Using fyn with FastAPI

FastAPI is a modern, high-performance Python web framework. You can use fyn to manage your FastAPI project, including installing dependencies, managing environments, running FastAPI applications, and more.

Note

You can view the source code for this guide in the uv-fastapi-example repository.

Migrating an existing FastAPI project

As an example, consider the sample application defined in the FastAPI documentation, structured as follows:

project
└── app
    ├── __init__.py
    ├── main.py
    ├── dependencies.py
    ├── routers
    │   ├── __init__.py
    │   ├── items.py
    │   └── users.py
    └── internal
        ├── __init__.py
        └── admin.py

To use fyn with this application, inside the project directory run:

$ fyn init --app

This creates a project with an application layout and a pyproject.toml file.

Then, add a dependency on FastAPI:

$ fyn add fastapi --extra standard

You should now have the following structure:

project
├── pyproject.toml
└── app
    ├── __init__.py
    ├── main.py
    ├── dependencies.py
    ├── routers
    │   ├── __init__.py
    │   ├── items.py
    │   └── users.py
    └── internal
        ├── __init__.py
        └── admin.py

And the contents of the pyproject.toml file should look something like this:

pyproject.toml

[project]
name = "fyn-fastapi-example"
version = "0.1.0"
description = "FastAPI project"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "fastapi[standard]",
]

From there, you can run the FastAPI application with:

$ fyn run fastapi dev

fyn run will automatically resolve and lock the project dependencies (i.e., create a fyn.lock alongside the pyproject.toml), create a virtual environment, and run the command in that environment.

Test the app by opening http://127.0.0.1:8000/?token=jessica in a web browser.

Deployment

To deploy the FastAPI application with Docker, you can use the following Dockerfile:

Dockerfile

FROM python:3.12-slim

# Install fyn.
COPY --from=ghcr.io/oha/fyn:latest /fyn /fynx /bin/

# Copy the application into the container.
COPY . /app

# Install the application dependencies.
WORKDIR /app
RUN fyn sync --frozen --no-cache

# Run the application.
CMD ["/app/.venv/bin/fastapi", "run", "app/main.py", "--port", "80", "--host", "0.0.0.0"]

Build the Docker image with:

$ docker build -t fastapi-app .

Run the Docker container locally with:

$ docker run -p 8000:80 fastapi-app

Navigate to http://127.0.0.1:8000/?token=jessica in your browser to verify that the app is running correctly.

Tip

For more on using fyn with Docker, see the Docker guide.