Streamlit Development Assistant

Expert assistant for working with the Streamlit open-source repository and building Streamlit applications.

Overview

[Streamlit](https://github.com/streamlit/streamlit) is an Apache 2.0 licensed Python library for creating interactive web applications and dashboards, focusing on data apps and internal tools.

Tech Stack Understanding

When working with Streamlit code:

Repository Structure

Navigate the codebase efficiently using these key directories:

Backend (`lib/`)

Frontend (`frontend/`)

Other Key Directories

Development Workflow

Running Commands

**CRITICAL**: Always follow these execution rules:

1. **Prefer `make` targets** for all development tasks (tests, lint, format, builds)

2. **Use `uv run`** to execute any Python command:

- ✅ `uv run streamlit`

- ✅ `uv run pytest`

- ✅ `uv run ruff`

- ✅ `uv run mypy`

3. **Python unit tests**: `uv run pytest` commands are allowed for running specific tests during development

4. **E2E tests**: Direct `uv run pytest` commands targeting `e2e_playwright/` files are **blocked by policy**

- ❌ `uv run pytest e2e_playwright/st_command_test.py`

- ✅ `make run-e2e-test st_command_test.py`

- ✅ `make debug-e2e-test st_command_test.py`

Essential Make Commands

Run these from the repository root:

#### General

#### Backend (Python)

#### Frontend (TypeScript)

#### E2E Testing (Playwright)

**Example**: `make run-e2e-test st_command_test.py`

Testing Strategy

Apply the appropriate test type based on what you're validating:

1. **Python Unit Tests**: Test internal backend behavior without frontend involvement

2. **Frontend Unit Tests**: Test React components, hooks, and related functionality using Vitest and React Testing Library

3. **E2E Tests**: Test complete app logic end-to-end with Playwright

4. **Python Type Tests**: Verify public API typing with mypy `assert_type`

**Best Practice**: When adding new features, run specific test files rather than the entire test suite for faster feedback.

Development Best Practices

1. **Follow existing patterns**: Always check neighboring files to understand and maintain existing conventions

2. **Use `work-tmp` directory**: Store temporary files, specs, and scripts in this directory

3. **Run from root**: If a `make` command fails, verify you're running it from the repository's top-level directory

4. **Main branch**: The main development branch is `develop` (not `main`)

5. **Hot-reload**: Frontend dev server runs at http://localhost:3000 with automatic reload

Common Workflows

Before Committing Changes

```bash

make check

```

Runs format, lint, types, and unit tests on changed files only.

After Modifying Protobufs

```bash

make protobuf

```

Recompiles Protocol Buffer definitions for both Python and frontend.

Frontend Development

```bash

make frontend-dev

```

Starts the development server with hot-reload at http://localhost:3000.

Running Specific Tests

```bash

Python unit test

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

E2E test (use make, not pytest directly)

make run-e2e-test st_button_test.py

```

Constraints