Python on Previewops

Previewops builds whatever your Dockerfile produces and runs it as a container, so Python needs no special support. What it does need is what every stack needs: read the injected PORT and listen on 0.0.0.0.
Read the port os.environ["PORT"]
Bind all interfaces gunicorn -b 0.0.0.0:$PORT

Dockerfile

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt ./
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
ENV PORT=8080
CMD ["sh", "-c", "gunicorn -b 0.0.0.0:$PORT app:app"]

ENV PORT=8080 is only a default. Previewops overrides it at deploy time, so the app must read the value rather than assume it.

Application code

A typical setup with Flask or FastAPI behind Gunicorn:

import os
from flask import Flask

app = Flask(__name__)

@app.get("/")
def index():
    return "ok"

if __name__ == "__main__":
    # Only used for local runs — in the container Gunicorn binds the socket.
    port = int(os.environ.get("PORT", 8080))
    app.run(host="0.0.0.0", port=port)

Python-specific pitfalls

Configuration

If your Dockerfile is not at the repository root, point at it from .previewops.yaml:

provider: cloud-run
dockerfile: ./docker/Dockerfile
port: 8080
ttl_hours: 24

See configuration.md for every available option.

Do not gate startup behind a migration

Language-agnostic, and the hardest failure here to diagnose:

# Avoid this
CMD ["sh", "-c", "run-migrations && start-server"]

If the migration cannot reach the database — a paused instance, an expired connection string, a cold-start timeout — it never exits, the server never starts, and the port never opens. The deploy itself already succeeded, so the preview shows as Live while nothing answers. Run migrations as a separate, visible step instead.