Regression With Pyodide And Panel: Troubleshooting Pn.io.hold()

October 4, 2025 by StackCamp Team 64 views

Introduction

Hey guys! Today, we're diving into an interesting issue encountered while using Panel with Pyodide, specifically concerning the @pn.io.hold() decorator. This decorator is super handy for controlling when Panel updates its output, but it seems like it's causing some unexpected behavior in Pyodide. Let's break down the problem, explore the code, and figure out what's going on and how to tackle it. If you're working with Panel and Pyodide, you'll definitely want to stick around to see how we can ensure our apps work smoothly across different environments.

Understanding the Issue: Regression with Pyodide and Panel

So, what's the problem we're tackling today? When using Panel with Pyodide, there's an issue where changing certain parameters doesn't trigger the expected updates in the user interface. Specifically, this problem arises when we use the @pn.io.hold() decorator in our Panel code. To really understand what's happening, let's break it down step by step. First, we have a Panel application that's designed to run both using panel serve and within a Pyodide environment. The application includes a class called InstallationMethod that uses param to define interactive parameters, such as the number of circuits and spacing. When running the app with panel serve, everything works perfectly fine. You can adjust the "Number of circuits," and the rest of the parameters respond as expected. However, when we convert this application to run in a browser using Pyodide (with Panel versions 1.7.2 and later), we hit a snag. The change in the "Number of circuits" parameter no longer triggers the intended updates. It's like the app is ignoring the change, which is definitely not what we want. Now, here's the kicker: this issue seems to be related to the @pn.io.hold() decorator. This decorator is designed to control when Panel updates its output, which can be useful when you're making multiple changes and only want to update the display once all changes are complete. But in this case, it's causing a regression in Pyodide, meaning the app isn't behaving as it should. When we remove the @pn.io.hold() decorator, the app starts working as expected in Pyodide. This gives us a clue that the decorator is somehow interfering with the update mechanism in the Pyodide environment. The issue appears to stem from a specific commit in Panel's history, which is 701e698bbcc01ecee1e837d578ead89975e9b54d. By examining this commit, we can see that it's indeed related to how @pn.io.hold() functions. This information is super helpful because it narrows down the scope of the problem and gives us a starting point for debugging. In the specific code example provided, the _on_method_change method is decorated with both @pn.io.hold() and @param.depends. This method is intended to update the k15_spacing parameter based on the selected number of circuits. The decorator @param.depends is crucial here because it tells Panel to watch for changes in k15_number_of_circuit and trigger the _on_method_change method whenever that value changes. The @pn.io.hold() decorator, in theory, should simply delay the update until the method has finished executing, but in practice, it's preventing the update from happening at all in Pyodide. This behavior is particularly problematic because in more complex applications, you might be updating multiple parameters at once, and you'd want to use @pn.io.hold() to ensure a smooth and efficient update process. However, if it's not working correctly in Pyodide, it can lead to a broken user experience. So, to summarize, the core issue is that @pn.io.hold() seems to be blocking parameter updates in Pyodide when it should only be delaying them. This is a regression because it wasn't happening in earlier versions of Panel (specifically, it works in 1.7.1 but not in 1.7.2 and later). The problem is triggered by a specific commit related to @pn.io.hold(), and it prevents the user interface from reflecting changes made to certain parameters, which can be a major headache when building interactive applications.

Code Example: Minimal Reproducible Example

To really nail down the issue, it's essential to have a minimal, self-contained code example that reproduces the problem. This makes it easier for others to understand and debug the issue, and it helps ensure that any proposed solutions actually solve the problem. Let's dive into the code snippet that demonstrates the regression with Pyodide and Panel:

import panel as pn
import param
from panel.viewable import Viewable


class InstallationMethod(pn.viewable.Viewer):
    k15_number_of_circuit: int = param.Selector(
        label="Number of circuits", objects=list(range(1, 13))
    )  # type:ignore
    k15_spacing: float = param.Number(
        label="Spacing", default=0, bounds=(0, 1.0)
    )  # type:ignore

    @pn.io.hold()
    @param.depends(
        "k15_number_of_circuit",
        watch=True,
        on_init=True,
    )
    def _on_method_change(self):
        self.param.k15_spacing.constant = self.k15_number_of_circuit == 1

    def __panel__(self) -> Viewable:
        return pn.Param(
            self.param,
            widgets={
                "k15_spacing": {
                    "type": pn.widgets.Select,
                    "options": {f"{v} m": v for v in [0, 0.25, 0.5, 1.0]},
                },
            },
            show_name=False,
            hide_constant=True,
        )


a = InstallationMethod()

pn.Column(a).servable()

This code defines a Panel application with a class InstallationMethod that includes two key parameters: k15_number_of_circuit and k15_spacing. The k15_number_of_circuit parameter is a selector that allows users to choose a number of circuits from 1 to 12. The k15_spacing parameter is a floating-point number representing the spacing, with a default value of 0 and bounds between 0 and 1. The crucial part of this code is the _on_method_change method, which is decorated with both @pn.io.hold() and @param.depends. This method is designed to be triggered whenever the k15_number_of_circuit parameter changes. Inside this method, we set the constant attribute of the k15_spacing parameter based on whether the selected number of circuits is equal to 1. This is a simple example of a reactive behavior where one parameter's properties are updated in response to changes in another parameter. The __panel__ method defines how the InstallationMethod class is displayed in the Panel application. It uses pn.Param to create a Panel widget based on the class's parameters. We also specify a custom widget for k15_spacing, using pn.widgets.Select to provide a dropdown menu with predefined spacing options (0, 0.25, 0.5, and 1.0). The show_name=False and hide_constant=True arguments are used to customize the appearance of the Panel widget, hiding the name of the class and any constant parameters. Finally, we create an instance of the InstallationMethod class and display it in a Panel column using pn.Column(a).servable(). This makes the application accessible through a Panel server. Now, let's talk about how this code demonstrates the issue. When you run this code with panel serve, everything works as expected. You can change the number of circuits, and the spacing options will update correctly. However, when you convert this code to run in Pyodide using panel convert index.py and serve it with python -m http.server, you'll notice that changing the "Number of circuits" has no effect on the spacing options. This is the regression in action. The @pn.io.hold() decorator, which is supposed to delay updates, is instead preventing them from happening altogether in the Pyodide environment. Removing the @pn.io.hold() decorator makes the code work as expected in Pyodide, confirming that it's the root cause of the problem. This minimal example is incredibly valuable because it isolates the issue and provides a clear way to reproduce it. Anyone can take this code snippet, run it in their environment, and see the problem for themselves. This is the first step towards finding a solution and ensuring that Panel applications work reliably across different platforms.

Reproduction Steps: How to Trigger the Issue

To effectively troubleshoot and resolve this issue, it's crucial to have clear, repeatable steps that demonstrate how to trigger the problem. This ensures that anyone can verify the issue and that any proposed solutions actually address the root cause. Let's outline the exact steps you need to follow to reproduce the regression with Pyodide and Panel.

Set Up Your Environment: First off, you'll want to make sure you have Panel installed. If you don't already, you can install it using pip:
```
pip install panel
```
It's also a good idea to create a virtual environment to keep your project dependencies isolated. You can do this using venv:
```
python -m venv venv
source venv/bin/activate  # On Linux/macOS
venv\Scripts\activate  # On Windows
```
Install Panel Versions: To reproduce the issue, you'll need to test with specific Panel versions. The regression occurs in Panel 1.7.2 and later, but not in 1.7.1. You can install these versions using pip:
```
pip install panel==1.7.1  # To test the working version
pip install panel==1.7.2  # To test the broken version
```
Save the Code: Copy the code example provided in the previous section (the InstallationMethod class) and save it as a Python file, for example, index.py.
Run with Panel Serve: First, let's verify that the code works correctly with panel serve. Open a terminal, navigate to the directory where you saved index.py, and run:
```
panel serve index.py
```
This will start a Panel server, and you should be able to access the application in your web browser. Verify that changing the "Number of circuits" updates the spacing options as expected. This confirms that the code works correctly when run with panel serve.
Convert to Pyodide: Now, let's convert the application to run in a browser using Pyodide. Make sure you have the panel convert command available. If not, you might need to install pyodide-http:
```
pip install pyodide-http
```
Then, run the conversion:
```
panel convert index.py --to pyodide --out .  # The dot specifies the current directory as the output
```
This command will generate the necessary HTML, JavaScript, and Pyodide files in the current directory.
Serve the Pyodide Application: To serve the Pyodide application, you can use Python's built-in HTTP server. In the same directory where you ran the panel convert command, run:
```
python -m http.server
```
This will start a simple web server, usually on port 8000. Open your web browser and navigate to http://localhost:8000/index.html (or whatever port your server is running on).
Observe the Issue: In the Pyodide application, try changing the "Number of circuits." You'll notice that the spacing options do not update, demonstrating the regression. This confirms that the issue occurs when running the application in a Pyodide environment with Panel 1.7.2 or later.
Test with Panel 1.7.1: To further confirm the regression, uninstall Panel 1.7.2 and install 1.7.1:
```
pip uninstall panel
pip install panel==1.7.1
```
Repeat steps 5-7. You should find that the application works correctly in Pyodide with Panel 1.7.1, which highlights that the issue was introduced in a later version.
Remove @pn.io.hold() decorator: Remove the @pn.io.hold() decorator from the method _on_method_change and repeat steps 5-7 with Panel 1.7.2. You should find that the application works correctly in Pyodide after this change.

By following these steps, you can consistently reproduce the issue and verify any potential solutions. This is an essential part of the debugging process and ensures that we're all on the same page when discussing the problem.

Root Cause Analysis: Diving into Commit 701e698bbcc01ecee1e837d578ead89975e9b54d

To really understand why this regression is happening, we need to dig deeper into the commit that's causing the issue. The original report pointed to commit 701e698bbcc01ecee1e837d578ead89975e9b54d in the Panel repository. This commit is a key piece of the puzzle, and by examining it, we can gain valuable insights into what might be going wrong. So, let's put on our detective hats and dive into the details of this commit. The first thing to notice is the commit message itself, which gives us a high-level overview of the changes. It's likely related to how @pn.io.hold() functions, which, as we've already seen, is at the heart of the issue. The commit message might mention changes to the way updates are handled or how the state is managed within Panel. This is a good starting point, but to truly understand the impact of the commit, we need to look at the code changes. By examining the diff (the changes made to the code), we can see exactly what lines were added, modified, or removed. This will give us a more granular view of how the commit affects Panel's behavior. When we look at the code changes, we should pay close attention to any modifications related to event handling, parameter updates, and the @pn.io.hold() decorator itself. Are there any changes to how events are queued or processed? Are there any new conditions or checks that might be preventing updates from being triggered in certain environments? Are there any changes to how the state is managed when using @pn.io.hold()? These are the kinds of questions we want to answer by examining the code. It's also helpful to consider the context in which the commit was made. What problem was the commit trying to solve? Was it a bug fix, a performance improvement, or a new feature? Understanding the motivation behind the commit can shed light on why certain changes were made and how they might be interacting with other parts of the codebase. For example, if the commit was intended to improve performance by reducing unnecessary updates, it's possible that it introduced a bug that prevents updates from happening in Pyodide. Once we have a good understanding of the code changes and the context in which they were made, we can start to form hypotheses about why the regression is occurring. We can think about how the changes might be interacting with the Pyodide environment differently than with a traditional server environment. Are there differences in how events are handled? Are there differences in how the state is managed? Are there differences in the timing of updates? By carefully considering these questions and examining the code, we can narrow down the potential causes of the regression and develop a targeted solution. This process might involve some trial and error, but by starting with a solid understanding of the commit that's causing the issue, we'll be in a much better position to find a fix.

Workaround: Removing `@pn.io.hold()`

Okay, so we've pinpointed the issue and have a good handle on what's going wrong with @pn.io.hold() in Pyodide. Now, let's talk about a quick and dirty workaround to get your Panel apps running smoothly in the meantime. The simplest solution, as we've already touched on, is to remove the @pn.io.hold() decorator from your code. I know, I know, it's not ideal, especially if you're relying on it to batch updates and optimize performance. But bear with me – this workaround can help you keep your project moving forward while we figure out a more permanent fix. When you remove @pn.io.hold(), Panel will update the UI immediately after each parameter change. This means that if you're updating multiple parameters in a single function, the UI might flicker or update multiple times before settling on the final state. This is the main downside of this workaround, but in many cases, it's an acceptable trade-off for getting the app to work in Pyodide. To apply this workaround, simply go through your code and remove the @pn.io.hold() decorator from any methods that are experiencing the issue. In the example code we've been using, this means removing it from the _on_method_change method. After removing the decorator, you'll need to rebuild your Pyodide application using panel convert and redeploy it. Once you've done that, the app should work correctly in Pyodide, with changes to the "Number of circuits" updating the spacing options as expected. Now, let's be clear: this workaround isn't a long-term solution. It's a temporary fix to get you unstuck while we investigate the underlying issue. If you're updating a large number of parameters or performing complex calculations, removing @pn.io.hold() might lead to performance issues or a less-than-ideal user experience. In those cases, you might need to explore other strategies for optimizing updates, such as manually controlling when updates occur or using other Panel features to batch changes. But for many applications, this workaround will be sufficient to get things working in Pyodide. And the good news is that it's a relatively easy change to make and revert once a proper fix is available. So, if you're blocked by this issue, give the workaround a try. It might just be the thing you need to keep your project on track.

Next Steps: Contributing to a Solution

Alright, we've identified the problem, reproduced it, and even found a temporary workaround. But what's next? Well, the real goal is to get a proper fix into Panel so that everyone can use @pn.io.hold() with Pyodide without any headaches. And that's where contributing to the solution comes in. If you're up for it, there are several ways you can help make Panel even better. First off, one of the most valuable things you can do is to report the issue clearly and comprehensively on the Panel GitHub repository. This helps the Panel maintainers understand the problem and prioritize it for fixing. When you report the issue, be sure to include all the details we've discussed in this article: a clear description of the problem, steps to reproduce it, the code example, and the relevant commit. The more information you provide, the easier it will be for the maintainers to diagnose and fix the issue. You can also link to this article or any other relevant discussions to provide additional context. Another great way to contribute is to dig deeper into the code and try to identify the root cause of the regression. We've already done some initial analysis by examining the problematic commit, but there's always more to learn. You can use debugging tools, logging, and other techniques to trace the execution of the code and see exactly what's happening when @pn.io.hold() is used in Pyodide. If you're able to pinpoint the exact line of code that's causing the issue, you'll be in a much better position to propose a solution. And speaking of solutions, if you have an idea for how to fix the regression, you can create a pull request with your proposed changes. This is a fantastic way to directly contribute to Panel and help other users. When you create a pull request, be sure to include a clear description of the problem you're solving, the changes you've made, and any tests you've added to verify the fix. The Panel maintainers will review your pull request and provide feedback, and if everything looks good, your changes will be merged into the codebase. If you're not quite ready to dive into the code, there are still plenty of ways you can help. You can test pull requests from other contributors, review code, and provide feedback on the Panel issue tracker or on the HoloViz Discourse forum. These contributions are incredibly valuable and help ensure that Panel remains a high-quality library. Contributing to open-source projects like Panel can be a rewarding experience. It's a chance to learn new skills, collaborate with other developers, and make a real impact on the community. So, if you're passionate about Panel and want to help make it even better, don't hesitate to get involved.

Conclusion

Alright guys, we've taken a pretty deep dive into this regression issue with Panel and Pyodide, specifically focusing on the @pn.io.hold() decorator. We've walked through how to reproduce the problem, discussed a temporary workaround, and explored ways to contribute to a real solution. To recap, the issue is that @pn.io.hold() seems to be preventing parameter updates in Pyodide when it should only be delaying them. This is a regression that was introduced in Panel 1.7.2 and is related to a specific commit that modified how @pn.io.hold() functions. While we've identified a workaround – removing the decorator – it's not ideal for all situations, especially when you're updating multiple parameters and want to avoid UI flickering. The best path forward is to contribute to a proper fix in Panel. This means reporting the issue clearly, digging into the code to understand the root cause, and potentially creating a pull request with a proposed solution. If you're not a code whiz, no worries! Testing pull requests and providing feedback are also super valuable ways to help out. Hopefully, this article has given you a solid understanding of the issue and how to tackle it. Whether you're a seasoned Panel pro or just getting started, your contributions can make a big difference in the HoloViz ecosystem. So, let's get out there and make Panel even more awesome!