[DynamicContainers] Callback support for expanders by sfc-gh-lwilby · Pull Request #14008 · streamlit/streamlit

sfc-gh-lwilby · 2026-02-18T20:37:03Z

Describe your changes

Adds callable on_change callback support to st.expander, extending the existing "ignore" / "rerun" 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
Requires a key when using a callable (consistent with existing expander behavior where key is required for "rerun")
The callback fires on any state change (both expand and collapse). Users can check st.session_state[key] inside the callback to distinguish between expand (True) and collapse (False) events.

GitHub Issue Link (if applicable)

Testing Plan

Unit Tests (Python) — lib/tests/streamlit/elements/layouts_test.py
E2E Tests — e2e_playwright/st_expander_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.

snyk-io · 2026-02-18T20:37:14Z

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github-actions · 2026-02-18T20:37:21Z

✅ PR preview is ready!

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sfc-gh-lwilby · 2026-02-18T20:37:21Z

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github-actions · 2026-02-18T20:47:52Z

📉 Frontend coverage change detected

The frontend unit test (vitest) coverage has decreased by 0.0000%

Current PR: 87.4100% (14197 lines, 1787 missed)
Latest develop: 87.4100% (14197 lines, 1787 missed)

✅ Coverage change is within normal range.

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Copilot

Pull request overview

Adds callable on_change callback support to st.expander, including passing args/kwargs and enforcing key when a callback is used.

Changes:

Extend st.expander(on_change=...) to accept a callable plus optional args/kwargs.
Add/expand unit, typing, and Playwright E2E coverage for callback behavior.
Update st.expander docstring to describe the new callback mode.

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	Adds callback + args/kwargs support to `st.expander` and updates validation/docs.
lib/tests/streamlit/elements/layouts_test.py	Adds unit tests covering callback requirements, firing behavior, and args/kwargs passthrough.
lib/tests/streamlit/typing/expander_container_types.py	Adds typing assertions for the new `on_change` callable and args/kwargs.
e2e_playwright/st_expander.py	Adds an E2E demo app section exercising callback expanders.
e2e_playwright/st_expander_test.py	Adds E2E tests verifying callback fires on toggle and receives args/kwargs.

lib/streamlit/elements/layouts.py

Copilot · 2026-02-23T16:06:32Z

lib/streamlit/elements/layouts.py

                serializer=serde.serialize,
                ctx=ctx,
                value_type="bool_value",
+                on_change_handler=on_change if callable(on_change) else None,
+                args=args,
+                kwargs=kwargs,


args/kwargs are forwarded to register_widget even when on_change_handler is None (e.g., on_change="rerun"). This makes it look like args/kwargs are supported for non-callback modes and can create inconsistent state/config. Consider only passing args/kwargs when on_change is callable (otherwise pass None) and/or raising a StreamlitAPIException if args/kwargs are provided without a callable.

lib/tests/streamlit/elements/layouts_test.py

lib/streamlit/elements/layouts.py

sfc-gh-bnisco · 2026-02-23T20:38:48Z

e2e_playwright/st_expander_test.py

+def test_expander_callback_with_args_kwargs(app: Page):
+    """Test that a callback with args and kwargs receives them correctly."""
+    # Initially no result
+    expect(app.get_by_text("Callback args result:")).to_be_visible()
+


suggestion: Consider doing a direct string match rather than a substring match here. I believe as written this will pass for both Callback args result: and Callback args result: hello-toggled-world.

Made this update to AGENTS.md about this as well #14102

sfc-gh-bnisco · 2026-02-23T20:42:01Z

lib/streamlit/elements/layouts.py


-        if on_change not in {"ignore", "rerun"}:
+        if not callable(on_change) and on_change not in {"ignore", "rerun"}:
            raise StreamlitValueError("on_change", ["'rerun'", "'ignore'"])


suggestion: Consider adding "a callable" to the StreamlitValueError.

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

This was referenced Feb 18, 2026

[DynamicContainers] Dynamic Expanders #13888

Merged

[refactor] extract Expander animation logic into useDetailsAnimation hook #13933

Merged

sfc-gh-lwilby mentioned this pull request Feb 18, 2026

[refactor] improve Expander animation with ResizeObserver, smooth interruption, and nullable expanded handling #13934

Merged

2 tasks

sfc-gh-lwilby changed the base branch from 02-10-_dynamiccontainers_dynamic_expanders to graphite-base/14008 February 18, 2026 20:48

sfc-gh-lwilby force-pushed the graphite-base/14008 branch from 8346511 to b1364c6 Compare February 23, 2026 15:55

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_expanders branch from b3c1e40 to 94a0eba Compare February 23, 2026 15:55

sfc-gh-lwilby changed the base branch from graphite-base/14008 to develop February 23, 2026 15:55

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 marked this pull request as ready for review February 23, 2026 16:01

Copilot AI review requested due to automatic review settings February 23, 2026 16:01

Copilot AI reviewed Feb 23, 2026

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Copilot started reviewing on behalf of sfc-gh-lwilby February 23, 2026 16:21 View session

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_expanders branch from 94a0eba to aaa79d8 Compare February 23, 2026 18:28

graphite-app bot reviewed Feb 23, 2026

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lib/streamlit/elements/layouts.py Outdated Show resolved Hide resolved

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_expanders branch 2 times, most recently from ce0cc59 to adfae87 Compare February 23, 2026 20:30

sfc-gh-bnisco approved these changes Feb 23, 2026

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[DynamicContainers] Callback support for expanders

62f405e

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

sfc-gh-lwilby force-pushed the 02-18-_dynamiccontainers_callback_support_for_expanders branch from adfae87 to 62f405e Compare February 24, 2026 07:49

sfc-gh-lwilby merged commit 5deeeff into develop Feb 24, 2026
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sfc-gh-lwilby deleted the 02-18-_dynamiccontainers_callback_support_for_expanders branch February 24, 2026 10:43

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[DynamicContainers] Callback support for expanders#14008

[DynamicContainers] Callback support for expanders#14008
sfc-gh-lwilby merged 1 commit intodevelopfrom
02-18-_dynamiccontainers_callback_support_for_expanders

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