gevent — Coroutine-Based Concurrency for Python

Introduction

gevent is a coroutine-based Python networking library that uses greenlet to provide lightweight cooperative concurrency. By monkey-patching the standard library, gevent lets developers write ordinary synchronous code that automatically yields during I/O operations, achieving concurrent performance without callbacks or async/await syntax. It powers high-concurrency workloads in web servers, scrapers, and network services.

What gevent Does

• Runs thousands of lightweight greenlets in a single thread with cooperative scheduling
• Monkey-patches the standard library so socket, ssl, threading, and subprocess become non-blocking
• Provides a fast event loop built on libev (or libuv) for I/O multiplexing
• Includes gevent.pool, gevent.queue, and gevent.event for structured concurrency patterns
• Ships a WSGI server (gevent.pywsgi) for serving web applications

Architecture Overview

gevent is built on two core components: greenlet, a C extension that provides stackful coroutines, and libev, a high-performance event loop. When a greenlet performs a blocking I/O call (e.g., socket.recv), gevent intercepts it, registers the file descriptor with the event loop, and switches to another greenlet. When the I/O completes, the event loop resumes the original greenlet. This model gives the simplicity of threads with the low overhead of an event loop.

Self-Hosting & Configuration

• Install with pip install gevent
• Call monkey.patch_all() at the top of your application to make stdlib non-blocking
• Use gevent.spawn() to launch greenlets and gevent.joinall() to wait for them
• Deploy web apps with gevent.pywsgi.WSGIServer or Gunicorn's gevent worker
• Tune the GEVENT_RESOLVER environment variable to choose between ares and dnspython resolvers

Key Features

• Write synchronous code that runs concurrently without async/await
• Handle tens of thousands of concurrent connections in a single process
• Compatible with most existing Python libraries through monkey-patching
• Built-in connection pooling, timeouts, and DNS resolution
• Mature project with over a decade of production use

Comparison with Similar Tools

• asyncio — Python standard library async; gevent lets you avoid async/await syntax entirely
• Tornado — async web framework; gevent can serve any WSGI app concurrently
• Twisted — callback-based networking; gevent uses synchronous style
• uvloop — fast asyncio event loop; gevent uses greenlets instead of coroutines
• threading — OS threads with GIL overhead; gevent greenlets are much lighter

FAQ

Q: Does monkey-patching break third-party libraries? A: Most I/O libraries work fine. Libraries with C extensions that bypass the Python socket layer may not be patched automatically.

Q: Can I use gevent with Django? A: Yes. Run Django under Gunicorn with the gevent worker class: gunicorn -k gevent myapp.wsgi.

Q: How does gevent compare to asyncio for new projects? A: asyncio is the standard and has broader library support. gevent excels when you need to make existing synchronous code concurrent without rewriting it.

Q: What happens if a greenlet blocks on CPU work? A: CPU-bound work blocks the event loop. Use gevent.threadpool to offload CPU tasks to OS threads.

Sources

• https://github.com/gevent/gevent
• https://www.gevent.org/