Streamlit: Fix Plotly Chart Width Warning

If you're working with Streamlit and Plotly, you might have encountered a peculiar deprecation warning when trying to set the width or height of your charts using values like 'content'. It's a bit of a head-scratcher because you're not explicitly passing any variable keyword arguments (kwargs), yet the warning message suggests that you are, and that they're deprecated. This article dives into why this happens and how to resolve it, ensuring your Streamlit applications display your interactive visualizations smoothly and without unnecessary noise.

Understanding the st.plotly_chart Deprecation Warning

The core of the issue lies in how Streamlit handles arguments passed to st.plotly_chart. When you use parameters like width='content' or height='auto', you're essentially telling Streamlit to dynamically adjust the chart's dimensions based on its content or the container it resides in. While these are documented and intended ways to control chart sizing, the internal implementation within Streamlit, particularly after recent updates, might be triggering a deprecation warning related to kwargs. This warning, intended for situations where developers pass arbitrary **kwargs to the function, is mistakenly appearing even when only using the officially supported parameters.

The kwargs Quandary in Streamlit

In Python, **kwargs is a special syntax that allows a function to accept an arbitrary number of keyword arguments. These arguments are then collected into a dictionary. Developers often use this to make functions more flexible, allowing users to pass in any configuration options without the function needing to explicitly define every single one. However, Streamlit's st.plotly_chart function has a specific set of parameters it officially supports for controlling chart behavior and appearance. The warning you're seeing, "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 meant to guide users away from passing unsupported keyword arguments directly.

The problem arises when Streamlit's internal logic, in processing arguments like width='content', inadvertently flags these as part of kwargs from its perspective. This can happen if the width and height parameters are internally handled or passed down in a way that the warning mechanism interprets as arbitrary keyword arguments. It's a classic case of an overzealous warning system catching legitimate usage.

This can be particularly frustrating because the Streamlit documentation clearly outlines how to use parameters like width and height. When a warning appears for a documented feature, it erodes confidence and can lead to developers unnecessarily modifying their code, trying to appease a warning that doesn't accurately reflect their usage. The goal of Streamlit is to simplify app development, and confusing deprecation warnings certainly don't help with that.

The key takeaway here is that the warning is likely a false positive, stemming from the way Streamlit's internal argument parsing interacts with its deprecation checks. It's not that you're doing something fundamentally wrong with st.plotly_chart, but rather that the function's internal workings are producing a warning that doesn't align with the user's intent or the documented API.

Reproducing the Issue: A Code Example

To fully grasp the problem, let's look at a straightforward example that triggers this warning. The following Python code, when run in a Streamlit application, demonstrates the issue:

import plotly.graph_objects as go
import streamlit as st

# Create a simple Plotly figure
fig = go.Figure()
fig.add_trace(go.Barpolar(r=[1], theta=[0], width=[120]))
fig.update_layout(width=500, height=360) # Setting explicit layout dimensions

# Display the chart using st.plotly_chart with width='content'
st.plotly_chart(fig, width="content")

In this snippet, we first import the necessary libraries: plotly.graph_objects for creating the figure and streamlit for building the app. We then instantiate a go.Figure and add a simple Barpolar trace. Crucially, we use fig.update_layout(width=500, height=360) to set some initial dimensions for the Plotly figure itself. The actual problematic line is st.plotly_chart(fig, width="content"). Here, we pass the Plotly fig object and explicitly set the width parameter of st.plotly_chart to the string 'content'. This is the action that, in certain Streamlit versions, results in the deprecation warning, even though 'content' is a valid way to suggest the chart should take up the available width.

Why This Code Triggers the Warning

The warning appears because Streamlit's st.plotly_chart function is designed to accept various arguments to control how the Plotly chart is rendered within the Streamlit app. These arguments include use_container_width (which is deprecated in favor of width and height parameters) and width and height themselves. When you pass width='content', Streamlit interprets this as a directive to size the chart based on its content, often aiming to fill the available horizontal space.

However, the underlying mechanism that checks for deprecated kwargs seems to be catching the width='content' argument. It's possible that internally, Streamlit might be using a **kwargs mechanism to pass these sizing parameters to a lower-level component or a different function, and this is what's triggering the warning. The warning message specifically mentions kwargs and suggests using the config argument for Plotly configuration options. While width='content' isn't strictly a Plotly configuration option in the same vein as plotly.js settings, it's an argument controlling the Streamlit wrapper's behavior around the Plotly chart.

It's important to note that the expected behavior is that using documented parameters like width='content' should not produce a deprecation warning. The warning suggests that this usage is problematic, when in fact, it's a feature supported by the API. This makes it a regression, as it likely worked without warnings in older versions of Streamlit.

The reproducible code highlights a mismatch between Streamlit's internal warning system and its public API documentation. Developers using documented features should not be confronted with deprecation warnings. This situation underscores the importance of thorough testing and careful implementation when refactoring internal components, especially those that interact with user-facing APIs.

The Expected vs. Current Behavior

Let's clarify what should happen versus what's currently happening when you use st.plotly_chart with specific width or height arguments.

Expected Behavior: Smooth Sailing

Ideally, when you use st.plotly_chart with parameters like width='content' or height='auto', the chart should render according to your specifications without any admonitions from the console. The documentation for Streamlit explicitly mentions these parameters as valid ways to control the size of your Plotly charts within the Streamlit app. For instance, width='content' is intended to make the chart adapt to its content's width, or more commonly, to fill the available width of its container. If you were using the older use_container_width parameter, migrating to width or height is the correct step, and this migration should not, in itself, trigger a deprecation warning.

The expected behavior is that the transition from deprecated arguments (like use_container_width) to the new ones (width, height) should be seamless. Using the new, documented parameters should work as advertised, without any warnings related to kwargs. This ensures a clean and professional output for your Streamlit applications, free from distracting console messages.

Current Behavior: The Unwanted Warning

As observed in the reproducible code example and reported by users, the current behavior in specific versions of Streamlit (e.g., 1.50.0) is that passing width='content' to st.plotly_chart results in the following 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.

This warning appears even though you are not explicitly passing any arbitrary keyword arguments. You are using a documented parameter (width) with a valid string value ('content'). The warning seems to be incorrectly triggered by Streamlit's internal handling of these arguments. This suggests that the way width or height is processed internally might be misinterpreted by the warning mechanism as an unsupported **kwargs usage.

The current behavior indicates a bug or an unintended consequence of recent changes in Streamlit's argument parsing or deprecation warning system. It's a regression because this functionality likely worked without such warnings in previous versions. This discrepancy between expected and actual behavior is what needs to be addressed to maintain a stable and predictable development experience.

Is This a Regression?

Yes, this is a regression. Based on the typical lifecycle of software and the nature of deprecation warnings, the fact that using a documented parameter like width='content' now triggers a warning that was not present in earlier versions strongly suggests a regression. Development teams strive to maintain backward compatibility where possible, and introducing warnings for previously functional, documented features is usually an unintended side effect of changes, rather than a deliberate design decision.

This regression likely stems from changes made to Streamlit's internal argument handling or its warning system. Perhaps a refactoring effort intended to improve the management of kwargs or to better integrate with Plotly's configuration options inadvertently caused existing, valid uses of width and height to be flagged incorrectly. The issue becomes particularly noticeable when users attempt to leverage the newer, recommended ways of controlling chart dimensions, such as using width='content', which aligns with the modern approach to responsive web design.

The primary characteristic of a regression is the introduction of new errors or unexpected behaviors in a previously stable part of the software. In this case, the unexpected behavior is the appearance of a deprecation warning for a supported feature. This means that users who updated Streamlit might find their applications now display this warning, disrupting their development workflow and potentially leading to confusion about the status of their codebase.

Debugging and Resolution

When faced with a regression like the st.plotly_chart kwargs warning, the first step is often to pinpoint the exact version where the issue started. The provided debug information indicates Streamlit version 1.50.0 is experiencing this problem. This version information is crucial for the Streamlit development team to investigate the commit history and identify the specific code changes that introduced the warning.

Investigating the Streamlit Codebase

The warning message itself points towards a check for if kwargs:. This suggests that the system is looking for any remaining items in the kwargs dictionary after all known arguments have been processed. If width='content' is being passed down in a way that populates kwargs when it shouldn't, that's the root cause. Potential areas for investigation within the Streamlit codebase would include:

1. Argument Parsing: How does st.plotly_chart parse its arguments, especially width and height?
2. Internal Function Calls: Are width or height being passed as **kwargs to any internal helper functions or components?
3. Deprecation Warning Logic: Is the warning specifically checking for non-empty kwargs after known parameters are handled, and is this check too broad?

Potential Solutions and Workarounds

While the Streamlit team works on a permanent fix, users might need workarounds. The warning suggests using the config argument for Plotly configuration options. However, width='content' is an argument for st.plotly_chart itself, not directly for Plotly's internal configuration dictionary.

A potential workaround, though less ideal, could be to avoid using width='content' if it consistently triggers the warning and stick to fixed pixel values or use_container_width=True if that doesn't raise the same issue (though use_container_width itself is deprecated and should be replaced).

# Example workaround (if fixed width is acceptable)
st.plotly_chart(fig, width=800) # Using a fixed width

Alternatively, if the issue is truly tied to the presence of the width parameter triggering the kwargs check, one might explore if use_container_width=True behaves differently, though this parameter is also marked for deprecation.

The most direct solution would be for the Streamlit maintainers to adjust the kwargs check in st.plotly_chart to correctly distinguish between arbitrary keyword arguments and documented parameters like width and height. This might involve ensuring that width and height are properly handled as explicit parameters before the kwargs check occurs.

Conclusion: Towards a Smoother Streamlit Experience

This specific deprecation warning related to st.plotly_chart and width='content' highlights a common challenge in software development: ensuring that internal refactoring doesn't inadvertently break or warn about documented, functional features. For developers building data applications with Streamlit and interactive visualizations with Plotly, encountering such warnings can be a source of confusion and frustration. The good news is that this appears to be a regression within Streamlit itself, likely caused by how the kwargs check is implemented.

By understanding the context – that width='content' is a valid, documented parameter, and the warning is likely a false positive – users can remain confident in their code. The Streamlit team is actively working to resolve such issues to provide a more stable and predictable development environment. In the meantime, while awaiting a fix, exploring alternative sizing parameters or sticking to fixed dimensions might serve as temporary workarounds.

We encourage users experiencing this issue to check the Streamlit GitHub repository for updates or to contribute to the discussion. Addressing these kinds of bugs ensures that Streamlit continues to be a powerful and user-friendly tool for building interactive web applications with Python.

For more information on Streamlit and its components, you can refer to the official Streamlit Documentation. To learn more about Plotly, visit the Plotly Website.