Using the Python Runtime with Vercel Functions

The Python runtime enables you to write Python code, including using FastAPI, Django, and Flask, with Vercel Functions. You can use a specific Python version as well as use a requirements.txt file to install dependencies.

You can create your first function, available at the /api route, as follows:

from http.server import BaseHTTPRequestHandler
 
class handler(BaseHTTPRequestHandler):
 
    def do_GET(self):
        self.send_response(200)
        self.send_header('Content-type','text/plain')
        self.end_headers()
        self.wfile.write('Hello, world!'.encode('utf-8'))
        return

By default, new projects will use the latest Python version available on Vercel. The current available version is Python 3.12.

You can specify which of the available Python versions to use by specifying it with a pyproject.toml file. The latest available Python version satisfying the constraint will be used.

[project]
name = "my-python-api"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.11"
dependencies = []

You can also use Pipfile:

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"
 
[packages]
flask = "*"
 
[requires]
python_version = "3.12"

Vercel Functions support streaming responses when using the Python runtime. This allows you to render parts of the UI as they become ready, letting users interact with your app before the entire page finishes loading.

You can install dependencies for your Python projects by defining them in a pyproject.toml with or without a corresponding uv.lock, requirements.txt, or a Pipfile with corresponding Pipfile.lock.

fastapi==0.117.1

[project]
name = "my-python-api"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.11"
dependencies = [
    "fastapi>=0.117.1",
]

By default, Python Vercel Functions include all files from your project that are reachable at build time. Unlike the Node.js runtime, there is no automatic tree-shaking to remove dead code or unused dependencies.

You should make sure your pyproject.toml or requirements.txt only lists packages necessary for runtime and you should also explicitly exclude files you don't need in your functions to keep bundles small and avoid hitting size limits.

To exclude unnecessary files (for example: tests, static assets, and test data), configure excludeFiles in vercel.json under the functions key. The pattern is a glob relative to your project root.

{
  "$schema": "https://openapi.vercel.sh/vercel.json",
  "functions": {
    "api/**/*.py": {
      "excludeFiles": "{tests/**,__tests__/**,**/*.test.py,**/test_*.py,fixtures/**,__fixtures__/**,testdata/**,sample-data/**,static/**,assets/**}"
    }
  }
}

FastAPI is a modern, high-performance, web framework for building APIs with Python. For information on how to use FastAPI with Vercel, review this guide.

Flask is a lightweight WSGI web application framework. For information on how to use Flask with Vercel, review this guide.

For FastAPI, Flask, or basic usage of the Python runtime, no configuration is required. Usage of the Python runtime with other frameworks, including Django, requires some configuration.

The entry point of this runtime is a glob matching .py source files with one of the following variables defined:

• handler that inherits from the BaseHTTPRequestHandler class
• app that exposes a WSGI or ASGI Application

Python uses the current working directory when a relative file is passed to open().

The current working directory is the base of your project, not the api/ directory.

For example, the following directory structure:

├── README.md
├── api
|  ├── user.py
├── data
|  └── file.txt
└── requirements.txt

With the above directory structure, your function in api/user.py can read the contents of data/file.txt in a couple different ways.

You can use the path relative to the project's base directory.

 
from http.server import BaseHTTPRequestHandler
from os.path import join
 
class handler(BaseHTTPRequestHandler):
 
    def do_GET(self):
        self.send_response(200)
        self.send_header('Content-type','text/plain')
        self.end_headers()
        with open(join('data', 'file.txt'), 'r') as file:
          for line in file:
            self.wfile.write(line.encode())
        return

Or you can use the path relative to the current file's directory.

 
from http.server import BaseHTTPRequestHandler
from os.path import dirname, abspath, join
dir = dirname(abspath(__file__))
 
class handler(BaseHTTPRequestHandler):
 
    def do_GET(self):
        self.send_response(200)
        self.send_header('Content-type','text/plain')
        self.end_headers()
        with open(join(dir, '..', 'data', 'file.txt'), 'r') as file:
          for line in file:
            self.wfile.write(line.encode())
        return

The Web Server Gateway Interface (WSGI) is a calling convention for web servers to forward requests to web applications written in Python. You can use WSGI with frameworks such as Flask or Django.

• Deploy an example with Flask
• Deploy an example with Django

The Asynchronous Server Gateway Interface (ASGI) is a calling convention for web servers to forward requests to asynchronous web applications written in Python. You can use ASGI with frameworks such as Sanic.

Instead of defining a handler, define an app variable in your Python file.

For example, define a api/index.py file as follows:

from sanic import Sanic
from sanic.response import json
app = Sanic()
 
 
@app.route('/')
@app.route('/<path:path>')
async def index(request, path=""):
    return json({'hello': path})

Inside requirements.txt define:

sanic==19.6.0