Streamlit Plotly Chart Warning: Width Parameter Issue

When you're building interactive dashboards and data visualizations with Streamlit, you often rely on libraries like Plotly to create stunning charts. Streamlit's st.plotly_chart is a fantastic tool for embedding these Plotly figures directly into your apps. However, some users have recently encountered a puzzling deprecation warning that pops up even when they're not explicitly passing any unusual arguments. This warning specifically mentions kwargs and suggests using the config argument instead. It’s quite confusing when you're just trying to set the width of your chart, for instance, by using width="content", a perfectly documented and intended parameter. This article dives into why this is happening and what it means for your Streamlit projects.

Understanding the kwargs Deprecation Warning in Streamlit

The kwargs deprecation warning in Streamlit's st.plotly_chart function is a bit of a head-scratcher for many developers. Let's break down what's going on. In Python, **kwargs is a special syntax that allows a function to accept an arbitrary number of keyword arguments. These are arguments passed in the format key=value. When a function uses **kwargs, it essentially means it can catch and process any keyword arguments that aren't explicitly defined in the function's signature. Streamlit, in its effort to keep its API clean and future-proof, has been refining how it handles arguments passed to its components, including st.plotly_chart.

Historically, some arguments that controlled the appearance or behavior of the chart might have been passed directly as keyword arguments. However, as the Streamlit library evolves, the recommended way to pass configuration options for Plotly charts is now through the config parameter, which is a dictionary where you can specify various Plotly settings. This shift is a good thing in principle, as it makes the API more organized and less prone to conflicts with Plotly's own extensive set of arguments.

The issue arises when you use a parameter like width="content" or height="auto" within st.plotly_chart. While these are valid and documented ways to control the size of your Plotly chart within Streamlit, the underlying code that handles these arguments might be inadvertently triggering the kwargs warning. This happens because the warning is designed to catch any keyword arguments that are not explicitly handled by Streamlit's core function signature, and it seems that the logic for handling width and height parameters might be falling into this trap. It's as if Streamlit is seeing width="content" as an unexpected keyword argument, even though it’s a parameter it’s supposed to understand and process for your chart.

This can be particularly frustrating because it feels like a regression. You're using the tool as intended, following the documentation, yet you're presented with a warning that suggests you're doing something wrong. The warning message itself, "Variable keyword arguments for st.plotly_chart have been deprecated and will be removed in a future release. Use the config argument instead to specify Plotly configuration options," is accurate in its broader context but is misfiring in this specific scenario. The intention behind this warning was to guide users away from passing arbitrary Plotly-specific settings directly to st.plotly_chart and towards the more structured config dictionary. However, the implementation seems to be too broad, flagging even standard Streamlit parameters that control chart dimensions.

Reproducible Code Example and Expected Behavior

To fully grasp the issue, let's look at a reproducible code example. This snippet demonstrates how the warning appears:

import plotly.graph_objects as go
import streamlit as st

fig = go.Figure()
fig.add_trace(go.Barpolar(r=[1], theta=[0], width=[120]))
fig.update_layout(width=500, height=360)

st.plotly_chart(fig, width="content")

In this code, we create a simple Plotly figure and then display it using st.plotly_chart. The crucial part is the width="content" argument. According to Streamlit's documentation, this should instruct the chart to adjust its width to fill the available content area. This is a common and useful setting for responsive web applications.

The Expected Behavior is straightforward: the Plotly chart should render correctly, filling the content width, and no deprecation warnings should be shown. The width="content" argument is a documented feature of st.plotly_chart for controlling layout, and it should be handled without triggering a warning about deprecated kwargs. The intention of the documentation is to guide users towards the config parameter for Plotly-specific configurations, not for Streamlit's own layout parameters like width or height.

The Current Behavior, however, is that running this code, even with Streamlit version 1.50.0 (as indicated in the debug info), throws the kwargs deprecation warning. This happens because the internal mechanism that checks for unexpected keyword arguments seems to be misinterpreting width="content" as an arbitrary kwargs parameter rather than a recognized Streamlit argument. This is precisely why the issue is considered a regression – it indicates that a functionality that previously worked without issues is now producing unwanted warnings, potentially confusing users and obscuring genuine API deprecations.

It's important to distinguish between passing Plotly configuration options (which should go into the config dictionary) and passing Streamlit layout parameters (like width and height). The current warning seems to be conflating these two, leading to a less-than-ideal developer experience. The fix, therefore, would involve Streamlit's development team ensuring that its own documented layout parameters are correctly recognized and not flagged by the generic kwargs deprecation check.

Why This Matters and How to Potentially Mitigate

This deprecation warning issue might seem minor, but it impacts the clarity and professionalism of your Streamlit applications. Unexpected warnings can:

• Confuse new users: Developers new to Streamlit might see the warning and think they are using the API incorrectly, leading to unnecessary troubleshooting.
• Mask real warnings: If your application generates many of these false warnings, it becomes harder to spot genuine deprecation warnings that might indicate future compatibility issues.
• Degrade the user experience: While not a functional bug, a console filled with warnings isn't ideal.

As mentioned, the underlying cause seems to be how Streamlit's st.plotly_chart handles its own layout arguments (like width and height) versus how it handles Plotly's internal configuration options. The warning is intended to steer developers towards using the config dictionary for things like enabling/disabling modes, tooltips, or other Plotly-specific behaviors, rather than passing them directly as keyword arguments. However, in this case, arguments like width="content" are not Plotly configurations; they are Streamlit parameters that control how the Plotly chart is displayed within the Streamlit app's layout.

A potential mitigation strategy, though not ideal, would be to avoid using width="content" or height="auto" if the warning is problematic for your use case. You could try setting explicit pixel values for width and height if that's acceptable, or perhaps try to manage the container width through other Streamlit layout elements like columns or containers, although this might not always yield the desired responsive behavior.

Another approach could be to explore the config parameter, but it's crucial to understand that config is for Plotly's settings, not Streamlit's. For example, you might use config={'scrollZoom': True} to enable scroll zoom on your Plotly chart. It's unlikely that width="content" can be directly translated into the config dictionary because it's a Streamlit-level directive.

The best course of action is to hope for a fix from the Streamlit team. As this issue has been reported and acknowledged, it's likely that a future release of Streamlit will correctly differentiate between its own layout parameters and Plotly's configuration options, thus resolving this misleading deprecation warning. In the meantime, developers should be aware of this quirk and not be overly alarmed if they see it when using documented width and height parameters.

Looking Ahead: Streamlit and Plotly Integration

The interaction between Streamlit and visualization libraries like Plotly is a cornerstone of modern data app development. Streamlit excels at providing a simple Pythonic way to build web applications, while Plotly offers sophisticated charting capabilities. The st.plotly_chart function acts as a bridge, allowing these two powerful tools to work seamlessly together. Understanding how arguments are passed and managed within this bridge is crucial for efficient development.

The evolution of the st.plotly_chart function, including changes related to argument handling like the kwargs deprecation warning, reflects Streamlit's ongoing commitment to providing a robust and well-organized API. While the current issue with width="content" is a temporary hiccup, it highlights the importance of clear documentation and precise implementation in API design. The warning's intent – to guide users toward the config argument for Plotly-specific settings – is valid. The goal is to prevent a situation where users might pass Plotly-specific arguments directly to st.plotly_chart that could conflict with Streamlit's own internal workings or future updates.

By consolidating Plotly configurations within the config dictionary, developers ensure that their charts behave predictably and remain compatible with future versions of both Streamlit and Plotly. This approach also makes it easier to manage complex chart configurations, as the config dictionary can hold various settings related to interactivity, theming, and data display.

However, as seen in this particular case, overly broad warning mechanisms can sometimes flag legitimate usage. The developers behind Streamlit are constantly working to refine these checks. When issues like the width="content" warning arise, reporting them through the appropriate channels (like GitHub issues) is vital. This feedback loop allows the Streamlit team to identify and address these nuances, ensuring that the developer experience remains as smooth as possible. It's a collaborative process where community input helps shape a better tool for everyone.

As Streamlit continues to mature, we can expect further enhancements to its integration with popular visualization libraries. The focus will likely remain on providing a clean, intuitive API that balances flexibility with predictability. For now, developers encountering the width="content" warning can be assured that it's a known issue and that a resolution is likely on the horizon. Continuing to use Streamlit and Plotly together offers a powerful combination for creating dynamic and insightful data applications.

For more information on Streamlit's charting capabilities and best practices for integrating Plotly, you can refer to the official Streamlit Documentation. The Plotly Python documentation is also an invaluable resource for understanding the full range of customization options available for your charts.