py-vercel

A Vercel builder for Python WSGI applications

Quickstart

If you have an existing WSGI app, getting this builder to work for you is a piece of 🍰!

1. Add a Vercel configuration

Add a vercel.json file to the root of your application:

{
    "builds": [{
        "src": "index.py",
        "use": "@potatohd/py-vercel",
        "config": { "maxLambdaSize": "15mb" }
    }]
}

This configuration is doing a few things in the "builds" part:

1. "src": "index.py" This tells Now that there is one entrypoint to build for. index.py is a file we'll create shortly.
2. "use": "@potatohd/py-vercel" Tell Now to use this builder when deploying your application
3. "config": { "maxLambdaSize": "15mb" } Bump up the maximum size of the built application to accommodate some larger python WSGI libraries (like Django or Flask). This may not be necessary for you.

2. Add a Now entrypoint

Add index.py to the root of your application. This entrypoint should make available an object named application that is an instance of your WSGI application. E.g.:

# For a Dango app
from django_app.wsgi import application
# Replace `django_app` with the appropriate name to point towards your project's
# wsgi.py file

Look at your framework documentation for help getting access to the WSGI application.

If the WSGI instance isn't named application you can set the wsgiApplicationName configuration option to match your application's name (see the configuration section below).

3. Deploy

That's it, you're ready to go:

$ vercel
> Deploying python-wsgi-app
...
> Success! Deployment ready [57s]

Requirements

Linux requirements

Your project may optionally include a setup.sh or a post-install.sh file to declare any dependencies, e.g.:

# setup.sh
yum install -y gcc mysql-devel

ln -s /usr/lib64/libmariadbclient.a /usr/lib64/libmariadb.a

pip install -t $srcDir $srcDir/django-storages

(setup.sh runs before python packages installation and post-install.sh after)

As you can see, there is default variable $srcDir which contains path to your project folder. Also, yoy may notice that the server is on special Amazon Linux 2 runtime. So keep that in mind when you are writing scripts.

Also be aware that on the runtime you must use the -t argument when installing python packages.

Python requirements

Your project may optionally include a requirements.txt file to declare any dependencies, e.g.:

# requirements.txt
Django >=2.2,<2.3

Be aware that the builder will install Werkzeug as a requirement of the handler. This can cause issues if your project requires a different version of Werkzeug than the handler.

Configuration options

runtime

Select the lambda runtime. Defaults to python3.8.

{
    "builds": [{
        "config": { "runtime": "python3.8" }
    }]
}

wsgiApplicationName

Select the WSGI application to run from your entrypoint. Defaults to application.

{
    "builds": [{
        "config": { "wsgiApplicationName": "application" }
    }]
}

production

Select whether the application is in production mode or not. When set to false, the debugger will appear on error. Defaults to false.

{
    "builds": [{
        "config": { "production": true }
    }]
}

Additional considerations

Routing

You'll likely want all requests arriving at your deployment url to be routed to your application. You can do this by adding a route rewrite to the Now configuration:

{
    "builds": [{
        "src": "index.py",
        "use": "@potatohd/py-vercel"
    }],
    "routes" : [{
        "src" : "/(.*)", "dest":"/"
    }]
}

Avoiding the index.py file

If having an extra file in your project is troublesome or seems unecessary, it's also possible to configure Now to use your application directly, without passing it through index.py.

If your WSGI application lives in vercel_app/wsgi.py and is named application, then you can configure it as the entrypoint and adjust routes accordingly:

{
    "builds": [{
        "src": "vercel_app/wsgi.py",
        "use": "@potatohd/py-vercel"
    }],
    "routes" : [{
        "src" : "/(.*)", "dest":"/vercel_app/wsgi.py"
    }]
}

Lambda environment limitations

At the time of writing, Vercel runs on AWS Lambda. This has a number of implications on what libraries will be available to you, notably:

• PostgreSQL, so psycopg2 won't work out of the box
• MySQL, so MySQL adapters won't work out of the box either
• Sqlite, so the built-in Sqlite adapter won't be available

Attribution

This implementation draws upon work from:

• vercel-python-wsgi by @jayhale