18.4. selectors
— High-level I/O multiplexing¶
Nuevo en la versión 3.4.
Source code: Lib/selectors.py
18.4.1. Introduction¶
This module allows high-level and efficient I/O multiplexing, built upon the
select
module primitives. Users are encouraged to use this module
instead, unless they want precise control over the OS-level primitives used.
It defines a BaseSelector
abstract base class, along with several
concrete implementations (KqueueSelector
, EpollSelector
…),
that can be used to wait for I/O readiness notification on multiple file
objects. In the following, «file object» refers to any object with a
fileno()
method, or a raw file descriptor. See file object.
DefaultSelector
is an alias to the most efficient implementation
available on the current platform: this should be the default choice for most
users.
Nota
The type of file objects supported depends on the platform: on Windows, sockets are supported, but not pipes, whereas on Unix, both are supported (some other types may be supported as well, such as fifos or special file devices).
Ver también
select
- Low-level I/O multiplexing module.
18.4.2. Classes¶
Classes hierarchy:
BaseSelector
+-- SelectSelector
+-- PollSelector
+-- EpollSelector
+-- DevpollSelector
+-- KqueueSelector
In the following, events is a bitwise mask indicating which I/O events should be waited for on a given file object. It can be a combination of the modules constants below:
Constant Meaning EVENT_READ
Available for read EVENT_WRITE
Available for write
-
class
selectors.
SelectorKey
¶ A
SelectorKey
is anamedtuple
used to associate a file object to its underlying file descriptor, selected event mask and attached data. It is returned by severalBaseSelector
methods.-
fileobj
¶ File object registered.
-
fd
¶ Underlying file descriptor.
-
events
¶ Events that must be waited for on this file object.
-
data
¶ Optional opaque data associated to this file object: for example, this could be used to store a per-client session ID.
-
-
class
selectors.
BaseSelector
¶ A
BaseSelector
is used to wait for I/O event readiness on multiple file objects. It supports file stream registration, unregistration, and a method to wait for I/O events on those streams, with an optional timeout. It’s an abstract base class, so cannot be instantiated. UseDefaultSelector
instead, or one ofSelectSelector
,KqueueSelector
etc. if you want to specifically use an implementation, and your platform supports it.BaseSelector
and its concrete implementations support the context manager protocol.-
abstractmethod
register
(fileobj, events, data=None)¶ Register a file object for selection, monitoring it for I/O events.
fileobj is the file object to monitor. It may either be an integer file descriptor or an object with a
fileno()
method. events is a bitwise mask of events to monitor. data is an opaque object.This returns a new
SelectorKey
instance, or raises aValueError
in case of invalid event mask or file descriptor, orKeyError
if the file object is already registered.
-
abstractmethod
unregister
(fileobj)¶ Unregister a file object from selection, removing it from monitoring. A file object shall be unregistered prior to being closed.
fileobj must be a file object previously registered.
This returns the associated
SelectorKey
instance, or raises aKeyError
if fileobj is not registered. It will raiseValueError
if fileobj is invalid (e.g. it has nofileno()
method or itsfileno()
method has an invalid return value).
-
modify
(fileobj, events, data=None)¶ Change a registered file object’s monitored events or attached data.
This is equivalent to
BaseSelector.unregister(fileobj)()
followed byBaseSelector.register(fileobj, events, data)()
, except that it can be implemented more efficiently.This returns a new
SelectorKey
instance, or raises aValueError
in case of invalid event mask or file descriptor, orKeyError
if the file object is not registered.
-
abstractmethod
select
(timeout=None)¶ Wait until some registered file objects become ready, or the timeout expires.
If
timeout > 0
, this specifies the maximum wait time, in seconds. Iftimeout <= 0
, the call won’t block, and will report the currently ready file objects. If timeout isNone
, the call will block until a monitored file object becomes ready.This returns a list of
(key, events)
tuples, one for each ready file object.key is the
SelectorKey
instance corresponding to a ready file object. events is a bitmask of events ready on this file object.Nota
This method can return before any file object becomes ready or the timeout has elapsed if the current process receives a signal: in this case, an empty list will be returned.
Distinto en la versión 3.5: The selector is now retried with a recomputed timeout when interrupted by a signal if the signal handler did not raise an exception (see PEP 475 for the rationale), instead of returning an empty list of events before the timeout.
-
close
()¶ Close the selector.
This must be called to make sure that any underlying resource is freed. The selector shall not be used once it has been closed.
-
get_key
(fileobj)¶ Return the key associated with a registered file object.
This returns the
SelectorKey
instance associated to this file object, or raisesKeyError
if the file object is not registered.
-
abstractmethod
get_map
()¶ Return a mapping of file objects to selector keys.
This returns a
Mapping
instance mapping registered file objects to their associatedSelectorKey
instance.
-
abstractmethod
-
class
selectors.
DefaultSelector
¶ The default selector class, using the most efficient implementation available on the current platform. This should be the default choice for most users.
-
class
selectors.
SelectSelector
¶ select.select()
-based selector.
-
class
selectors.
PollSelector
¶ select.poll()
-based selector.
-
class
selectors.
EpollSelector
¶ select.epoll()
-based selector.-
fileno
()¶ This returns the file descriptor used by the underlying
select.epoll()
object.
-
-
class
selectors.
DevpollSelector
¶ select.devpoll()
-based selector.-
fileno
()¶ This returns the file descriptor used by the underlying
select.devpoll()
object.
Nuevo en la versión 3.5.
-
-
class
selectors.
KqueueSelector
¶ select.kqueue()
-based selector.-
fileno
()¶ This returns the file descriptor used by the underlying
select.kqueue()
object.
-
18.4.3. Examples¶
Here is a simple echo server implementation:
import selectors
import socket
sel = selectors.DefaultSelector()
def accept(sock, mask):
conn, addr = sock.accept() # Should be ready
print('accepted', conn, 'from', addr)
conn.setblocking(False)
sel.register(conn, selectors.EVENT_READ, read)
def read(conn, mask):
data = conn.recv(1000) # Should be ready
if data:
print('echoing', repr(data), 'to', conn)
conn.send(data) # Hope it won't block
else:
print('closing', conn)
sel.unregister(conn)
conn.close()
sock = socket.socket()
sock.bind(('localhost', 1234))
sock.listen(100)
sock.setblocking(False)
sel.register(sock, selectors.EVENT_READ, accept)
while True:
events = sel.select()
for key, mask in events:
callback = key.data
callback(key.fileobj, mask)