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Merge pull request #281 from philipaconrad/master
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Documentation fixups
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denik committed Jun 3, 2013
2 parents b3a9ff1 + c3559fe commit 7d142d1
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56 changes: 28 additions & 28 deletions doc/intro.rst
Expand Up @@ -43,7 +43,7 @@ After the jobs have been spawned, :func:`gevent.joinall` waits for them to compl
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 to proceed with their requests as well.
the whole interpreter and thus lets the other greenlets proceed with their requests unhindered.

.. _monkey-patching:

Expand All @@ -52,14 +52,14 @@ Monkey patching
---------------

The example above used :mod:`gevent.socket` for socket operations. If the standard :mod:`socket`
module was used it would took it 3 times longer to complete because the DNS requests would
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 for. The functions in :mod:`gevent.monkey` carefully
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 multi-greenlet environment.
in a multi-greenlet environment.

>>> from gevent import monkey; monkey.patch_socket()
>>> import urllib2 # it's usable from multiple greenlets now
Expand All @@ -71,10 +71,10 @@ __ https://github.com/surfly/gevent/blob/master/examples/concurrent_download.py#
Event loop
----------

Unlike other network libraries and similar to eventlet, gevent starts
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 ``run()`` or ``dispatch()`` function to call. When a function from
gevent API wants to block, it obtains the :class:`Hub` instance - a greenlet
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.

Expand All @@ -88,24 +88,24 @@ kqueue backend. Please read the `libev documentation`_ for more
information.

The Libev API is available under :mod:`gevent.core` module. Note, that
the callbacks supplied to libev API are run in the :class:`Hub`
greenlet and thus cannot use synchronous gevent API. It is possible to
use asynchronous API there, like :func:`spawn` and :meth:`Event.set`.
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
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 calling blocking I/O functions that bypass libev event loop.
of this when doing something CPU intensive, or when calling blocking I/O functions that bypass the libev event loop.

Synchronizing access to the objects shared across the greenlets is unnecessary in most cases, thus
:class:`Lock` and :class:`Semaphore` classes although present aren't used as often. Other abstractions
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 to wake up a number of greenlets that are calling :meth:`Event.wait` method.
- :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`.

Expand All @@ -120,8 +120,8 @@ method. (The :func:`spawn` function is a shortcut that does exactly that). The :
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 was an error during execution it won't escape greenlet's boundaries. An unhandled error results
in a stacktrace being printed complemented by failed function signature and arguments:
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)
Expand All @@ -132,13 +132,13 @@ in a stacktrace being printed complemented by failed function signature and argu

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

:class:`Greenlet` instances has a number of useful methods:
: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 :class:`Greenlet` class
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__``::
Expand All @@ -164,11 +164,11 @@ of continuing execution, a :exc:`GreenletExit` will be raised.
>>> g.dead
True

The :exc:`GreenletExit` exception and its subclasses are handled differently then other exceptions.
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 was returned by the greenlet, not raised.
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 an exception to raise:
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"))
Expand All @@ -178,29 +178,29 @@ The :meth:`kill <Greenlet.kill>` method can accept an exception to raise:
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 and thus it's a good idea always to pass a timeout to :meth:`kill <Greenlet.kill>`.
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 ``block=False`` argument.
functions can be made asynchronous by passing the argument ``block=False``.

Furthermore, many of the synchronous functions accept *timeout* argument, which specifies a limit on how long the function
could block (examples: :meth:`Event.wait`, :meth:`Greenlet.join`, :meth:`Greenlet.kill`, :meth:`AsyncResult.get` and many more).
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 :meth:`settimeout <gevent.socket.socket.settimeout>` method.
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 :class:`Pool` class (see `example: dns_mass_resolve.py`_).
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`.

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