feat(blocks): add Card, Carousel, and Alert block types

[srtaalej]

Summary

• Adds CardBlock — a rich display block for presenting structured content (hero image, icon, title, subtitle, body, actions)
• Adds CarouselBlock — a horizontally scrollable collection of card blocks (1–10 cards)
• Adds AlertBlock — a prominent notice block for warnings, status updates, or important information with severity levels (default, info, warning, error, success)

All three blocks include deserialization support via Block.parse(), exports in __init__.py, and round-trip tests.

Note: The AlertBlock is only supported in modal views (views_open / views_update). It is not supported in chat_postMessage or other message surfaces.

Test plan

Test app.py (sends carousel + cards via chat_postMessage on startup)

import os
import logging

from slack_bolt import App
from slack_bolt.adapter.socket_mode import SocketModeHandler
from slack_sdk.models.blocks import CarouselBlock, CardBlock, ButtonElement, ImageElement

from listeners import register_listeners

logging.basicConfig(level=logging.DEBUG)

# Initialization
app = App(token=os.environ.get("SLACK_BOT_TOKEN"))

# Register Listeners
register_listeners(app)

# Start Bolt app
if __name__ == "__main__":
    app.client.chat_postMessage(
        channel="#test-blockkit-new-types",
        blocks=[
            CarouselBlock(elements=[
                CardBlock(
                    hero_image=ImageElement(image_url="https://picsum.photos/seed/a/400/200", alt_text="Card 1 hero"),
                    title="Card Block",
                    subtitle="Rich structured content",
                    body="Recommendations, results, or work items.",
                    actions=[ButtonElement(text="Learn more", action_id="card_1_btn")],
                ),
                CardBlock(
                    hero_image=ImageElement(image_url="https://picsum.photos/seed/b/400/200", alt_text="Card 2 hero"),
                    title="Alert Block",
                    subtitle="Prominent notices",
                    body="Warnings, status updates, and more.",
                    actions=[ButtonElement(text="Learn more", action_id="card_2_btn")],
                ),
                CardBlock(
                    hero_image=ImageElement(image_url="https://picsum.photos/seed/c/400/200", alt_text="Card 3 hero"),
                    title="Carousel Block",
                    subtitle="Scrollable card collections",
                    body="Up to 10 cards, side by side.",
                    actions=[ButtonElement(text="Learn more", action_id="card_3_btn")],
                ),
            ]),
        ],
        text="New Block Kit types demo",
    )
    SocketModeHandler(app, os.environ.get("SLACK_APP_TOKEN")).start()

Test shortcut handler (opens modal with alert blocks via global shortcut)

def sample_shortcut_callback(body: dict, ack: Ack, client: WebClient, logger: Logger):
    try:
        ack()
        blocks = [
            AlertBlock(text="This is a *default* alert block").to_dict(),
            AlertBlock(text="This is an *info* alert block", level="info").to_dict(),
            AlertBlock(text="This is a *warning* alert block", level="warning").to_dict(),
            AlertBlock(text="This is an *error* alert block", level="error").to_dict(),
            AlertBlock(text="This is a *success* alert block", level="success").to_dict(),
            {
                "type": "input",
                "block_id": "input_block_id",
                "label": {
                    "type": "plain_text",
                    "text": "What are your hopes and dreams?",
                },
                "element": {
                    "type": "plain_text_input",
                    "action_id": "sample_input_id",
                    "multiline": True,
                },
            },
            {
                "block_id": "select_channel_block_id",
                "type": "input",
                "label": {
                    "type": "plain_text",
                    "text": "Select a channel to message the result to",
                },
                "element": {
                    "type": "conversations_select",
                    "action_id": "sample_dropdown_id",
                    "response_url_enabled": True,
                },
            },
        ]
        client.views_open(
            trigger_id=body["trigger_id"],
            view={
                "type": "modal",
                "callback_id": "sample_view_id",
                "title": {"type": "plain_text", "text": "Alert Block Demo"},
                "blocks": blocks,
                "submit": {"type": "plain_text", "text": "Submit"},
            },
        )
    except Exception as e:
        logger.error(e)

• Round-trip serialization tests for all three block types
• Block.parse() deserialization tests
• Validation tests (required fields, length constraints, enum values)
• Integration test: send a message with CardBlock to Slack
• Integration test: send a message with CarouselBlock to Slack
• Integration test: open a modal with AlertBlock in Slack (alert blocks are only supported in modal views)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.09%. Comparing base (60bd43d) to head (2140576).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1865      +/-   ##
==========================================
+ Coverage   84.00%   84.09%   +0.08%     
==========================================
  Files         117      117              
  Lines       13257    13325      +68     
==========================================
+ Hits        11137    11205      +68     
  Misses       2120     2120              

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[zimeg]

@srtaalej Might I request an example snippet using the CarouselBlock imports? I'm hoping we can put this effort toward the slack-samples/bolt-python-examples project after release 📚

[srtaalej]

@srtaalej Might I request an example snippet using the CarouselBlock imports? I'm hoping we can put this effort toward the slack-samples/bolt-python-examples project after release 📚

ah yes ofc!

[zimeg]

@srtaalej LGTM! Thanks for adding the example code! Those imports make a big difference in testing 🎁 ✨

I'm approving now with a few notes but nothing too important:

• Let's alphabeticize these new additions if ordering allows!
• Our inline docs become more different from the API reference in some of these descriptions

The second point might require wider changes across blocks but I'm wanting to know if we're aligned with docs for some of these block shapes? 🪬

[zimeg]

⏰ suggestion: If dependencies aren't required this order, can we use applied alphabetics on these blocks?

[zimeg]

🔭 question: I notice the current docs suggest string but we're finding the image block is expected? I'm curious if the docs team has seen these differences with this changeset?

[zimeg]

Suggested change

	elements: Sequence[Union[dict, "CardBlock"]],
	elements: Sequence[Union[dict, CardBlock]],

👾 question(non-blocking): Does the stringified class avoid formatting issue or can we reference this direct?

[zimeg]

🔮 note: The release milestone 3.41.1 should be 3.42.0 or "next" but we can adjust this at time of release for this change!