Quick Reference

Platform	Cold Start	Packages	Best For
Workers (Pyodide)	~50ms	Limited	API endpoints
Containers	~10s	Any	Heavy compute, AI

Worker Pattern	Code
Basic handler	class Default(WorkerEntrypoint):
FastAPI	await asgi.fetch(app, request, self.env)
Env vars	self.env.API_KEY
Service binding	await self.env.WORKER_B.method()

Command	Purpose
pywrangler init	Create Python Worker
pywrangler dev	Local development
pywrangler deploy	Deploy to Cloudflare

Container vs Worker	Recommendation
Simple API	Worker
pandas/numpy	Container
GPU/AI	Container
<50ms latency	Worker

When to Use This Skill

Use for Cloudflare edge deployment:

• Deploying Python APIs to Cloudflare Workers
• Running FastAPI on Cloudflare edge
• Using Containers for heavy compute
• Setting up service-to-service RPC
• Optimizing cold starts

Related skills:

• For FastAPI: see python-fastapi
• For async patterns: see python-asyncio
• For Docker: see python-github-actions

---

Python on Cloudflare (Workers & Containers)

Overview

Cloudflare provides two ways to run Python:

1. Python Workers - Serverless functions using Pyodide (WebAssembly)
2. Cloudflare Containers - Full Docker containers (beta, June 2025)

Python Workers

How It Works

• Python runs via Pyodide (CPython compiled to WebAssembly)
• Executes inside V8 isolates on Cloudflare's edge network
• Memory snapshots enable fast cold starts
• Supports many pure Python packages

Quick Start

# Install pywrangler (Python Workers CLI)
pip install pywrangler

# Create new Python Worker
pywrangler init my-worker
cd my-worker

# Project structure
my-worker/
├── src/
│   └── entry.py
├── pyproject.toml
└── wrangler.toml

Basic Worker

# src/entry.py
from workers import Response, WorkerEntrypoint

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        return Response("Hello from Python Worker!")

Configuration

# wrangler.toml
name = "my-python-worker"
main = "src/entry.py"
compatibility_date = "2024-12-01"

[build]
command = ""

# pyproject.toml
[project]
name = "my-python-worker"
version = "0.1.0"
requires-python = ">=3.12"
dependencies = [
    "fastapi",
    "pydantic",
]

[dependency-groups]
dev = ["workers-py"]

FastAPI on Workers

# src/entry.py
from workers import WorkerEntrypoint
from fastapi import FastAPI, Request
from pydantic import BaseModel

app = FastAPI()

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        import asgi
        return await asgi.fetch(app, request, self.env)

@app.get("/")
async def root():
    return {"message": "Hello from FastAPI on Cloudflare!"}

@app.get("/env")
async def get_env(req: Request):
    env = req.scope.get("env")
    if env and hasattr(env, "API_KEY"):
        return {"has_api_key": True}
    return {"has_api_key": False}

class Item(BaseModel):
    name: str
    price: float
    description: str | None = None

@app.post("/items/")
async def create_item(item: Item):
    return item

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id}

Using Environment Variables

from workers import WorkerEntrypoint, Response

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        # Access environment variables via self.env
        api_key = self.env.API_KEY
        database_url = self.env.DATABASE_URL

        return Response(f"API Key exists: {bool(api_key)}")

# wrangler.toml
[vars]
API_KEY = "your-api-key"
DATABASE_URL = "your-database-url"

Service Bindings (RPC)

# Worker A - Caller
from workers import WorkerEntrypoint, Response

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        # Call Worker B's add method
        result = await self.env.WORKER_B.add(1, 2)
        return Response(f"Result: {result}")

# Worker B - Callee
from workers import WorkerEntrypoint, Response

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        return Response("Hello from Worker B")

    def add(self, a: int, b: int) -> int:
        return a + b

# Worker A wrangler.toml
[[services]]
binding = "WORKER_B"
service = "worker-b"

Caching

from workers import WorkerEntrypoint
from pyodide.ffi import to_js as _to_js
from js import Response, URL, Object, fetch

def to_js(x):
    return _to_js(x, dict_converter=Object.fromEntries)

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        request_url = URL.new(request.url)
        params = request_url.searchParams
        tags = params["tags"].split(",") if "tags" in params else []
        url = params["uri"] or None

        if url is None:
            return Response.json(to_js({"error": "URL required"}), status=400)

        # Fetch with cache tags
        options = {"cf": {"cacheTags": tags}}
        result = await fetch(url, to_js(options))

        cache_status = result.headers["cf-cache-status"]
        return Response.json(to_js({
            "cache": cache_status,
            "status": result.status
        }))

Supported Packages

# HTTP clients (async only)
import aiohttp
import httpx

# Data processing
import numpy  # Limited support
import pandas  # Limited support

# Standard library works well
import json
import re
import urllib
import base64
import hashlib

# Check Pyodide package list for full support
# https://pyodide.org/en/stable/usage/packages-in-pyodide.html

Cold Start Optimization

# Imports at module level are cached in memory snapshot
import json
import hashlib
from pydantic import BaseModel

# This code runs during snapshot creation
CACHED_CONFIG = load_config()

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        # Fast access to pre-loaded modules and data
        return Response(json.dumps(CACHED_CONFIG))

Cloudflare Containers (Beta)

Overview (June 2025)

• Full Docker container support
• Run any language/runtime (Python, Go, Java, etc.)
• FFmpeg, Pandas, AI toolchains supported
• Pay-per-use pricing
• Global edge deployment

When to Use Containers vs Workers

Feature	Workers	Containers
Cold start	~50ms	~10s (with prewarming)
Package support	Pyodide-compatible	Any
Memory	Limited	Configurable
File system	No	Yes
Native binaries	No	Yes
Best for	API endpoints	Batch jobs, AI, heavy compute

Container Setup

# Dockerfile
FROM python:3.12-slim

WORKDIR /app

# Install dependencies
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# Copy application
COPY . .

# Run application
CMD ["python", "main.py"]

# main.py
import pandas as pd
import numpy as np
from fastapi import FastAPI
import uvicorn

app = FastAPI()

@app.get("/process")
async def process_data():
    # Heavy computation that wouldn't work in Workers
    df = pd.DataFrame(np.random.randn(10000, 4))
    result = df.describe().to_dict()
    return result

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8080)

Wrangler Commands

# Build container
wrangler containers build . --tag my-app:latest

# Push to registry
wrangler containers push my-app:latest

# Deploy
wrangler containers deploy my-app

# List containers
wrangler containers list

# Delete
wrangler containers delete my-app

Container Optimization

# Multi-stage build for smaller images
FROM python:3.12-slim as builder

WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt --target=/app/deps

FROM python:3.12-slim

WORKDIR /app
COPY --from=builder /app/deps /app/deps
COPY . .

ENV PYTHONPATH=/app/deps
CMD ["python", "main.py"]

# Optimize for container cold starts
import asyncio

# Lazy imports for faster startup
def get_pandas():
    import pandas as pd
    return pd

async def process_large_data():
    pd = get_pandas()  # Import only when needed
    # Process data...

GPU Support (Preview)

# Container with GPU support
import torch

def run_inference(data):
    device = "cuda" if torch.cuda.is_available() else "cpu"
    model = load_model().to(device)

    with torch.no_grad():
        result = model(data.to(device))

    return result.cpu().numpy()

Python Workflows (Durable Execution)

Overview

Cloudflare Workflows now supports Python for multi-step, long-running applications with automatic retries and state persistence.

from cloudflare.workflows import Workflow, step

class DataPipeline(Workflow):
    @step
    async def fetch_data(self, url: str) -> dict:
        """Fetch data from external API."""
        async with aiohttp.ClientSession() as session:
            async with session.get(url) as response:
                return await response.json()

    @step
    async def process_data(self, data: dict) -> dict:
        """Process the fetched data."""
        # Heavy processing - will retry if fails
        processed = transform(data)
        return processed

    @step
    async def save_results(self, data: dict) -> str:
        """Save to database."""
        result_id = await save_to_db(data)
        return result_id

    async def run(self, input_url: str) -> str:
        data = await self.fetch_data(input_url)
        processed = await self.process_data(data)
        result_id = await self.save_results(processed)
        return f"Saved as {result_id}"

Best Practices

1. Minimize Cold Starts

# Do expensive imports at module level
import json
import hashlib
from pydantic import BaseModel

# Pre-compute static data
STATIC_CONFIG = {"version": "1.0", "features": ["a", "b"]}

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        # Fast path - no initialization needed
        return Response(json.dumps(STATIC_CONFIG))

2. Use Async HTTP Clients

# Workers only support async HTTP
import httpx

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        async with httpx.AsyncClient() as client:
            response = await client.get("https://api.example.com/data")
            return Response(response.text)

3. Handle Errors Gracefully

from workers import WorkerEntrypoint, Response

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        try:
            result = await self.process_request(request)
            return Response(json.dumps(result), headers={
                "Content-Type": "application/json"
            })
        except ValueError as e:
            return Response(json.dumps({"error": str(e)}), status=400)
        except Exception as e:
            # Log error (sent to Workers Logs)
            print(f"Error: {e}")
            return Response(json.dumps({"error": "Internal error"}), status=500)

4. Structure for Testability

# src/handlers.py - Pure Python logic
async def handle_create_item(data: dict) -> dict:
    # Testable without Worker runtime
    validated = validate_item(data)
    return {"id": generate_id(), **validated}

# src/entry.py - Worker entrypoint
from workers import WorkerEntrypoint, Response
from handlers import handle_create_item

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        if request.method == "POST":
            data = await request.json()
            result = await handle_create_item(data)
            return Response(json.dumps(result))

5. Local Development

# Start local development server
pywrangler dev

# Test locally
curl http://localhost:8787/

# Deploy to Cloudflare
pywrangler deploy