Define app.callback in a separate file

Dash Python
1

My app is defined with a layout, followed by an @app.callback(), and then a function, as is standard practice. Is it possible, however, to define the callback statement and the function in a separate file?

I would like to be able to have my script read something like this:

my_app.py

import external_file

app.layout = [LAYOUT HERE]
@app.callback(external_file().callback)
def do_stuff():
  ...

external_file.py

class callback():
  def __init__(self):
    callback = Output(), [Input()])

I didn’t show it above for brevity, but it would be ideal to have the function definition following the app.callback in the external file as well.

2

Sure, you can do that. Here’s an example:

main.py

from dash import Dash, html, dcc, callback
from external_file import PongCallback

app = Dash(__name__)

app.layout = html.Div([
    dcc.Interval(id='interval', interval=1000),
    html.Div('PING', id='main-div')
])

# Pong callback
callback(*PongCallback.callback_params)(PongCallback())

if __name__ == "__main__":
    app.run(debug=True)

external_file.py

from dash import Input, Output
from dash.exceptions import PreventUpdate


class PongCallback:
    callback_params = (
        [Output('main-div', 'children')],
        [Input('interval', 'n_intervals')]
    )

    @staticmethod
    def callback(n_intervals):
        if n_intervals is None:
            raise PreventUpdate

        if n_intervals % 2 == 0:
            return ['PONG']
        return ['PING']

    def __call__(self, *args):
        return self.callback(*args)
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