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Understanding the Issue with NiceGUI and Plotly

When using NiceGUI to create interactive applications involving Plotly visualizations, you may encounter perplexing behavior, particularly when you attempt to update a plot from within a function. This issue often stems from how the plotting context is managed within the NiceGUI framework, especially when compared to updating directly in the main script body.

In your case, the problem arises when adding new data to a Plotly Scattermap within a function. This is a common scenario when trying to fetch new data or refresh a visualization interactively, but it can lead to confusion when things don’t work as expected. Let's dive into the details and understand why the plot is not updating correctly when encapsulated within a function.

Why Doesn't It Work?

The core of the problem lies in how the fig object is being accessed and modified. When you create the plot and add traces outside of a function, you are directly manipulating the plot instance. However, once you define the update_plt() function, the fig object might not behave the same way due to how local and global variables work in Python.

The Context of Variable Scoping

In Python, variables defined in a function are local to that function unless explicitly marked as global. When the update_plt() function attempts to add new traces to fig, it’s possible that it doesn’t have access to the globally scoped fig object unless you tell it to refer to the global variable.

Correcting the Code: Step-by-Step Guide

To resolve the issue and ensure that the update occurs correctly, you can follow these steps:

Step 1: Define the Function Correctly

Make sure your function has access to the fig variable. You can achieve this by declaring fig as a global variable within the function.

Step 2: Update Your Code Structure

You should modify your code to access the global fig correctly. Here’s how the updated code looks:

import plotly.graph_objects as go import dataretrieval.nwis as nwis from nicegui import ui fig = go.Figure(go.Scattermap( fill="toself", lon=[-90,-89,-89,-90], lat=[45,45,44,44], marker={'size': 10, 'color': "orange"}, name='BBox')) fig.update_layout(margin=dict(l=0,r=0,t=0,b=0), width=400, showlegend=False, map={ 'style': 'carto-darkmatter', 'center': {'lon': -90, 'lat': 44}, 'zoom': 5 }) plot = ui.plotly(fig) def update_plt(): global fig # Declare fig as global siteBBOX, md = nwis.get_info(bBox=[-90,44,-89,45]) select_y = siteBBOX['dec_lat_va'] select_x = siteBBOX['dec_long_va'] names = siteBBOX['station_nm'] fig.add_trace(go.Scattermap(lon=select_x, lat=select_y, fill=None, mode='markers', marker=dict(size=15, color='blue'), text=names, name='sites')) plot.update() ui.button('Update', on_click=update_plt) ui.run(title='Test') 

Step 3: Explanation of Changes

• By including the global fig statement at the top of the update_plt() function, the function now knows which fig object you are referring to, allowing modifications to be applied correctly.
• When the button is clicked, update_plt() now adds the new traces based on the data retrieved successfully.

Frequently Asked Questions (FAQ)

Q: Why do I need to declare global fig?
A: Declaring fig as global lets your function access and modify the variable defined outside its scope.

Q: What happens if I don't declare it?
A: Without the global declaration, your function will create a new local variable named fig, leading to confusion and failure to update the actual plot.

Q: Can I use other methods instead of global?
A: Yes, you can also pass fig as an argument to the function, but using global can simplify access in this scenario.

Conclusion

By understanding the difference in variable scoping and ensuring that your function can properly access the fig object, you can successfully update your Plotly traces within a NiceGUI application. If you continue to explore dynamic data and visualizations, be mindful of how context and scope can affect your plots. Happy coding!