README.md

sveltekit-python-vercel

Write Python endpoints in SvelteKit and seamlessly deploy them to Vercel.

• Current Features
• Installing
• Testing Locally
• Deploying to Vercel
• Example
  • Backend Caveats
• Fork of sveltekit-modal
• Possible future plans

This is very much in beta.

Current Features

• Write +server.py files nearly the same way you would write +server.js files
• Deploy (quasi) automatically to Vercel Serverless

Installing

• Open or set up your SvelteKit project

• Install SvelteKit's Vercel adapter: pnpm i -D @sveltejs/adapter-vercel

• Install with pnpm i -D sveltekit-python-vercel

• Update your vite.config.js

  import { defineConfig } from "vite";
  import { sveltekit } from "@sveltejs/kit/vite";
  import { sveltekit_python_vercel } from "sveltekit-python-vercel/vite";
  
  export default defineConfig(({ command, mode }) => {
    return {
      plugins: [sveltekit_python_vercel(), sveltekit()],
    };
  });

• Update your svelte.config.js:

  import adapter from "@sveltejs/adapter-vercel"; // Use the vercel adapter
  import { vitePreprocess } from "@sveltejs/kit/vite";
  
  /** @type {import('@sveltejs/kit').Config} */
  const config = {
    preprocess: vitePreprocess(),
    kit: {
      adapter: adapter(),
      moduleExtensions: [".js", ".ts", ".py"], // add ".py" to resolve +server.py endpoints
    },
  };
  
  export default config;

• Update your vercel.json

  • The build command prepares all your endpoints and copies them to the /api directory where Vercel looks for functions
  • Functions and Routes tell Vercel how to run and redirect function calls

  {
    "buildCommand": "node ./node_modules/sveltekit-python-vercel/esm/src/vite/sveltekit_python_vercel/bin.mjs; vite build",
    "routes": [
      {
        "src": "/api/(.*)",
        "dest": "api/index.py"
      }
    ]
  }

• Write some +server.py endpoints. See the example section below.

Testing Locally

Using uv or Poetry to manage your virtual environments with this package is recommended.

• Run uv init or poetry init to create a new virtual environment to create a pyproject.toml
• Required packages are python3.9 (that is what Vercel's runtime uses), fastapi, and uvicorn.
• Install whatever other dependencies you need from pypi using poetry add package-name or uv add package-name.
• Run your npm or pnpm dev server within uv or poetry. For example:
  • uv run pnpm dev
  • poetry run npm dev etc
• You should see both the usual SvelteKit server start as well as the unvicorn server (by default on http://0.0.0.0:8000) in the console.

Deploying to Vercel

• At the moment this requires a tiny bit of extra labor besides just pushing to your repository. I believe this is because of the way Vercel looks for serverless functions, but I hope to make this a bit easier in the future.

• When you make changes to your python endpoints, you have to manually regenerate the /api folder by running one of:

  1. poetry export -f requirements.txt --output requirements.txt --without-hashes
  2. uv pip compile pyproject.toml -o requirements.txt

• Followed by:

  • node ./node_modules/sveltekit-python-vercel/esm/src/vite/sveltekit_python_vercel/bin.mjs`

• Then commit requirements.txt and the changes in /api and push.

Note:

• To make this a bit smoother, you can add a script to you package.json:

  "scripts": {
    ...
    "py:poetry": "poetry export -f requirements.txt --output requirements.txt --without-hashes; node ./node_modules/sveltekit-python-vercel/esm/src/vite/sveltekit_python_vercel/bin.mjs",
    "py:uv": "uv pip compile pyproject.toml -o requirements.txt; node ./node_modules/sveltekit-python-vercel/esm/src/vite/sveltekit_python_vercel/bin.mjs"
  }

  • and then just run pnpm py:poetry or pnpm py:uv

Example

• Frontend: /src/routes/py/+page.svelte

  <script lang="ts">
    let a = 0;
    let b = 0;
    let total = 0;
  
    async function pyAddPost() {
      const response = await fetch("/py", {
        method: "POST",
        body: JSON.stringify({ a, b }),
        headers: {
          "content-type": "application/json",
        },
      });
      let res = await response.json();
      total = res.sum;
    }
  
    async function pyAddGet() {
      const response = await fetch(`/py?a=${a}&b=${b}`, {
        method: "GET",
        headers: {
          "content-type": "application/json",
        },
      });
  
      let res = await response.json();
      total = res.sum;
    }
  </script>
  
  <h1>This is a SvelteKit page with a python backend.</h1>
  
  <h3>POST Example</h3>
  <form>
    <input type="number" name="a" placeholder="Number 1" bind:value="{a}" />
    <input type="number" name="b" placeholder="Number 2" bind:value="{b}" />
    <button on:click|preventDefault="{pyAddPost}">Add</button>
  </form>
  <h4>Total: {total}</h4>
  
  <br />
  
  <h3>GET Example</h3>
  <form>
    <input type="number" name="a" placeholder="Number 1" bind:value="{a}" />
    <input type="number" name="b" placeholder="Number 2" bind:value="{b}" />
    <button on:click|preventDefault="{pyAddGet}">Add</button>
  </form>
  <h4>Total: {total}</h4>

• Backend: /src/routes/py/+server.py

  from pydantic import BaseModel
  
  
  class NumberSet(BaseModel):
      a: float
      b: float
  
  
  async def POST(data: NumberSet):
      return {"sum": data.a + data.b}
  
  
  async def GET(a: float, b: float):
      return {"sum": a + b}

Backend Caveats

There are currently a few things that have to be worked around.

• GET endpoints are directly fed the parameters from the url, so when you define an endpoint
• All other endpoints are fed the body as a JSON. The recommended way to deal with this is to use a pydantic model and pass it as the singular input to the function.

See the example above.

Fork of sveltekit-modal

Check out the awesome sveltekit-modal package by @semicognitive, the original way to get your python code running in SvelteKit. Modal even has GPU support for running an entire ML stack within SvelteKit.

Possible future plans

• Add hot reloading in dev mode
• Generate endpoints (/api folder) automatically during build
  • Auto create requirements.txt from pyproject.toml (both related to vercel functions being checked/handled before build)
• Add form actions
• Add load functions
• Add helper functions to automatically call API endpoints in project\