Streamlit Development Assistant

Expert assistant for developing [Streamlit](https://github.com/streamlit/streamlit), the open-source Python library for building interactive data applications and dashboards.

What This Skill Does

Guides you through Streamlit's codebase, build system, testing strategy, and development workflows. Helps you navigate the monorepo structure, run the correct build commands, write tests, and follow established patterns.

Tech Stack

Repository Structure

```

streamlit/

├── lib/ # Backend Python code

│ ├── streamlit/ # Main package

│ │ ├── elements/ # Elements and widgets (backend)

│ │ ├── runtime/ # App runtime logic

│ │ └── web/ # Web server and CLI

│ └── tests/ # Python unit tests (pytest)

├── frontend/ # Frontend TypeScript/React code

│ ├── app/ # Main UI application

│ ├── lib/ # Shared library (elements, widgets, layouts)

│ ├── connection/ # WebSocket handling

│ └── utils/ # Shared utilities

├── proto/streamlit/proto/ # Protobuf definitions

├── e2e_playwright/ # E2E tests (Playwright)

├── scripts/ # Dev and CI/CD utilities

├── component-lib/ # Custom component library

└── wiki/ # Development documentation

```

Instructions

1. Command Execution Policy

**Always use `make` targets for development tasks:**

2. Common Development Commands

**Verification and Checks:**

**Backend (Python):**

**Frontend (TypeScript/React):**

**E2E Testing:**

3. Development Workflow

**Before starting work:**

1. Check the `develop` branch (main branch for this repo)

2. Study neighboring files to understand existing patterns

3. Use `work-tmp/` directory for temporary files and scripts

**When writing code:**

1. Follow existing conventions in nearby files

2. Run `make check` before committing to verify changes

3. Add appropriate tests (unit or E2E) for new features

4. Update protobuf definitions if modifying client-server communication

**Frontend development:**

1. Start dev server with `make frontend-dev`

2. Access hot-reload UI at http://localhost:3000

3. Write unit tests with Vitest and React Testing Library

4. Check types with `make frontend-types`

**Backend development:**

1. Use `uv run streamlit run <script.py>` to test apps

2. Add pytest unit tests in `lib/tests/`

3. Run specific tests with `uv run pytest <test_file.py>`

4. Verify types with `make python-types`

4. Testing Strategy

**Choose the right test type:**

**Best practices:**

5. Common Pitfalls

6. When You're Stuck

1. Run `make help` to see all available commands

2. Check `wiki/` directory for development documentation

3. Look at `.github/workflows/` for CI/CD examples

4. Study existing tests to understand testing patterns

5. Check protobuf definitions in `proto/streamlit/proto/` for API contracts

Examples

**Run specific Python test:**

```bash

uv run pytest lib/tests/streamlit/elements/button_test.py

```

**Start frontend development:**

```bash

make frontend-dev

Access at http://localhost:3000

```

**Run E2E test:**

```bash

make run-e2e-test st_button_test.py

```

**Verify changes before commit:**

```bash

make check

```

**Fix all auto-fixable issues:**

```bash

make autofix

```

Important Notes