[BUG] numpy array not correctly read back from figure data

[stdkoehler]

Please provide us your environment, so we can easily reproduce the issue:

Windows 10, python 3.12

dash                 3.0.4
dash-core-components 2.0.0
dash-html-components 2.0.0
plotly               6.1.1
numpy                2.2.5

Describe the bug

When I create a go.Figure and add a trace with x and/or y being numpy arrays, the plot is created correctly.
However, when I want to read back the data, some representation of the numpy array is provided as dictionary: {‘dtype’: ‘f8’, ‘bdata’: ‘AAAAAAAA8D8AAAAAAAAAQAAAAAAAAAhAAAAAAAAAEEAAAAAAAAAUQA==’, ‘_inputArray’: {‘0’: 1, ‘1’: 2, ‘2’: 3, ‘3’: 4, ‘4’: 5, ‘bdata’: ‘AAAAAAAA8D8AAAAAAAAAQAAAAAAAAAhAAAAAAAAAEEAAAAAAAAAUQA==’, ‘dtype’: ‘f8’, ‘shape’: ‘5’}}

As also described in https://community.plotly.com/t/changed-behaviour-in-dash-2-18-2-using-numpy-1-26-4-when-reading-figure-into-callback-on-linux/90614

import dash
from dash import dcc, html, Input, Output, State
import plotly.graph_objs as go
import numpy as np

# Initialize the Dash app
app = dash.Dash(__name__)

# Create some sample data using numpy
x_data = np.array([1.0, 2.0, 3.0, 4.0, 5.0])
y_data = np.array([2.0, 1.5, 3.5, 2.0, 5.0])

# Define the layout of the app
app.layout = html.Div([
    dcc.Graph(id='scatter-plot'),
    html.Button('Generate Plot', id='generate-button', n_clicks=0),
    html.Button('Print Data', id='print-button', n_clicks=0)
])

# Callback to generate the ScatterGL plot
@app.callback(
    Output('scatter-plot', 'figure'),
    Input('generate-button', 'n_clicks')
)
def update_figure(n_clicks):
    # Create a ScatterGL trace
    trace = go.Scattergl(
        x=x_data,
        y=y_data,
        mode='markers',
        marker=dict(size=10, color='blue')
    )
    
    # Create the figure
    fig = go.Figure(data=[trace])
    fig.update_layout(title='ScatterGL Plot', xaxis_title='X Axis', yaxis_title='Y Axis')
    
    return fig

# Callback to print the x and y data from the figure
@app.callback(
    Output('print-button', 'children'),
    Input('print-button', 'n_clicks'),
    State('scatter-plot', 'figure')
)
def print_data(n_clicks, figure):
    if n_clicks > 0 and figure is not None:
        # Extract the x and y data from the figure
        x_values = figure['data'][0]['x']
        y_values = figure['data'][0]['y']
        
        # Print the x and y values
        print("X values:", x_values)
        print("Y values:", y_values)
    
    return "Print Data"

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

Expected behavior

A) Throw an Exception when numpy arrays are not supported
B) If numpy arrays are supported return the numpy array or the numpy array converted to a valid list

[gvwilson]

@T4rk1n is this an issue with Dash or is this lower down the stack?

[T4rk1n]

This is from plotly.py typed array, I think this will keep on coming back so we might have to do something. The data get serialized to those for optimized transfer and end up as an encoded string in the renderer state.
There should be a plotly.py function to transform that back into readable or we should add one.

[gvwilson]

@marthacryan is there already a function in plotly.py to do the required transformation? if not, I'll create an issue for getting it written.