index.py

from dash import Dash, html, dcc, callback, Output, Input
from app import app
import pandas as pd

server = app.server
df = pd.read_csv('https://raw.githubusercontent.com/plotly/datasets/master/gapminder_unfiltered.csv')

# def serve_content():
    # """
    # :return: html div component
    # """
    # return html.Div([
    #     dcc.Location(id='url', refresh=False),
    #     html.Div(id='page-content')
    # ])


# app.layout = serve_content()

app.layout = html.Div([
    html.H1(children='Title of Dash App', style={'textAlign':'center'}),
    dcc.Dropdown(df.country.unique(), 'Canada', id='dropdown-selection'),
    dcc.Graph(id='graph-content')
])

@callback(
    Output('graph-content', 'figure'),
    Input('dropdown-selection', 'value')
)
def update_graph(value):
    dff = df[df.country==value]
    return px.line(dff, x='year', y='pop')


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