# Basic Usage

If it’s your first time to Eventlet, you may find the illuminated examples in the Design Patterns document to be a good starting point.

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.

# 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.

## Greenthread Spawn

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 `greenthread.GreenThread` object, which can be used to retrieve the return value of func. See `spawn` for more details.

eventlet.spawn_n(func, *args, **kw)

  The same as `spawn()`, but it’s not possible to know how the function terminated (i.e. no return value or exceptions). This makes execution faster. See `spawn_n` for more details.

eventlet.spawn_after(seconds, func, *args, **kw)

  Spawns func after seconds have elapsed; a delayed version of `spawn()`. To abort the spawn and prevent func from being called, call `GreenThread.cancel()` on the return value of `spawn_after()`. See `spawn_after` for more details.

## Greenthread Control

eventlet.sleep(seconds=0)

  Suspends the current greenthread and allows others a chance to process. See `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 `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 `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 `Queue` for more details.

class eventlet.Timeout

  This class is a way to add timeouts to anything. It raises exception in the current greenthread after timeout seconds. When exception is omitted or `None`, the Timeout instance itself is raised.

  Timeout objects are context managers, and so can be used in with statements. See `Timeout` for more details.

## Patching Functions

eventlet.import_patched(modulename, *additional_modules, **kw_additional_modules)

  Imports a module in a way that ensures that the module uses “green” versions of the standard library modules, so that everything works nonblockingly. The only required argument is the name of the module to be imported. For more information see Import Green.

eventlet.monkey_patch(all=True, os=False, select=False, socket=False, thread=False, time=False)

  Globally patches certain system modules to be greenthread-friendly. The keyword arguments afford some control over which modules are patched. If all is True, then all modules are patched regardless of the other arguments. If it’s False, then the rest of the keyword arguments control patching of specific subsections of the standard library. Most patch the single module of the same name (os, time, select). The exceptions are socket, which also patches the ssl module if present; and thread, which patches thread, threading, and Queue. It’s safe to call monkey_patch multiple times. For more information see Monkeypatching the Standard Library.

## Network Convenience Functions

eventlet.connect(addr, family=AddressFamily.AF_INET, bind=None)

  Convenience function for opening client sockets.

  Parameters:

    - addr – Address of the server to connect to. For TCP sockets, this is a (host, port) tuple.

    - family – Socket family, optional. See `socket` documentation for available families.

    - bind – Local address to bind to, optional.

  Returns:

    The connected green socket object.

eventlet.listen(addr, family=AddressFamily.AF_INET, backlog=50, reuse_addr=True, reuse_port=None)

  Convenience function for opening server sockets. This socket can be used in `serve()` or a custom `accept()` loop.

  Sets SO_REUSEADDR on the socket to save on annoyance.

  Parameters:

    - addr – Address to listen on. For TCP sockets, this is a (host, port) tuple.

    - family – Socket family, optional. See `socket` documentation for available families.

    - backlog – The maximum number of queued connections. Should be at least 1; the maximum value is system-dependent.

  Returns:

    The listening green socket object.

eventlet.wrap_ssl(sock, *a, **kw)

  Convenience function for converting a regular socket into an SSL socket. Has the same interface as `ssl.wrap_socket()`, but can also use PyOpenSSL. Though, note that it ignores the cert_reqs, ssl_version, ca_certs, do_handshake_on_connect, and suppress_ragged_eofs arguments when using PyOpenSSL.

  The preferred idiom is to call wrap_ssl directly on the creation method, e.g., `wrap_ssl(connect(addr))` or `wrap_ssl(listen(addr), server_side=True)`. This way there is no “naked” socket sitting around to accidentally corrupt the SSL session.

  :return Green SSL object.

eventlet.serve(sock, handle, concurrency=1000)

  Runs a server on the supplied socket. Calls the function handle in a separate greenthread for every incoming client connection. handle takes two arguments: the client socket object, and the client address:

      def myhandle(client_sock, client_addr):
          print("client connected", client_addr)

      eventlet.serve(eventlet.listen(('127.0.0.1', 9999)), myhandle)

  Returning from handle closes the client socket.

  `serve()` blocks the calling greenthread; it won’t return until the server completes. If you desire an immediate return, spawn a new greenthread for `serve()`.

  Any uncaught exceptions raised in handle are raised as exceptions from `serve()`, terminating the server, so be sure to be aware of the exceptions your application can raise. The return value of handle is ignored.

  Raise a `StopServe` exception to gracefully terminate the server – that’s the only way to get the server() function to return rather than raise.

  The value in concurrency controls the maximum number of greenthreads that will be open at any time handling requests. When the server hits the concurrency limit, it stops accepting new connections until the existing ones complete.

class eventlet.StopServe

  Exception class used for quitting `serve()` gracefully.

These are the basic primitives of Eventlet; there are a lot more out there in the other Eventlet modules; check out the Module Reference.