Streamlit Plotly Chart Width Deprecation Warning

When you're building interactive dashboards with Streamlit, you often want to fine-tune the appearance of your visualizations. One common task is adjusting the width and height of your charts. Recently, users have encountered a peculiar deprecation warning when using st.plotly_chart with the width="content" argument, even when they weren't explicitly passing any variable keyword arguments. This can be quite confusing, especially when you're just trying to use documented parameters. Let's dive into what's happening, why it's occurring, and how to navigate it effectively.

The Root of the Warning: How Streamlit Handles plotly_chart Arguments

At its core, the warning stems from how Streamlit processes the arguments passed to st.plotly_chart. The function is designed to be flexible, accepting various parameters to customize how your Plotly figures are displayed within your Streamlit app. Historically, some of these parameters, like width and height, were treated as general keyword arguments (kwargs). However, as Streamlit evolves, it's becoming best practice to use a dedicated config dictionary for Plotly-specific configurations. This shift is intended to provide clearer separation and a more organized way to manage Plotly chart settings.

When you use st.plotly_chart(fig, width="content"), Streamlit's internal logic might still be processing width as a general keyword argument before it's correctly interpreted and applied to the Plotly chart's layout. The deprecation warning is a signal from Streamlit that this way of handling width (and potentially height) is becoming outdated. It's a heads-up that in future versions, relying on these as direct keyword arguments might break your code, and you should instead use the config parameter. The warning is triggered even if you're not passing other arbitrary kwargs, because the width parameter itself is being caught by the logic that checks for deprecated kwargs usage. It's a bit like a security guard seeing someone using an old keycard slot, even if they're not trying to sneak into any other restricted areas. The slot itself is flagged as deprecated.

Reproducing the Issue: A Simple Code Example

To help illustrate this, let's look at a straightforward code snippet that triggers this warning. This example demonstrates a common scenario where you create a basic Plotly figure and then attempt to set its width using the "content" value within Streamlit:

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) # Standard Plotly layout updates

# Attempt to set the width of the chart in Streamlit
st.plotly_chart(fig, width="content")

When you run this code in a Streamlit application (specifically with versions around 1.50.0 or earlier, on Python 3.14.2 with macOS 26.1 and Chrome), you'll likely see a warning message in your console or Streamlit's output. This warning isn't indicating that you've done something fundamentally wrong with your Plotly figure or its layout; rather, it's highlighting how Streamlit is interpreting the width="content" argument. It's a subtle but important distinction. The deprecation warning is specifically about the method of passing the width, not the validity of the width value itself. The intended behavior is for width="content" to work seamlessly, adapting the chart's width to its container.

Expected vs. Current Behavior: What Should Happen and What Is Happening

The expected behavior, as documented and intended, is that using arguments like width="content" or height="content" with st.plotly_chart should work without issuing a deprecation warning. These parameters are designed to provide convenient ways to control the display size of your charts, making them responsive to the surrounding layout. The "content" value, in particular, is meant to instruct Streamlit to automatically adjust the chart's width to fill the available space in its container. This is a crucial feature for creating adaptable and user-friendly dashboards.

However, the current behavior, as observed in recent versions, is that passing width="content" triggers a deprecation warning. The warning message often states something like: "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 width="content" is a documented and intended parameter, not an arbitrary kwargs addition. The underlying issue seems to be that the internal mechanism responsible for detecting and warning about deprecated kwargs is mistakenly flagging the width parameter when it's used in this manner. It's a case where the warning system is a bit too enthusiastic and is flagging a legitimate, albeit evolving, usage pattern.

The Evolution of st.plotly_chart Arguments

Streamlit's API for displaying Plotly charts has evolved over time to better align with Plotly's own best practices and to provide a more robust user experience. Previously, it was common to pass various Plotly chart configurations directly as keyword arguments to st.plotly_chart. This approach worked, but it could lead to ambiguity, especially as the number of available configuration options grew. Plotly itself recommends using a config dictionary to manage these settings, which offers a more structured and explicit way to define how a chart should behave and appear.

To facilitate this transition and to prepare for future changes, Streamlit introduced the config parameter. This parameter accepts a dictionary where you can place Plotly-specific settings. For instance, instead of st.plotly_chart(fig, width=500), the recommended way to set the width would be st.plotly_chart(fig, config={'width': 500}) or, for responsiveness, st.plotly_chart(fig, config={'autosize': True}) or similar configurations that influence size. The width="content" behavior is a specific instance where the direct argument was a convenient shortcut. The deprecation warning signals that this shortcut is being phased out in favor of the more structured config approach. It's a proactive measure to ensure that Streamlit apps remain compatible with future Plotly and Streamlit updates.

Why Is This Happening Now? The Deprecation and Its Impact

The deprecation warning is a consequence of ongoing development and refactoring within the Streamlit library. As mentioned, the primary driver is the move towards using the config argument for Plotly-specific settings, rather than relying on direct keyword arguments like width and height. This change aims to standardize how Plotly charts are configured within Streamlit, making the API cleaner and more predictable. By consolidating these settings into a config dictionary, Streamlit can better manage and pass these options to the underlying Plotly rendering engine.

The immediate impact of this warning is primarily a notification. Your Streamlit application will likely continue to function as expected, with the width="content" argument still controlling the chart's width. However, this warning is a strong indicator that you should update your code to use the config parameter for future compatibility. Ignoring it might lead to your application breaking when a future Streamlit version removes direct support for these arguments. It's a standard software development practice to provide ample warning before introducing breaking changes.

The developers are essentially guiding users to adopt a more modern and maintainable way of configuring their Plotly charts in Streamlit. While the width="content" argument was a useful feature, its implementation as a direct keyword argument is being superseded. The goal is to ensure that Streamlit remains a cutting-edge tool for data visualization, seamlessly integrating with the latest advancements in libraries like Plotly. This proactive approach helps prevent larger issues down the line and keeps the library ecosystem healthy.

The Recommended Solution: Using the config Argument

The most effective way to address the deprecation warning and ensure your Streamlit applications are future-proof is to migrate from using direct width and height arguments to the config dictionary. For the specific case of width="content", you'll want to configure Plotly to automatically adjust its size based on the container. This is typically achieved by setting 'autosize': True within the config dictionary. While "content" isn't a direct Plotly config option in the same way, autosize often achieves a similar responsive behavior.

Here's how you can modify the reproducible code example to use the config argument:

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) # You might keep or remove this depending on desired default behavior

# Use the config argument for responsive sizing
# 'autosize': True is often the closest equivalent to width="content" for responsiveness
st.plotly_chart(fig, config={'autosize': True})

It's important to note that the exact behavior of "content" might be slightly different from 'autosize': True, depending on the specifics of how Streamlit handles element sizing versus Plotly's internal autosizing. However, 'autosize': True is the standard Plotly way to make a chart responsive to its container, which is usually the desired outcome when using width="content".

If you need to specify a fixed width and height, you can do so within the config dictionary as well:

# For fixed dimensions using config
st.plotly_chart(fig, config={'width': 500, 'height': 360})

By adopting this approach, you eliminate the deprecation warning and align your code with Streamlit's recommended practices for integrating Plotly charts. This ensures that your applications will continue to work smoothly as Streamlit and Plotly evolve. It's a small change that brings significant benefits in terms of code maintainability and future compatibility.

Conclusion: Embracing Evolving APIs

Deprecation warnings, while sometimes inconvenient, are a vital part of the software development lifecycle. They serve as signposts, guiding developers towards more robust, efficient, and future-proof coding practices. The warning associated with st.plotly_chart's width="content" argument is a prime example of this. It highlights a shift in how Streamlit intends for users to configure Plotly charts, moving towards a more centralized config dictionary.

By understanding the reasons behind this warning – the transition from direct keyword arguments to a dedicated configuration object – you can confidently adapt your code. Migrating to the config argument, particularly using options like 'autosize': True, is the recommended path forward. This not only resolves the immediate warning but also ensures that your Streamlit applications remain compatible with future updates.

As you continue to build sophisticated data applications with Streamlit, embrace these API evolutions. They are designed to enhance your development experience and the performance of your applications. For further insights into Streamlit's capabilities and best practices, exploring the official Streamlit documentation is always a great resource. Additionally, for deeper understanding of Plotly configurations, the Plotly.js documentation offers comprehensive details on available options.