You probably already know about nonblocking I/O. Recently widely used nonblocking I/O libraries like node.js use callbacks to interop with event loops (we exactly call it continuation-passing style).

The philosophy of gevent (and eventlet) is that nonblockong codes have to be seem alike ordinary blocking codes though these actually block nothing. To achieve that gevent provides monkey patcher (gevent.monkey) which replaces all built-in networking libraries of Python (e.g. socket, urllib2) with nonblocking version provided by gevent.

As a result most of blocking-looking codes become nonblocking by monkey patcher:

from gevent.monkey import patch_all; patch_all()
import urllib2
import json


def get_followers(screen_name):
    url = 'http://api.twitter.com/1/followers/ids.json' \
          '?screen_name={0}&cursor={1}'
    next_cursor = -1
    ids = []
    while next_cursor:
        u = urllib2.urlopen(url.format(screen_name, next_cursor))
        data = json.load(u)
        for id_ in data['ids']:
            yield id_
        next_cursor = data['next_cursor']


print list(get_followers(screen_name='lily199iu'))

The above code is equivalent to the following node.js code which is harder to read:

var http = require('http');

function getFollowers(screenName, complete, cursor) {
  var path = '/1/followers/ids.json?screen_name=' + screenName
           + '&cursor=' + (cursor || '-1');
  http.get({ host: 'api.twitter.com', path: path }, function (res) {
    res.setEncoding('utf8');
    var buffer = '';
    res.on('data', function (chunk) {
      buffer += chunk;
    })
    res.on('end', function () {
      var data = JSON.parse(buffer);
      if (data['next_cursor']) {
        getFollowers(screenName, function (d) {
          complete(data['ids'] + d);
        }, data['next_cursor_str']);
      } else {
        complete(data['ids']);
      }
    });
  });
}

getFollowers('lily199iu', function (data) {
  console.log(data);
});

(Point: iteration v. recursion)