serve Dash app as a subdirectory

app.py

# -*- coding: utf-8 -*-

# Run this app with `DASH_URL_BASE_PATHNAME=/MY_PREFIX/ python app.py` and
# visit http://127.0.0.1:8050/MY_PREFIX/ in your web browser.

import dash
import dash_core_components as dcc
import dash_html_components as html
import plotly.express as px
import pandas as pd

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

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

# assume you have a "long-form" data frame
# see https://plotly.com/python/px-arguments/ for more options
df = pd.DataFrame({
    "Fruit": ["Apples", "Oranges", "Bananas", "Apples", "Oranges", "Bananas"],
    "Amount": [4, 1, 2, 2, 4, 5],
    "City": ["SF", "SF", "SF", "Montreal", "Montreal", "Montreal"]
})

fig = px.bar(df, x="Fruit", y="Amount", color="City", barmode="group")

app.layout = html.Div(children=[
    html.H1(children='Hello Dash'),

    html.Div(children='''
        Dash: A web application framework for Python.
    '''),

    dcc.Graph(
        id='example-graph',
        figure=fig
    )
])

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

default.nix

let
  nixpkgs = builtins.fetchGit {
      name = "nixos-20.03";
      url = "https://github.com/nixos/nixpkgs-channels/";
      # `git ls-remote https://github.com/nixos/nixpkgs-channels nixos-20.03`
      ref = "refs/heads/nixos-20.03";
      rev = "ab3adfe1c769c22b6629e59ea0ef88ec8ee4563f";
  };
  pkgs = import nixpkgs {};

  python = pkgs.python37.withPackages(ps: with ps; [ pandas dash ]);
in
pkgs.stdenv.mkDerivation {
    name = "dash-sub-folder";
    src = ./.;
    buildInputs = [python];

    installPhase = ''
      mkdir -p $out/bin
      echo -e "#!/bin/sh\n\n${python.interpreter} $src/app.py" > $out/bin/server
      chmod +x $out/bin/server
    '';
}