Add script that can serve a bytesIO link to a file that is not saved on the server

dash-download-file-link-bytesio.py

@@ -0,0 +1,72 @@
+# Adapted from https://community.plotly.com/t/allowing-users-to-download-csv-on-click/5550/19
+# example sends Excel .xlsx file type, for other mimetypes, see https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types
+
+import dash
+import dash_core_components as dcc
+import dash_html_components as html
+from datetime import datetime as dt
+import flask
+import io
+
+import pandas as pd
+
+df = pd.DataFrame({
+    'Gender': ['m', 'f', 'o', 'm'],
+    'JoinDate': ['19970309', '14200420', '19870930', '03421206'],
+    'ZipState': ['x', 'x', 'y', 'y']
+})
+
+app = dash.Dash()
+app.css.append_css({'external_url': 'https://codepen.io/chriddyp/pen/bWLwgP.css'})
+app.layout = html.Div([
+
+    html.Label('Filter gender'),
+    dcc.Input(id='gender-picker', value='m', type='text'),
+
+    html.Label('Filter join date'),
+    dcc.Input(id='joindate', value='14200420', type='text'),
+
+    html.Label('Filter state'),
+    dcc.Input(id='state-picker', value='x', type='text'),
+
+    html.A(html.Button('Download', id='download-button'), id='download-link')
+    ])
+
+@app.callback(
+    dash.dependencies.Output('download-link', 'href'),
+    [dash.dependencies.Input('gender-picker', 'value'),
+     dash.dependencies.Input('joindate', 'value'),
+     dash.dependencies.Input('state-picker', 'value')])
+def update_link(selected_gender, join_start_date, selected_state):
+    return '/DownloadData?value={}/{}/{}'.format('-'.join(selected_gender),
+                                                                  dt.strptime(join_start_date,'%Y%m%d'),
+                                                                  '-'.join(selected_state))
+
+@app.server.route('/DownloadData')
+def download_csv():
+    # get parsing details from url that was just updated
+    value = flask.request.args.get('value')
+    value = value.split('/')
+
+    selected_gender = value[0].split('-')
+    selected_state = value[2].split('-')
+    join_start_date = value[1]
+
+    filtered_df = df[df['Gender'].isin(selected_gender)]
+    filtered_df = filtered_df[filtered_df['ZipState'].isin(selected_state)]
+    filtered_df = filtered_df.loc[(demographics['JoinDate'] >= join_start_date),]
+
+    str_io = io.BytesIO()
+    filtered_df.to_excel(str_io)
+
+    mem = io.BytesIO()
+    mem.write(str_io.getvalue()
+    mem.seek(0)
+    str_io.close()
+    return flask.send_file(mem,
+                       mimetype='application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+                       attachment_filename='downloadFile.xlsx',
+                       as_attachment=True)
+
+if __name__ == '__main__':
+    app.run_server()