Use app.run_server in callback

Dash Python
1

Hi all,

I hope someone can help me. I’m developing an app with Dash, in which one can upload some files at runtime using dcc.Upload. Then, I need to create some callbacks, depending on the uploaded file. Unfortunately, I can’t define the callbacks before running the server (app.run_server(debug=True)), since they depend on the uploaded data.

My idea was to “restart” the whole app in a callback, e.g. create a new app with all the callbacks, and then run the new app by app.run_server(debug=True). Unfortunately, I always get a ValueError: signal only works in main thread.

The following callback is triggered when I selected the files and hit an “Upload Button”.

@app.callback(Output("dummy-div", "children"),
              [Input("upload-button", "n_clicks")])
    def update(upload_button):
        # A new layout is calculated from the uploaded data
        layout = setup_new_dash_layout()
        # A new app is created in my dash_app_handler
        dash_app_handler.create_new_app()
        # The layout of this new app will be set
        dash_app_handler.set_layout(layout)
        # The callbacks depending on the new data will be created
        add_callbacks(dash_app_handler)
        # The new app should be restarted again
        dash_app_handler.run_server()

This is how dash_app_handler looks like:

class DashAppHandler():
    def __init__(self):
        self.app = dash.Dash(name=__name__,
                             external_stylesheets=external_stylesheets,
                             )
        self._add_empty_layout()

    def run_server(self, debug=True):
        self.app.run_server(
            debug=debug,
        )

    def set_layout(self, layout):
        self.app.layout.children = layout

    def create_new_app(self):
        self.app = dash.Dash(name=__name__, external_stylesheets=external_stylesheets)
        self._add_empty_layout()

    def _add_empty_layout(self):
        self.app.layout = html.Div(children=[], id='app_layout')

The easiest solution I think would be to add callbacks to the existing running app. But I haven’t found a way to modify the callbacks in a running server (app.run_server).

Any ideas how to get it work? Or any other ideas how to solve my problem?

Any help is highly appreciated! Thanks in advance!

2

I think there probably is some simpler solution than restarting the server.

Could you give a minimal (toy) example of a callback that would depend on the uploaded data in a such way that it would be impossible to define it before running the server?

3

Hi np8,

Thank you very much for the response and for trying to help with the issue.

Some background
I can upload up to 4 different files. Each file contains different x,y data with a corresponding title. For simplicity, after decoding the uploaded files and storing them in e.g. uploaded_file, the structure of one file looks like this: {'data_name1': [x_values1, y_values1], 'data_name2': [x_values2, y_values2], ...}. The files which will be uploaded have often different data_names, and, of course, different data values (x_values, y_values).

I now display the graphs (which will be created from the data of the files) in columns. (i.e. 1 file == 1 colum, 2 files == 2 columns, etc.).

For each file, I create a dashtable that displays all the data_names of an uploaded file. What I want is, that when clicking on one of the cells and hitting a “Plot” button, the corresponding data of the selected cell will be plotted using Plotly in a graph. So when having e.g. four files, four tables are displayed in 4 columns. Below the tables, a graph is displayed.
Now, when selecting a cell of a file (that contains the name) and then hitting the “Plot” button, I will plot the corresponding data in the corresponding graph.
The problem is now, that I have a varying amount of files in Dash. For each file, I need to add a “Plot” button.

Of course, I could before running the server add always the callbacks for four log files, but then I always got “id not found errors”, because the ids of the buttons do not exist yet.

(This is only one example, I have some more cases of callbacks that are dependent on the files, but I hope it is clear what I want to achieve)

So, I assume I need to somehow restart the server in order to be able to change the app itself…

Thanks for helping!

4

Hi @gaaroai ,

At first glance to me it seems that this should be doable without any special tricks in Dash.

Could you put here a simple MWE that produces the id not found error, and also some example data files?