Streamlit Development Expert

This skill provides expert guidance for developing features, components, and tests in the Streamlit open-source project. Streamlit is an Apache 2.0 licensed Python library for creating interactive web applications and dashboards focused on data apps and internal tools.

Architecture Overview

Streamlit uses a client-server architecture:

Repository Structure

- `elements/` — Backend code for elements and widgets

- `runtime/` — App runtime and execution logic

- `web/` — Web server and CLI implementation

- `tests/` — Python unit tests

- `app/` — Streamlit application UI

- `lib/` — Shared TypeScript library (elements, widgets, layouts)

- `connection/` — WebSocket connection handling

Development Workflow

1. Use Make Commands for All Tasks

Always prefer `make` targets over direct tool invocation:

```bash

make help # Show all available commands

make check # Run all checks on changed files

make autofix # Auto-fix linting and formatting

make protobuf # Recompile Protocol Buffers

```

2. Python Development

**CRITICAL:** Always use `uv run` prefix for Python commands:

```bash

Testing

uv run pytest path/to/test_file.py # Run specific tests

make python-tests # Run all Python tests

Code Quality

make python-lint # Lint with ruff

make python-format # Format with ruff

make python-types # Type check with mypy & ty

```

**Never run Python commands without `uv run` prefix.**

3. Frontend Development

```bash

make frontend-dev # Start dev server (hot-reload)

make frontend-fast # Build frontend with Vite

make frontend-lint # Lint with eslint

make frontend-format # Format with eslint

make frontend-types # Type check with tsc

make frontend-tests # Run Vitest unit tests

```

4. E2E Testing

**CRITICAL:** Do NOT run `uv run pytest` directly on E2E tests. Use make instead:

```bash

✅ CORRECT

make run-e2e-test st_command_test.py

❌ WRONG (blocked by policy)

uv run pytest e2e_playwright/st_command_test.py

```

5. Debugging

Use `make debug` to run both backend and frontend with hot-reload:

```bash

make debug my_app.py

```

This command:

Logs are written to:

Logs persist after exit for debugging but are cleared on next run.

Development Guidelines

Follow Existing Patterns

Temporary Files

Before Committing

Run `make check` to verify all changes pass:

Branch Strategy

Testing Strategy

Python Unit Tests

Frontend Unit Tests

E2E Tests

Type Tests

Testing Best Practices

Common Pitfalls

1. **Running Python without `uv run`** — Always prefix Python commands

2. **Running E2E tests with pytest directly** — Use `make run-e2e-test` instead

3. **Running make from wrong directory** — Always run from repository root

4. **Forgetting to recompile protobufs** — Run `make protobuf` after proto changes

5. **Not restarting debug after lib changes** — Backend changes to `lib/streamlit/` require restart

Quick Reference

| Task | Command |

|------|---------|

| Run specific Python test | `uv run pytest path/to/test.py` |

| Run E2E test | `make run-e2e-test test_name.py` |

| Start debugging | `make debug app.py` |

| Check all changes | `make check` |

| Auto-fix issues | `make autofix` |

| Recompile protobufs | `make protobuf` |

| Frontend dev server | `make frontend-dev` |

| View all commands | `make help` |

When to Use This Skill

Use this skill when:

This skill provides the knowledge and conventions needed to work effectively within the Streamlit codebase while following project standards and best practices.