Streamlit Plotly Chart Width Deprecation Warning

Hey there, fellow Streamlit enthusiasts! Have you ever been happily building your interactive dashboards, dropping in some beautiful Plotly charts, and then BAM! – you're greeted with a deprecation warning that seems to come out of nowhere? Yeah, me too. It’s particularly confusing when you’re sure you’re not using any weird, undeclared keyword arguments. Today, we’re diving deep into a common head-scratcher: the kwargs deprecation warning when using st.plotly_chart with specific width or height settings. Let’s untangle this, understand why it happens, and ensure your Streamlit apps remain sleek and warning-free.

The Mystery of the Unwanted Warning

So, you’ve got your Plotly figure ready to go, and you want to control its size within your Streamlit app. The documentation points you towards using arguments like width='content' or height='content' directly within the st.plotly_chart function. It seems straightforward, right? You add st.plotly_chart(fig, width='content'), expecting your chart to perfectly fill its container or adjust as needed. However, instead of a clean render, you’re presented with a warning message. This 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 is where the confusion sets in. You might be thinking, "But I’m not using any variable keyword arguments! I’m just using width='content', which is clearly documented!"

This behavior was particularly noticeable around Streamlit version 1.50.0. The issue arises from how Streamlit internally handles the arguments passed to st.plotly_chart. Even when you pass documented parameters like width or height, Streamlit's internal logic might be checking for the presence of any additional kwargs that haven't been explicitly handled. When it finds these, it triggers the deprecation warning. Essentially, the warning is a bit too eager, flagging the potential for using undeclared kwargs rather than strictly enforcing it only when they are actually misused for Plotly configuration.

It’s important to remember that Streamlit is constantly evolving. Developers work hard to streamline the API and ensure consistency. In this case, the introduction of the config argument for Plotly-specific settings was a move towards better organization. However, the transition period can sometimes lead to these kinds of slightly overzealous warnings. The good news is that understanding the root cause helps us navigate it and find the best practices moving forward. Let's look at a simple example that demonstrates this.

A Concrete Example of the Behavior

To really see this in action, let’s consider the reproducible code example provided:

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 code in an environment where this deprecation warning is active (like Streamlit version 1.50.0), you’ll notice that even though width='content' is a valid and documented parameter for st.plotly_chart, the warning about kwargs still pops up. This is because Streamlit's internal mechanism for checking kwargs is triggered by the presence of parameters that aren't part of its core, explicitly defined arguments for plotly_chart in that specific context. The width and height arguments, while intuitive, were being processed in a way that inadvertently triggered the kwargs check, leading to the deprecation notice.

This behavior is particularly frustrating because it doesn't necessarily indicate a bug in your code, but rather a nuance in how the Streamlit library is handling its arguments and warnings during a transitional phase. The core functionality of displaying the Plotly chart with the specified width still works, but the warning can be a distraction, potentially masking other, more critical warnings or simply cluttering the console output. It’s a classic case of a warning being a bit too broad in its scope, catching legitimate uses along with the intended targets for deprecation.

Why the Change? Embracing the config Argument

Streamlit’s decision to warn about kwargs and steer users towards the config argument is rooted in a desire for clearer API design and better separation of concerns. In the past, Plotly charts might have accepted a wide range of configuration options directly as keyword arguments. While this can be convenient for simple cases, it can lead to ambiguity and make it harder to manage complex configurations.

By introducing the config argument, Streamlit encourages a more structured approach. The config argument expects a dictionary, allowing you to pass Plotly-specific configurations, including displayModeBar, responsive, scrollZoom, and crucially for our case, width and height when you want them to behave in a Plotly-native, responsive manner. This separation helps:

1. Improve Clarity: It becomes immediately obvious which arguments are Streamlit-specific (like use_container_width in older versions, or how width/height are now handled) and which are Plotly’s internal configurations.
2. Enhance Maintainability: As both Streamlit and Plotly evolve, having a dedicated config dictionary makes it easier to manage updates without breaking Streamlit’s core API. Streamlit can pass this dictionary through to Plotly’s underlying rendering engine more reliably.
3. Promote Best Practices: It guides users towards a more robust way of handling Plotly’s extensive customization options, especially when dealing with responsiveness and layout.

So, while the deprecation warning might feel like an annoyance when using width='content', it’s a nudge towards a more robust and future-proof way of configuring your Plotly charts within Streamlit.

The Expected vs. Current Behavior: A Tale of Two Outcomes

The expected behavior when migrating from older methods (like use_container_width) to the newer width and height arguments in st.plotly_chart is, quite simply, that it should just work without generating unnecessary warnings. If you’re using documented parameters like width='content', you shouldn’t be prompted about deprecated kwargs. The warning system should be precise, targeting only actual misuse of variable keyword arguments where Plotly-specific configurations are being passed incorrectly.

However, the current behavior, as observed in certain Streamlit versions, is that using width='content' (or height='content') does trigger this kwargs deprecation warning. This happens because Streamlit’s internal check for unused kwargs is triggered by the presence of these arguments, even though they are valid and intended for use. The code that checks for if kwargs: erroneously flags the situation, assuming that any argument not explicitly handled by Streamlit’s own core functions might be an intended Plotly configuration passed as a kwarg.

This discrepancy highlights a slight misalignment in the warning mechanism. It’s a regression because functionality that previously worked without warnings now generates them. This can be confusing for users, leading them to question their implementation or worry about future compatibility. The ideal scenario is a seamless transition where updated documentation and API changes are accompanied by a warning system that accurately reflects the user’s actions.

Understanding the Regression

Is this a regression? Yes, absolutely. In previous versions of Streamlit, you could comfortably use arguments like width='content' with st.plotly_chart without encountering the kwargs deprecation warning. The warning was specifically designed to catch users attempting to pass Plotly’s internal configuration options directly as keyword arguments, which is now discouraged in favor of the config dictionary. However, the implementation of this warning became a bit too aggressive, catching a valid Streamlit-specific argument (width='content') as if it were an incorrect Plotly kwarg.

This is a classic example of a regression bug. A regression occurs when a change in the software introduces a new bug or causes previously working functionality to stop working as expected. In this case, the functionality of setting chart width via st.plotly_chart(fig, width='content') remains intact – the chart still renders correctly with the desired width. However, the introduction of the warning message that wasn't present before constitutes a degradation in the user experience and API clarity. It’s a step backward in terms of the