Command Line Interface¶

Responder installs a responder command that lets you launch applications from the terminal. You can point it at a Python module, a local file, or even a URL — and it will find your API instance and start serving.

Launching from a Module¶

The most common way to run a Responder application in production. Use Python’s standard dotted module path:

$ responder run acme.app

This imports acme.app and looks for an attribute called api (a responder.API instance). It’s the same import system Python uses everywhere — your PYTHONPATH and virtual environment are respected.

Launching from a File¶

During development, you often have a single file you want to run:

$ responder run helloworld.py

This loads the file directly and starts the server. Quick and easy for prototyping and single-file applications.

You can test it with a simple HTTP request:

$ curl http://127.0.0.1:5042/hello
hello, world!

Launching from a URL¶

Responder can fetch and run a Python file from any URL — great for demos, sharing examples, and running code from GitHub:

$ responder run https://github.com/kennethreitz/responder/raw/refs/heads/main/examples/helloworld.py

This also works with github:// URLs and any filesystem protocol supported by fsspec:

$ responder run github://kennethreitz:responder@/examples/helloworld.py

Cloud storage is supported too — Azure Blob Storage, Google Cloud Storage, S3, HDFS, SFTP, and more. Install fsspec[full] for all protocols:

$ uv pip install 'fsspec[full]'

Custom Instance Names¶

By default, Responder looks for an attribute called api. If your application uses a different name, specify it with a colon:

$ responder run acme.app:service
$ responder run myapp.py:application

For URLs, use a fragment:

$ responder run https://example.com/app.py#service

Environment Variables¶

Responder automatically reads the PORT environment variable at runtime:

PORT — bind to 0.0.0.0 on this port (cloud platform convention)

When PORT is set, the server binds to all interfaces automatically. This is how cloud platforms like Fly.io, Railway, and Heroku inject the listen port.

For other settings like SECRET_KEY, read them in your application code and pass them to responder.API():

import os
api = responder.API(secret_key=os.environ["SECRET_KEY"])

Building Frontend Assets¶

If your project includes a JavaScript frontend with a package.json, the build subcommand runs npm run build:

$ responder build
$ responder build /path/to/frontend