# What is PEP 503?


PEP 503 defines the "Simple Repository API" — the interface that Python package repositories must implement to be compatible with package installers like [pip](https://pydevtools.com/handbook/reference/pip.md) and [uv](https://pydevtools.com/handbook/reference/uv.md). [PyPI](https://pydevtools.com/handbook/explanation/what-is-pypi.md) implements this API, and the specification allows anyone to host a compatible package index.

## How the Simple Repository API Works

The API is intentionally minimal. A compliant repository serves two types of HTML pages:

1. Root index (`/simple/`) — an HTML page with links to each available project:

    ```html
    <a href="/simple/requests/">requests</a>
    <a href="/simple/flask/">flask</a>
    <a href="/simple/numpy/">numpy</a>
    ```

2. Project page (`/simple/<project>/`) — an HTML page listing downloadable files for a specific package:

    ```html
    <a href="requests-2.31.0.tar.gz#sha256=abc123...">requests-2.31.0.tar.gz</a>
    <a href="requests-2.31.0-py3-none-any.whl#sha256=def456...">requests-2.31.0-py3-none-any.whl</a>
    ```

The hash fragment after `#` allows installers to verify download integrity.

## Using Alternative Indexes

Package installers can be pointed at any PEP 503-compliant index. This is how organizations host private packages:

```bash
# With pip
pip install --index-url https://my-company.example.com/simple/ my-package

# With uv
uv pip install --index-url https://my-company.example.com/simple/ my-package
```

## Impact

This standardization enabled the growth of private package repositories (like Artifactory, AWS CodeArtifact, and Google Artifact Registry) and alternative indexes like [TestPyPI](https://test.pypi.org/), all using the same interface that pip and uv already understand.

## Learn More

- [PEP 503 Specification](https://peps.python.org/pep-0503/)
- [Hosting Your Own Index](https://packaging.python.org/en/latest/guides/hosting-your-own-index/)
- [What is PyPI?](https://pydevtools.com/handbook/explanation/what-is-pypi.md)