dcc.Upload example not working

I was trying to understand how the error caching works for this method, and I couldn’t get an error message to pop up when I used the example on this page: https://dash.plotly.com/dash-core-components/upload

Anybody know why? I’m using Chrome Version 90.0.4430.212, and Javascript is allowed (i.e. I’m not using NoScript).


The example you point to doesn’t actually use a pop-up window to show an error state, it inserts the error text into an HTML div in the layout where the table would show up if the upload was successful.

However, to trigger that error state in the example you have to upload a malformed file, which isn’t very convenient.

Below I’ve added a check to the code from that example so that if you try to upload a file that is not a .csv, you will trigger the error state. So try to upload a picture for example, and you will see the error state.

I hope that this helps!

# app.py
import base64
import datetime
import io

import dash
from dash.dependencies import Input, Output, State
import dash_core_components as dcc
import dash_html_components as html
import dash_table

import pandas as pd

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div([
            'Drag and Drop or ',
            html.A('Select Files')
            'width': '100%',
            'height': '60px',
            'lineHeight': '60px',
            'borderWidth': '1px',
            'borderStyle': 'dashed',
            'borderRadius': '5px',
            'textAlign': 'center',
            'margin': '10px'
        # Allow multiple files to be uploaded

def parse_contents(contents, filename, date):
    content_type, content_string = contents.split(',')

    decoded = base64.b64decode(content_string)
        if 'csv' in filename:
            # Assume that the user uploaded a CSV file
            df = pd.read_csv(
        elif 'xls' in filename:
            # Assume that the user uploaded an excel file
            df = pd.read_excel(io.BytesIO(decoded))
    except Exception as e:
        return html.Div([
            'There was an error processing this file.'

    return html.Div([

            columns=[{'name': i, 'id': i} for i in df.columns]

        html.Hr(),  # horizontal line

        # For debugging, display the raw contents provided by the web browser
        html.Div('Raw Content'),
        html.Pre(contents[0:200] + '...', style={
            'whiteSpace': 'pre-wrap',
            'wordBreak': 'break-all'

@app.callback(Output('output-data-upload', 'children'),
              Input('upload-data', 'contents'),
              State('upload-data', 'filename'),
              State('upload-data', 'last_modified'))
def update_output(list_of_contents, list_of_names, list_of_dates):
    if list_of_contents is not None:
        if 'csv' not in list_of_names[0]:
            return html.Div(['There was an error processing this file.'])

        children = [
            parse_contents(c, n, d) for c, n, d in
            zip(list_of_contents, list_of_names, list_of_dates)]
        return children

if __name__ == '__main__':