error below is resolved by parsing df into a string via ´str(df)´

@callback(
    Output(component_id='figcontainer', component_property='figure'),
     Output(component_id='table', component_property='data'),
     Output(component_id="alert", component_property='is_open'),
     Output(component_id="alert",component_property='children'),
    Input(component_id='date_picker', component_property='start_date'),
    Input(component_id='date_picker', component_property='end_date'),
)
def update_graph(start, end):
    df = get_bookings_in_interval(start, end)
    # if there is no data, keep previous states and use alert
    if type(df) is AssertionError:
        return no_update, no_update, True, df

    #else simply display the information and dont update the alert
    all_appointments, best_days_with_numbers, cleaning_schedule = create_dfs(df)
    
    fig = create_figure(start, end, all_appointments, cleaning_schedule)

    
    return fig.to_dict(), best_days_with_numbers.to_dict(orient="records"), no_update, no_update


#either returns a df or an AssertionError
def get_bookings_in_interval(start, end):
    df = None
    try:
        data = get(#do something)
        assert data.status_code == 200 , "Failed to fetch bookings"
        parsed_data = dict(data.json())
        assert len(parsed_data["bookings"]) > 0, "No items in Response")
        #do something
            
    except AssertionError as e:
        print(e)
        return e

    return data

Environment used:

dash                      2.18.2
dash-bootstrap-components 1.6.0
dash-core-components      2.0.0
dash-html-components      2.0.0
dash-table                5.0.0

Error Log:

dash.exceptions.InvalidCallbackReturnValue: The callback for `[<Output `figcontainer.figure`>, <Output `table.data`>, <Output `alert.is_open`>, <Output `alert.children`>]`
returned a value having type `NoUpdate`
which is not JSON serializable.


The value in question is either the only value returned,
or is in the top level of the returned list,

and has string representation
`<dash._callback.NoUpdate object at 0x000002A7FA5ACBE0>`

In general, Dash properties can only be
dash components, strings, dictionaries, numbers, None,
or lists of those.

Expected behavior

As dash.no_update is explicitly mentioned, I expected it to be culprit. I think the error hint should either reflect the possibility of other returned values being the issue or explicitly state the correct one.

Screenshots

[yuvashrikarunakaran]

from dash import no_update
from dash.dependencies import Input, Output

@callback(
Output(component_id='figcontainer', component_property='figure'),
Output(component_id='table', component_property='data'),
Output(component_id="alert", component_property='is_open'),
Output(component_id="alert", component_property='children'),
Input(component_id='date_picker', component_property='start_date'),
Input(component_id='date_picker', component_property='end_date'),
)
def update_graph(start, end):
df = get_bookings_in_interval(start, end)

# Handle the case where there is an AssertionError
if isinstance(df, AssertionError):
    return no_update, no_update, True, str(df)  # Convert AssertionError to a string

# Otherwise, process the data normally
all_appointments, best_days_with_numbers, cleaning_schedule = create_dfs(df)
fig = create_figure(start, end, all_appointments, cleaning_schedule)

return (
    fig.to_dict(),
    best_days_with_numbers.to_dict(orient="records"),
    no_update,
    no_update,
)

def get_bookings_in_interval(start, end):
df = None
try:
data = get(# do something)
assert data.status_code == 200, "Failed to fetch bookings"
parsed_data = dict(data.json())
assert len(parsed_data["bookings"]) > 0, "No items in response"
# Process the data and return
df = process_data(parsed_data) # Replace with actual processing logic
except AssertionError as e:
print(e)
return e # Return AssertionError for handling in the callback
return df