Basic Usage ============= Eventlet is built around the concept of green threads (i.e. coroutines, we use the terms interchangeably) that are launched to do network-related work. Green threads differ from normal threads in two main ways: * Green threads are so cheap they are nearly free. You do not have to conserve green threads like you would normal threads. In general, there will be at least one green thread per network connection. * Green threads cooperatively yield to each other instead of preemptively being scheduled. The major advantage from this behavior is that shared data structures don't need locks, because only if a yield is explicitly called can another green thread have access to the data structure. It is also possible to inspect primitives such as queues to see if they have any pending data. There are a bunch of basic patterns that Eventlet usage falls into. Here are a few examples that show their basic structure. Client-side pattern -------------------- The canonical client-side example is a web crawler. This use case is given a list of urls and wants to retrieve their bodies for later processing. Here is a very simple example:: urls = ["http://www.google.com/intl/en_ALL/images/logo.gif", "https://wiki.secondlife.com/w/images/secondlife.jpg", "http://us.i1.yimg.com/us.yimg.com/i/ww/beta/y3.gif"] import eventlet from eventlet.green import urllib2 def fetch(url): return urllib2.urlopen(url).read() pool = eventlet.GreenPool() for body in pool.imap(fetch, urls): print "got body", len(body) There is a slightly more complex version of this in the :ref:`web crawler example `. Here's a tour of the interesting lines in this crawler. ``from eventlet.green import urllib2`` is how you import a cooperatively-yielding version of urllib2. It is the same in all respects to the standard version, except that it uses green sockets for its communication. ``pool = eventlet.GreenPool()`` constructs a :class:`GreenPool ` of a thousand green threads. Using a pool is good practice because it provides an upper limit on the amount of work that this crawler will be doing simultaneously, which comes in handy when the input data changes dramatically. ``for body in pool.imap(fetch, urls):`` iterates over the results of calling the fetch function in parallel. :meth:`imap ` makes the function calls in parallel, and the results are returned in the order that they were executed. Server-side pattern -------------------- Here's a simple server-side example, a simple echo server:: import eventlet from eventlet.green import socket def handle(client): while True: c = client.recv(1) if not c: break client.sendall(c) server = socket.socket() server.bind(('0.0.0.0', 6000)) server.listen(50) pool = eventlet.GreenPool(10000) while True: new_sock, address = server.accept() pool.spawn_n(handle, new_sock) The file :ref:`echo server example ` contains a somewhat more robust and complex version of this example. ``from eventlet.green import socket`` imports eventlet's socket module, which is just like the regular socket module, but cooperatively yielding. ``pool = eventlet.GreenPool(10000)`` creates a pool of green threads that could handle ten thousand clients. ``pool.spawn_n(handle, new_sock)`` launches a green thread to handle the new client. The accept loop doesn't care about the return value of the ``handle`` function, so it uses :meth:`spawn_n `, instead of :meth:`spawn `. Primary API =========== The design goal for Eventlet's API is simplicity and readability. You should be able to read its code and understand what it's doing. Fewer lines of code are preferred over excessively clever implementations. `Like Python itself `_, there should be one, and only one obvious way to do it in Eventlet! Though Eventlet has many modules, much of the most-used stuff is accessible simply by doing ``import eventlet``. Here's a quick summary of the functionality available in the ``eventlet`` module, with links to more verbose documentation on each. .. function:: eventlet.spawn(func, *args, **kw) This launches a greenthread to call *func*. Spawning off multiple greenthreads gets work done in parallel. The return value from ``spawn`` is a :class:`greenthread.GreenThread` object, which can be used to retrieve the return value of *func*. See :func:`spawn ` for more details. .. function:: eventlet.spawn_n(func, *args, **kw) The same as :func:`spawn`, but it's not possible to retrieve the return value. This makes execution faster. See :func:`spawn_n ` for more details. .. function:: eventlet.spawn_after(seconds, func, *args, **kw) Spawns *func* after *seconds* have elapsed; a delayed version of :func:`spawn`. To abort the spawn and prevent *func* from being called, call :meth:`GreenThread.cancel` on the return value of :func:`spawn_after`. See :func:`spawn_after ` for more details. .. function:: eventlet.sleep(seconds=0) Suspends the current greenthread and allows others a chance to process. See :func:`sleep ` for more details. .. class:: eventlet.GreenPool Pools control concurrency. It's very common in applications to want to consume only a finite amount of memory, or to restrict the amount of connections that one part of the code holds open so as to leave more for the rest, or to behave consistently in the face of unpredictable input data. GreenPools provide this control. See :class:`GreenPool ` for more on how to use these. .. class:: eventlet.GreenPile GreenPile objects represent chunks of work. In essence a GreenPile is an iterator that can be stuffed with work, and the results read out later. See :class:`GreenPile ` for more details. .. class:: eventlet.Queue Queues are a fundamental construct for communicating data between execution units. Eventlet's Queue class is used to communicate between greenthreads, and provides a bunch of useful features for doing that. See :class:`Queue ` for more details. .. class:: eventlet.Timeout Raises *exception* in the current greenthread after *timeout* seconds:: timeout = Timeout(seconds, exception) try: ... # execution here is limited by timeout finally: timeout.cancel() When *exception* is omitted or ``None``, the :class:`Timeout` instance itself is raised: >>> Timeout(0.1) >>> eventlet.sleep(0.2) Traceback (most recent call last): ... Timeout: 0.1 seconds In Python 2.5 and newer, you can use the ``with`` statement for additional convenience:: with Timeout(seconds, exception) as timeout: pass # ... code block ... This is equivalent to the try/finally block in the first example. There is an additional feature when using the ``with`` statement: if *exception* is ``False``, the timeout is still raised, but the with statement suppresses it, so the code outside the with-block won't see it:: data = None with Timeout(5, False): data = mysock.makefile().readline() if data is None: ... # 5 seconds passed without reading a line else: ... # a line was read within 5 seconds As a very special case, if *seconds* is None, the timer is not scheduled, and is only useful if you're planning to raise it directly. There are two Timeout caveats to be aware of: * If the code block in the try/finally or with-block never cooperatively yields, the timeout cannot be raised. In Eventlet, this should rarely be a problem, but be aware that you cannot time out CPU-only operations with this class. * If the code block catches and doesn't re-raise :class:`BaseException` (for example, with ``except:``), then it will catch the Timeout exception, and might not abort as intended. When catching timeouts, keep in mind that the one you catch may not be the one you have set; if you going to silence a timeout, always check that it's the same instance that you set:: timeout = Timeout(1) try: ... except Timeout, t: if t is not timeout: raise # not my timeout .. function:: eventlet.with_timeout(seconds, function, *args, **kwds) Wrap a call to some (yielding) function with a timeout; if the called function fails to return before the timeout, cancel it and return a flag value. :param seconds: seconds before timeout occurs :type seconds: int or float :param func: the callable to execute with a timeout; it must cooperatively yield, or else the timeout will not be able to trigger :param \*args: positional arguments to pass to *func* :param \*\*kwds: keyword arguments to pass to *func* :param timeout_value: value to return if timeout occurs (by default raises :class:`Timeout`) :rtype: Value returned by *func* if *func* returns before *seconds*, else *timeout_value* if provided, else raises :class:`Timeout`. :exception Timeout: if *func* times out and no ``timeout_value`` has been provided. :exception: Any exception raised by *func* Example:: data = with_timeout(30, urllib2.open, 'http://www.google.com/', timeout_value="") Here *data* is either the result of the ``get()`` call, or the empty string if it took too long to return. Any exception raised by the ``get()`` call is passed through to the caller. These are the basic primitives of Eventlet; there are a lot more out there in the other Eventlet modules; check out the :doc:`modules`.