Introduction
============

gevent is a coroutine-based Python networking library.

Features include:

* Fast event loop based on libev (epoll on Linux, kqueue on FreeBSD).
* Lightweight execution units based on greenlet.
* API that re-uses concepts from the Python standard library (e.g. :class:`Event`, :class:`Queue`).
* Cooperative :mod:`socket` and :mod:`ssl` modules.
* Ability to use standard library and 3rd party modules written for standard blocking sockets (:mod:`gevent.monkey`).
* DNS queries performed through threadpool (default) or through c-ares (enabled via GEVENT_RESOLVER=ares env var).
* TCP/UDP/HTTP servers
* Subprocess support (through :mod:`gevent.subprocess`)
* Thread pools


Installation
------------

gevent runs on Python 2.5 and newer and requires

* greenlet__ which can be installed with ``pip install greenlet``.

For ssl to work on Python older than 2.6, ssl_ package is required.

__ http://pypi.python.org/pypi/greenlet
.. _ssl: http://pypi.python.org/pypi/ssl


Example
-------

The following example shows how to run tasks concurrently.

    >>> import gevent
    >>> from gevent import socket
    >>> urls = ['www.google.com', 'www.example.com', 'www.python.org']
    >>> jobs = [gevent.spawn(socket.gethostbyname, url) for url in urls]
    >>> gevent.joinall(jobs, timeout=2)
    >>> [job.value for job in jobs]
    ['74.125.79.106', '208.77.188.166', '82.94.164.162']

After the jobs have been spawned, :func:`gevent.joinall` waits for them to complete,
no longer than 2 seconds though. The results are then collected by checking
:attr:`gevent.Greenlet.value` property. The :func:`gevent.socket.gethostbyname` function
has the same interface as the standard :func:`socket.gethostbyname` but it does not block
the whole interpreter and thus lets the other greenlets proceed with their requests unhindered.

.. _monkey-patching:


Monkey patching
---------------

The example above used :mod:`gevent.socket` for socket operations. If the standard :mod:`socket`
module was used the example would have taken 3 times longer to complete because the DNS requests would
be sequential. Using the standard socket module inside greenlets makes gevent rather
pointless, so what about module and packages that are built on top of :mod:`socket`?

That's what monkey patching is for. The functions in :mod:`gevent.monkey` carefully
replace functions and classes in the standard :mod:`socket` module with their cooperative
counterparts. That way even the modules that are unaware of gevent can benefit from running
in a multi-greenlet environment.

    >>> from gevent import monkey; monkey.patch_socket()
    >>> import urllib2 # it's usable from multiple greenlets now

See `examples/concurrent_download.py`__

__ https://github.com/surfly/gevent/blob/master/examples/concurrent_download.py#L1

Event loop
----------

Unlike other network libraries, in similar fashion to eventlet, gevent starts
the event loop implicitly in a dedicated greenlet. There's no ``reactor`` that
you must call a ``run()`` or ``dispatch()`` function on. When a function from
gevent's API wants to block, it obtains the :class:`Hub` instance - a greenlet
that runs the event loop - and switches to it. If there's no :class:`Hub`
instance yet, one is created on the fly.

The event loop provided by libev uses the fastest polling mechanism
available on the system by default. It is possible to command libev to
use a particular polling mechanism by setting the ``LIBEV_FLAGS```
environment variable. Possible values include ``LIBEV_FLAGS=1`` for
the select backend, ``LIBEV_FLAGS=2`` for the poll backend,
``LIBEV_FLAGS=4`` for the epoll backend and ``LIBEV_FLAGS=8`` for the
kqueue backend. Please read the `libev documentation`_ for more
information.

.. _`libev documentation`: http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#FUNCTIONS_CONTROLLING_EVENT_LOOPS

The Libev API is available under :mod:`gevent.core` module. Note, that
the callbacks supplied to the libev API are run in the :class:`Hub`
greenlet and thus cannot use the synchronous gevent API. It is possible to
use the asynchronous API there, like :func:`spawn` and :meth:`Event.set`.


Cooperative multitasking
------------------------

The greenlets all run in the same OS thread and are scheduled cooperatively. This means that until
a particular greenlet gives up control, (by calling a blocking function that will switch to the :class:`Hub`), other greenlets
won't get a chance to run. It is typically not an issue for an I/O bound app, but one should be aware
of this when doing something CPU intensive, or when calling blocking I/O functions that bypass the libev event loop.

Synchronizing access to objects shared across the greenlets is unnecessary in most cases, thus
:class:`Lock` and :class:`Semaphore` classes, although present, aren't used very often. Other abstractions
from threading and multiprocessing remain useful in the cooperative world:

- :class:`Event` allows one to wake up a number of greenlets that are calling :meth:`Event.wait` method.
- :class:`AsyncResult` is similar to :class:`Event` but allows passing a value or an exception to the waiters.
- :class:`Queue` and :class:`JoinableQueue`.


Lightweight pseudothreads
-------------------------

.. currentmodule:: gevent.greenlet

The greenlets are spawned by creating a :class:`Greenlet` instance and calling its :meth:`start <Greenlet.start>`
method. (The :func:`spawn` function is a shortcut that does exactly that). The :meth:`start <Greenlet.start>`
method schedules a switch to the greenlet that will happen as soon as the current greenlet gives up control.
If there is more than one active event, they will be executed one by one, in an undefined order.

If there is an error during execution it won't escape greenlet's boundaries. An unhandled error results
in a stacktrace being printed, complemented by the failed function's signature and arguments:

    >>> gevent.spawn(lambda : 1/0)
    >>> gevent.sleep(1)
    Traceback (most recent call last):
     ...
    ZeroDivisionError: integer division or modulo by zero
    <Greenlet at 0x7f2ec3a4e490: <function <lambda...>> failed with ZeroDivisionError

The traceback is asynchronously printed to ``sys.stderr`` when the greenlet dies.

:class:`Greenlet` instances have a number of useful methods:

- :meth:`join <Greenlet.join>` -- waits until the greenlet exits;
- :meth:`kill <Greenlet.kill>` -- interrupts greenlet's execution;
- :meth:`get <Greenlet.get>`  -- returns the value returned by greenlet or re-raised the exception that killed it.

It is possible to customize the string printed after the traceback by subclassing the :class:`Greenlet` class
and redefining its ``__str__`` method.

To subclass a :class:`Greenlet`, override its :meth:`_run` method and call ``Greenlet.__init__(self)`` in ``__init__``::

    class MyNoopGreenlet(Greenlet):

        def __init__(self, seconds):
            Greenlet.__init__(self)
            self.seconds = seconds

        def _run(self):
            gevent.sleep(self.seconds)

        def __str__(self):
            return 'MyNoopGreenlet(%s)' % self.seconds

Greenlets can be killed asynchronously. Killing will resume the sleeping greenlet, but instead
of continuing execution, a :exc:`GreenletExit` will be raised.

    >>> g = MyNoopGreenlet(4)
    >>> g.start()
    >>> g.kill()
    >>> g.dead
    True

The :exc:`GreenletExit` exception and its subclasses are handled differently than other exceptions.
Raising :exc:`GreenletExit` is not considered an exceptional situation, so the traceback is not printed.
The :exc:`GreenletExit` is returned by :meth:`get <Greenlet.get>` as if it were returned by the greenlet, not raised.

The :meth:`kill <Greenlet.kill>` method can accept a custom exception to be raised:

    >>> g = MyNoopGreenlet.spawn(5) # spawn() creates a Greenlet and starts it
    >>> g.kill(Exception("A time to kill"))
    Traceback (most recent call last):
     ...
    Exception: A time to kill
    MyNoopGreenlet(5) failed with Exception

The :meth:`kill <Greenlet.kill>` can also accept a *timeout* argument specifying the number of seconds to wait for the greenlet to exit.
Note, that :meth:`kill <Greenlet.kill>` cannot guarantee that the target greenlet will not ignore the exception, thus it's a good idea always to pass a timeout to :meth:`kill <Greenlet.kill>`.


Timeouts
--------

Many functions in the gevent API are synchronous, blocking the current greenlet until the operation is done. For example,
:meth:`kill <Greenlet.kill>` waits until the target greenlet is :attr:`dead` before returning [#f1]_. Many of those
functions can be made asynchronous by passing the argument ``block=False``.

Furthermore, many of the synchronous functions accept a *timeout* argument, which specifies a limit on how long the function
can block (examples: :meth:`Event.wait`, :meth:`Greenlet.join`, :meth:`Greenlet.kill`, :meth:`AsyncResult.get`, and many more).

The :class:`socket <gevent.socket.socket>` and :class:`SSLObject <gevent.ssl.SSLObject>` instances can also have a timeout,
set by the :meth:`settimeout <gevent.socket.socket.settimeout>` method.

When these are not enough, the :class:`Timeout` class can be used to add timeouts to arbitrary sections of (yielding) code.


Futher reading
--------------

To limit concurrency, use the :class:`Pool` class (see `example: dns_mass_resolve.py`_).

Gevent comes with TCP/SSL/HTTP/WSGI servers. See :doc:`servers`.

.. _`example: dns_mass_resolve.py`: http://bitbucket.org/denis/gevent/src/tip/examples/dns_mass_resolve.py#cl-17


External resources
------------------

`Gevent for working Python developer`__ is a comprehensive tutorial.

__ http://sdiehl.github.io/gevent-tutorial/

.. rubric:: Footnotes

.. [#f1] This was not the case before 0.13.0, :meth:`kill <Greenlet>` method in 0.12.2 and older was asynchronous by default.

