Downloading csv from browser

Dash Python
1

I have a some drop downs to filter my data and a download csv button to download the said filtered data. I have currently implemented a solution that is similar to the one in this link: https://github.com/plotly/dash-recipes/blob/master/dash-download-file-link-server.py

However, it slows down the app since my file is very large and it takes too long to save the csv file on the server side. Is there any way to directly download the filtered data from the client side itself without having to save it in a temporary folder on the server.

P.S.: I am very new to flask apps and dash, so if you could explain it in simple terms, I’d be really grateful

2

How large is the file? In most cases, the disk write speed is much faster than the network transfer speed, in which case writing the file to disk server side doesn’t impact performance significantly.

3

The file is about 160MB… it takes about 30s to download into the static folder.

4

Where does the data come from (could you share the code)? If the data is on the server in the first place, the write operation shouldn’t take more than a few seconds.

5

The data is stored in memory as a global variable. The csv to be downloaded is generated from a callback and saved in a hidden div.
I was able to cut out the server part by implementing something similar to this: https://github.com/plotly/dash-recipes/blob/master/download-raw-data.py

but the performance is still pretty slow.

6

The hidden div is client side, i.e. if you save the data there, they are sent from the server to the client for storage and back again for processing when you are accessing them in a callback. That approach will yield a bad performance for large datasets. Saving the data to disk should be much faster. It is possible to serve data from memory, which would probably be the fastest option. Here is an example, where a data frame (stored as a global variable) is served,

import io
import dash
import dash_html_components as html
import pandas as pd

from flask import Flask, send_file

df = pd.DataFrame({
    'a': [1, 2, 3, 4],
    'b': [2, 1, 5, 6],
    'c': ['x', 'x', 'y', 'y']
})

server = Flask(__name__)
app = dash.Dash(external_stylesheets=['https://codepen.io/chriddyp/pen/bWLwgP.css'], server=server)
app.layout = html.Div([html.A("Click me", href="/test_download")])


@server.route('/test_download')
def test_download():
    # Write data to StringIO Object (in memory).
    buffer = io.StringIO()
    df.to_csv(buffer)
    buffer.seek(0)
    # Send the data directly without writing to disk.
    return send_file(
        buffer,
        attachment_filename='test.csv',
    )


if __name__ == '__main__':
    app.run_server(debug=False)
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