[DynamicContainers] Callback support for tabs by sfc-gh-lwilby · Pull Request #14007 · streamlit/streamlit

sfc-gh-lwilby · 2026-02-18T20:36:52Z

Describe your changes

Adds callable on_change callback support to st.tabs, extending the existing "ignore" / "rerun" / None string literals.

on_change now accepts a callable in addition to the existing string options
Optional args and kwargs parameters pass through to the callback
The callback fires when the user switches to a different tab. st.session_state[key] contains the label of the newly active tab, so the callback can branch on which tab was selected.

GitHub Issue Link (if applicable)

Testing Plan

Unit Tests (Python) — lib/tests/streamlit/elements/layouts_test.py
E2E Tests — e2e_playwright/st_tabs_test.py
Any manual testing needed?
- Manually verified as well.

Contribution License Agreement

By submitting this pull request you agree that all contributions to this project are made under the Apache 2.0 license.

github-actions · 2026-02-18T20:37:05Z

✅ PR preview is ready!

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sfc-gh-lwilby · 2026-02-18T20:37:06Z

[DynamicContainers] Callback support for tabs #14007 👈 (View in Graphite)
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snyk-io · 2026-02-18T20:37:34Z

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Copilot

Pull request overview

Adds callable callback support to st.tabs so apps can run user code on tab switches (with optional args/kwargs), while keeping the existing "ignore" / "rerun" / None behaviors intact.

Changes:

Extend st.tabs(..., on_change=...) to accept a callable, plus new args/kwargs parameters that forward to the callback.
Add Python unit tests covering proto/id wiring, .open behavior, callback invocation semantics, and args/kwargs forwarding.
Add typing tests and Playwright E2E coverage for callback behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
`lib/streamlit/elements/layouts.py`	Extends `st.tabs` API + widget registration to support callable `on_change` with args/kwargs and updates docstring + validation.
`lib/tests/streamlit/elements/layouts_test.py`	Adds unit tests for callback wiring, state transitions, and args/kwargs forwarding.
`lib/tests/streamlit/typing/tab_container_types.py`	Updates typing tests to ensure callable `on_change` and args/kwargs type-check.
`e2e_playwright/st_tabs.py`	Adds app scenarios exercising `st.tabs` callbacks (including args/kwargs).
`e2e_playwright/st_tabs_test.py`	Adds E2E tests asserting callback execution on tab switches and updates tab component count.

Copilot · 2026-02-23T16:52:38Z

lib/streamlit/elements/layouts.py

+            - A callable: A callback function to execute before rerunning the
+              app when tabs are switched. Enables state tracking (equivalent to
+              ``"rerun"`` plus the callback). The callback receives no arguments
+              by default, but you can pass arguments using ``args`` and
+              ``kwargs``.


The docstring note near the top of st.tabs still says to use on_change="rerun" to enable lazy execution, but this PR adds callable on_change with the same stateful/lazy behavior. Please update that note to also mention the callable option so users don't miss the new capability.

Copilot · 2026-02-23T16:52:38Z

lib/streamlit/elements/layouts.py

+        args : tuple or None
+            An optional tuple of args to pass to the ``on_change`` callback.


args is typed as WidgetArgs (tuple or list) across Streamlit widgets, and other widget docstrings document this as “list or tuple”. Here it’s documented as “tuple or None”, which is inconsistent with the actual accepted types. Either update the docstring to “list or tuple” (and keep the WidgetArgs type), or add runtime/type-level validation to only accept tuples.

Suggested change

args : tuple or None

An optional tuple of args to pass to the ``on_change`` callback.

args : list or tuple or None

An optional list or tuple of args to pass to the ``on_change`` callback.

lib/tests/streamlit/elements/layouts_test.py

lukasmasuch

LGTM 👍

lukasmasuch · 2026-02-24T14:35:36Z

lib/streamlit/elements/layouts.py

+        if not callable(on_change) and (args is not None or kwargs is not None):
+            raise StreamlitAPIException(
+                "`args` and `kwargs` can only be used when `on_change` is a callable."
+            )


suggestion: I think it's slightly better here to just not throw the error because 1) for consistency, since I believe we are not doing it anywhere else, 2) with agents, it might be better to be a bit more forgiving.

OK, that sounds good to me. This was a suggestion from one of the AI reviews but it also seemed a bit heavy to me.

lib/streamlit/elements/layouts.py

Co-authored-by: lawilby <[email protected]>

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_tabs branch from 7c5b44b to 9aa118f Compare February 20, 2026 15:54

sfc-gh-lwilby added security-assessment-completed change:feature PR contains new feature or enhancement implementation impact:users PR changes affect end users labels Feb 23, 2026

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_tabs branch from 9aa118f to 9fa2210 Compare February 23, 2026 15:11

sfc-gh-lwilby marked this pull request as ready for review February 23, 2026 16:46

Copilot AI review requested due to automatic review settings February 23, 2026 16:46

Copilot started reviewing on behalf of sfc-gh-lwilby February 23, 2026 16:47 View session

Copilot AI reviewed Feb 23, 2026

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sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_tabs branch 2 times, most recently from 5d5fe20 to b119d39 Compare February 23, 2026 20:17

lukasmasuch approved these changes Feb 24, 2026

View reviewed changes

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_tabs branch 2 times, most recently from 8bae35c to 13f8f8f Compare February 25, 2026 07:11

graphite-app bot reviewed Feb 25, 2026

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[DynamicContainers] Callback support for tabs

d379ac7

Co-authored-by: lawilby <[email protected]>

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_tabs branch from 13f8f8f to d379ac7 Compare February 25, 2026 07:47

sfc-gh-lwilby merged commit c1ad756 into develop Feb 25, 2026
44 checks passed

sfc-gh-lwilby deleted the 02-18-_dynamiccontainers_callback_support_for_tabs branch February 25, 2026 09:06

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[DynamicContainers] Callback support for tabs#14007

[DynamicContainers] Callback support for tabs#14007
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02-18-_dynamiccontainers_callback_support_for_tabs

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		args : tuple or None
		An optional tuple of args to pass to the ``on_change`` callback.

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