spylt

This project is very experimental, expect bugs and caveats

Spylt (pronounced spilt, combination of Svelte and Python) is a connector for Python HTTP backends and Svelte frontends. Simple Python functions can be turned into HTTP APIs which can be directly called from Svelte code under a single codebase with auto-generated wrappers with documentation and types.

Why?

Normally, if you wanted to combine an API and a frontend framework, you would have to separate the apps into different codebases and different repositories.

The ideal solution would be to simply store frontend code in a single application, similar to how classic SSR sites work. Instead of storing HTML pages however, it would have Svelte pages, which allow for better modularity and more reactivity.

This is what Spylt aims to do. Spylt works best for demo apps (like Streamlit or Gradio) that need more control over JavaScript/CSS. Since Spylt also uses Svelte for the frontend, it can outperform libraries like Pynecone and Plotly Dash.

Simple usage

Install spylt with pip:

git clone https://github.com/ajskateboarder/spylt
cd spylt
pip install -r requirements.txt

Initialize a starter project with spylt new:

python3 -m spylt new <dir>
cd <dir>

Whenever you make changes to your API in src/App.py, you can build a typed JavaScript wrapper for it using spylt interface:

python3 -m spylt interface

which looks like this:

/**
 * {function docstring}
 * @param {type} param
 * @returns {type}
 */
export function name(param) {
  const res = fetchSync(`/api/route?param=${param}`)
  return res.response
}

After you make changes to your backend and frontend, you can use spylt build to compile them into a web API and static HTML respectively:

python3 -m spylt build
# start the server
python3 main.py

Check out the example project and documentation on using Pandas dataframes for more information.

Caveats

Most of the caveats can be fixed manually by dumping the API by fixing errors manually.

• Using multiple parameters for functions creates completely wrong code

• Return statements must be on one line. If you have list comprehensions or other multi-line statements, save them to a seperate variable before returning

• src.* imports are ignored when compiling backend routes (I think)

• Backend routes are strictly named after functions

• Backend route parameters, when compiled, expect values to be passed through query params. This is obviously unwieldy for certain cases. (Support for POSTing can be added in the future)

• Mentioned parameters require type annotations

• Using single-quotes for F-strings can cause syntax errors in the compiled API (just follow PEP8)

Inspired by PySvelte