Streamlit Plotly Chart Width Bug: Deprecation Warning Fix

The Unexpected kwargs Warning

If you've been working with Streamlit and Plotly, you might have encountered a rather annoying deprecation warning when trying to set the width of your charts using st.plotly_chart(fig, width="content"). This warning message, which pertains to variable keyword arguments (kwargs), pops up even when you're not explicitly passing any extra keyword arguments, and you're just using the documented parameters. It's a bit like getting a warning about loud music when you're just whispering – confusing and unnecessary! This issue has been reported and discussed within the Streamlit community, causing some confusion among developers who are just trying to make their visualizations look just right. The core of the problem lies in how Streamlit handles arguments passed to st.plotly_chart, particularly when specific arguments like width are used with certain values. Essentially, the warning is triggered internally, making it seem like you've done something wrong when, in fact, you've followed the documentation. This can be frustrating, especially when you're trying to deploy an application and want to keep the console output clean and free of unnecessary warnings. Let's dive deeper into why this happens and how it can be resolved, ensuring your Streamlit apps remain professional and your debugging process smooth.

Reproducing the plotly_chart Issue

To truly understand the problem, let's look at a simple, reproducible code example that triggers this unexpected deprecation warning. This example uses standard libraries, plotly.graph_objects and streamlit, to create a basic Plotly figure and then displays it using st.plotly_chart with the width="content" argument. Here’s the code snippet that demonstrates the issue:

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")

When you run this simple script in your Streamlit environment, you'll notice that even though width="content" is a valid and documented way to instruct st.plotly_chart to adjust the chart's width to fit its container, a deprecation warning related to kwargs still appears. This is particularly perplexing because we are not passing any variable keyword arguments that aren't explicitly handled by the function. The warning suggests that variable keyword arguments are being used and are deprecated, recommending the use of the config argument instead. However, in this scenario, width="content" is not a variable keyword argument in the sense that it's an undocumented, catch-all parameter; it's a specific, intended argument for controlling chart dimensions. The issue stems from an internal check within Streamlit's plotly_chart function that flags any kwargs being present, even if those kwargs are being used to pass recognized arguments like width. This oversight leads to the warning being displayed erroneously, creating a perception of incorrect usage when the code is, in fact, following established patterns.

The Underlying Cause: Internal kwargs Handling

The root cause of this deprecation warning stems from how Streamlit's st.plotly_chart function handles incoming arguments. In recent versions, Streamlit introduced changes to manage keyword arguments (kwargs) more strictly, aiming to deprecate the use of arbitrary kwargs and guide users towards using a dedicated config argument for Plotly-specific configurations. The intention behind this change is to provide a clearer and more organized way to pass settings to the underlying Plotly chart. However, the implementation of this check has a slight flaw when it comes to arguments like width and height when they are set to specific values like "content".

Here’s a peek at the logic that causes the issue:

if kwargs:
    show_deprecation_warning(
        "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."
    )

As you can see, the warning is triggered simply if the kwargs dictionary is not empty. When you pass width="content" (or height="content") to st.plotly_chart, Streamlit internally treats this as a keyword argument that might not be a standard, fixed parameter. Even though width and height are documented parameters for st.plotly_chart, the internal processing or how they are passed along might lead to them being included in the kwargs that the warning check intercepts. Therefore, the condition if kwargs: evaluates to True, and the deprecation warning is shown, even though the user is using documented functionality and not passing arbitrary, undefined kwargs. This creates a false positive, leading developers to believe they are misusing the API when, in reality, it's a subtle bug in the warning mechanism itself.

Expected vs. Current Behavior

Understanding the discrepancy between what we expect and what we are currently experiencing is crucial for troubleshooting and advocating for fixes. The expected behavior when using st.plotly_chart with width="content" is straightforward: the chart should simply render with its width adjusted to fit the Streamlit container, and no deprecation warnings should be shown. The documentation clearly indicates that width and height are valid parameters for st.plotly_chart, and using string values like "content" is a supported feature for responsive design.

Conversely, the current behavior, as observed and demonstrated in the reproducible example, is that calling st.plotly_chart(fig, width="content") triggers an erroneous deprecation warning. This warning message,

"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 displayed because the internal kwargs check in Streamlit incorrectly flags the width="content" argument as a variable keyword argument. This warning is misleading because width is a documented parameter, not an arbitrary one, and its use in this context is not causing any actual compatibility issues with Plotly itself. The intended use of the config argument is for finer-grained Plotly-specific settings that are passed directly to the Plotly.js library, not for basic layout adjustments like width and height that Streamlit aims to manage.

Is This a Regression?

Yes, this is indeed a regression. Users have reported that this behavior was not present in previous versions of Streamlit. The problem seems to have been introduced following changes related to the handling of kwargs in st.plotly_chart. Previously, developers could confidently use arguments like width="content" without encountering any deprecation warnings. The current behavior, where a warning is triggered for a documented and functional parameter, indicates a step backward in terms of user experience and API clarity. This regression can be particularly disruptive for applications that were functioning correctly and have been updated to newer Streamlit versions, leading to unexpected console noise and potential confusion for developers maintaining the codebase. It highlights the importance of thorough testing for backward compatibility when making changes to core functionalities and argument parsing within libraries. The community's expectation is that updates should either maintain existing functionality or provide clear migration paths, which is not the case with this specific warning.

Debugging Information and Future Steps

To aid in diagnosing and resolving this issue, the following debugging information has been provided:

• Streamlit version: 1.50.0
• Python version: 3.14.2
• Operating System: Mac OS 26.1
• Browser: Chrome

This information provides a specific context for the bug. The fact that it's reproducible across different environments (though versions are key) suggests a systemic issue within Streamlit's codebase related to argument handling for st.plotly_chart. The specific version 1.50.0 is important, as it helps pinpoint when this regression might have been introduced.

Moving forward, the primary goal is to address the flawed kwargs check. Ideally, Streamlit's st.plotly_chart should differentiate between arbitrary, undocumented kwargs and recognized, documented parameters like width and height. The warning should only be triggered when truly variable, unexpected keyword arguments are passed.

Potential solutions include:

1. Refining the kwargs check: Modify the condition to specifically exclude known, documented parameters like width and height from being flagged as problematic kwargs.
2. Improving argument parsing: Ensure that width and height are handled as distinct parameters before the general kwargs check is performed.
3. Updating documentation (if necessary): While the documentation currently supports width="content", ensuring this remains clear and accurate is vital. However, the core issue is the warning itself.

This issue underscores the importance of a clear and consistent API. While the move towards structured configuration is generally positive, it should not come at the expense of breaking existing, well-documented features or generating misleading warnings. Community feedback and detailed bug reports, like this one, are essential for identifying and rectifying such regressions, ensuring Streamlit continues to be a robust and user-friendly tool for building data applications.

For further insights into Streamlit development and best practices, you can always refer to the official Streamlit Documentation. Additionally, for discussions and community support, the Streamlit Community Cloud is an excellent resource.