implementing-new-features

name	implementing-new-features
description	Implementation guide for new Streamlit features. Use when adding new elements, widgets, or features that span backend, frontend, and protobufs.

New Feature Implementation Guide

Most features need implementation in three areas:

• Backend: lib/streamlit/
• Frontend: frontend/
• Protobufs: proto/

New features should include:

• Python unit tests in lib/tests
• Vitest unit tests
• E2E Playwright tests in e2e_playwright/

Order of Implementation

1. Protobuf changes in proto/ then run make protobuf

   • New elements: add to proto/streamlit/proto/Element.proto

2. Backend in lib/streamlit/

   • New elements: add to lib/streamlit/__init__.py

3. Python unit tests in lib/tests

   • Run: uv run pytest lib/tests/streamlit/the_test_name.py
   • New elements: add to lib/tests/streamlit/element_mocks.py

4. Frontend in frontend/

   • New elements: add to frontend/lib/src/components/core/Block/ElementNodeRenderer.tsx

5. Vitest tests in *.test.tsx

   • Run: cd frontend && yarn vitest lib/src/components/elements/NewElement/NewElement.test.tsx

6. E2E Playwright tests in e2e_playwright/

   • Run: make run-e2e-test e2e_playwright/name_of_the_test.py

7. Autofix formatting and linting: make autofix

8. Verify the implementation: make check