Source code for selectors
From Get docs
Python/docs/3.9/ modules/selectors
Source code for selectors
"""Selectors module.
This module allows high-level and efficient I/O multiplexing, built upon the
`select` module primitives.
"""
from abc import ABCMeta, abstractmethod
from collections import namedtuple
from collections.abc import Mapping
import math
import select
import sys
# generic events, that must be mapped to implementation-specific ones
EVENT_READ = (1 << 0)
EVENT_WRITE = (1 << 1)
def _fileobj_to_fd(fileobj):
"""Return a file descriptor from a file object.
Parameters:
fileobj -- file object or file descriptor
Returns:
corresponding file descriptor
Raises:
ValueError if the object is invalid
"""
if isinstance(fileobj, int):
fd = fileobj
else:
try:
fd = int(fileobj.fileno())
except (AttributeError, TypeError, ValueError):
raise ValueError("Invalid file object: "
"{!r}".format(fileobj)) from None
if fd < 0:
raise ValueError("Invalid file descriptor: {}".format(fd))
return fd
SelectorKey = namedtuple('SelectorKey', ['fileobj', 'fd', 'events', 'data'])
SelectorKey.__doc__ = """SelectorKey(fileobj, fd, events, data)
Object used to associate a file object to its backing
file descriptor, selected event mask, and attached data.
"""
if sys.version_info >= (3, 5):
SelectorKey.fileobj.__doc__ = 'File object registered.'
SelectorKey.fd.__doc__ = 'Underlying file descriptor.'
SelectorKey.events.__doc__ = 'Events that must be waited for on this file object.'
SelectorKey.data.__doc__ = ('''Optional opaque data associated to this file object.
For example, this could be used to store a per-client session ID.''')
class _SelectorMapping(Mapping):
"""Mapping of file objects to selector keys."""
def __init__(self, selector):
self._selector = selector
def __len__(self):
return len(self._selector._fd_to_key)
def __getitem__(self, fileobj):
try:
fd = self._selector._fileobj_lookup(fileobj)
return self._selector._fd_to_key[fd]
except KeyError:
raise KeyError("{!r} is not registered".format(fileobj)) from None
def __iter__(self):
return iter(self._selector._fd_to_key)
[docs]class BaseSelector(metaclass=ABCMeta):
"""Selector abstract base class.
A selector supports registering file objects to be monitored for specific
I/O events.
A file object is a file descriptor or any object with a `fileno()` method.
An arbitrary object can be attached to the file object, which can be used
for example to store context information, a callback, etc.
A selector can use various implementations (select(), poll(), epoll()...)
depending on the platform. The default `Selector` class uses the most
efficient implementation on the current platform.
"""
@abstractmethod
def register(self, fileobj, events, data=None):
"""Register a file object.
Parameters:
fileobj -- file object or file descriptor
events -- events to monitor (bitwise mask of EVENT_READ|EVENT_WRITE)
data -- attached data
Returns:
SelectorKey instance
Raises:
ValueError if events is invalid
KeyError if fileobj is already registered
OSError if fileobj is closed or otherwise is unacceptable to
the underlying system call (if a system call is made)
Note:
OSError may or may not be raised
"""
raise NotImplementedError
@abstractmethod
def unregister(self, fileobj):
"""Unregister a file object.
Parameters:
fileobj -- file object or file descriptor
Returns:
SelectorKey instance
Raises:
KeyError if fileobj is not registered
Note:
If fileobj is registered but has since been closed this does
*not* raise OSError (even if the wrapped syscall does)
"""
raise NotImplementedError
[docs] def modify(self, fileobj, events, data=None):
"""Change a registered file object monitored events or attached data.
Parameters:
fileobj -- file object or file descriptor
events -- events to monitor (bitwise mask of EVENT_READ|EVENT_WRITE)
data -- attached data
Returns:
SelectorKey instance
Raises:
Anything that unregister() or register() raises
"""
self.unregister(fileobj)
return self.register(fileobj, events, data)
@abstractmethod
def select(self, timeout=None):
"""Perform the actual selection, until some monitored file objects are
ready or a timeout expires.
Parameters:
timeout -- if timeout > 0, this specifies the maximum wait time, in
seconds
if timeout <= 0, the select() call won't block, and will
report the currently ready file objects
if timeout is None, select() will block until a monitored
file object becomes ready
Returns:
list of (key, events) for ready file objects
`events` is a bitwise mask of EVENT_READ|EVENT_WRITE
"""
raise NotImplementedError
[docs] def close(self):
"""Close the selector.
This must be called to make sure that any underlying resource is freed.
"""
pass
[docs] def get_key(self, fileobj):
"""Return the key associated to a registered file object.
Returns:
SelectorKey for this file object
"""
mapping = self.get_map()
if mapping is None:
raise RuntimeError('Selector is closed')
try:
return mapping[fileobj]
except KeyError:
raise KeyError("{!r} is not registered".format(fileobj)) from None
@abstractmethod
def get_map(self):
"""Return a mapping of file objects to selector keys."""
raise NotImplementedError
def __enter__(self):
return self
def __exit__(self, *args):
self.close()
class _BaseSelectorImpl(BaseSelector):
"""Base selector implementation."""
def __init__(self):
# this maps file descriptors to keys
self._fd_to_key = {}
# read-only mapping returned by get_map()
self._map = _SelectorMapping(self)
def _fileobj_lookup(self, fileobj):
"""Return a file descriptor from a file object.
This wraps _fileobj_to_fd() to do an exhaustive search in case
the object is invalid but we still have it in our map. This
is used by unregister() so we can unregister an object that
was previously registered even if it is closed. It is also
used by _SelectorMapping.
"""
try:
return _fileobj_to_fd(fileobj)
except ValueError:
# Do an exhaustive search.
for key in self._fd_to_key.values():
if key.fileobj is fileobj:
return key.fd
# Raise ValueError after all.
raise
def register(self, fileobj, events, data=None):
if (not events) or (events & ~(EVENT_READ | EVENT_WRITE)):
raise ValueError("Invalid events: {!r}".format(events))
key = SelectorKey(fileobj, self._fileobj_lookup(fileobj), events, data)
if key.fd in self._fd_to_key:
raise KeyError("{!r} (FD {}) is already registered"
.format(fileobj, key.fd))
self._fd_to_key[key.fd] = key
return key
def unregister(self, fileobj):
try:
key = self._fd_to_key.pop(self._fileobj_lookup(fileobj))
except KeyError:
raise KeyError("{!r} is not registered".format(fileobj)) from None
return key
def modify(self, fileobj, events, data=None):
try:
key = self._fd_to_key[self._fileobj_lookup(fileobj)]
except KeyError:
raise KeyError("{!r} is not registered".format(fileobj)) from None
if events != key.events:
self.unregister(fileobj)
key = self.register(fileobj, events, data)
elif data != key.data:
# Use a shortcut to update the data.
key = key._replace(data=data)
self._fd_to_key[key.fd] = key
return key
def close(self):
self._fd_to_key.clear()
self._map = None
def get_map(self):
return self._map
def _key_from_fd(self, fd):
"""Return the key associated to a given file descriptor.
Parameters:
fd -- file descriptor
Returns:
corresponding key, or None if not found
"""
try:
return self._fd_to_key[fd]
except KeyError:
return None
[docs]class SelectSelector(_BaseSelectorImpl):
"""Select-based selector."""
def __init__(self):
super().__init__()
self._readers = set()
self._writers = set()
def register(self, fileobj, events, data=None):
key = super().register(fileobj, events, data)
if events & EVENT_READ:
self._readers.add(key.fd)
if events & EVENT_WRITE:
self._writers.add(key.fd)
return key
def unregister(self, fileobj):
key = super().unregister(fileobj)
self._readers.discard(key.fd)
self._writers.discard(key.fd)
return key
if sys.platform == 'win32':
def _select(self, r, w, _, timeout=None):
r, w, x = select.select(r, w, w, timeout)
return r, w + x, []
else:
_select = select.select
def select(self, timeout=None):
timeout = None if timeout is None else max(timeout, 0)
ready = []
try:
r, w, _ = self._select(self._readers, self._writers, [], timeout)
except InterruptedError:
return ready
r = set(r)
w = set(w)
for fd in r | w:
events = 0
if fd in r:
events |= EVENT_READ
if fd in w:
events |= EVENT_WRITE
key = self._key_from_fd(fd)
if key:
ready.append((key, events & key.events))
return ready
class _PollLikeSelector(_BaseSelectorImpl):
"""Base class shared between poll, epoll and devpoll selectors."""
_selector_cls = None
_EVENT_READ = None
_EVENT_WRITE = None
def __init__(self):
super().__init__()
self._selector = self._selector_cls()
def register(self, fileobj, events, data=None):
key = super().register(fileobj, events, data)
poller_events = 0
if events & EVENT_READ:
poller_events |= self._EVENT_READ
if events & EVENT_WRITE:
poller_events |= self._EVENT_WRITE
try:
self._selector.register(key.fd, poller_events)
except:
super().unregister(fileobj)
raise
return key
def unregister(self, fileobj):
key = super().unregister(fileobj)
try:
self._selector.unregister(key.fd)
except OSError:
# This can happen if the FD was closed since it
# was registered.
pass
return key
def modify(self, fileobj, events, data=None):
try:
key = self._fd_to_key[self._fileobj_lookup(fileobj)]
except KeyError:
raise KeyError(f"{fileobj!r} is not registered") from None
changed = False
if events != key.events:
selector_events = 0
if events & EVENT_READ:
selector_events |= self._EVENT_READ
if events & EVENT_WRITE:
selector_events |= self._EVENT_WRITE
try:
self._selector.modify(key.fd, selector_events)
except:
super().unregister(fileobj)
raise
changed = True
if data != key.data:
changed = True
if changed:
key = key._replace(events=events, data=data)
self._fd_to_key[key.fd] = key
return key
def select(self, timeout=None):
# This is shared between poll() and epoll().
# epoll() has a different signature and handling of timeout parameter.
if timeout is None:
timeout = None
elif timeout <= 0:
timeout = 0
else:
# poll() has a resolution of 1 millisecond, round away from
# zero to wait *at least* timeout seconds.
timeout = math.ceil(timeout * 1e3)
ready = []
try:
fd_event_list = self._selector.poll(timeout)
except InterruptedError:
return ready
for fd, event in fd_event_list:
events = 0
if event & ~self._EVENT_READ:
events |= EVENT_WRITE
if event & ~self._EVENT_WRITE:
events |= EVENT_READ
key = self._key_from_fd(fd)
if key:
ready.append((key, events & key.events))
return ready
if hasattr(select, 'poll'):
[docs] class PollSelector(_PollLikeSelector):
"""Poll-based selector."""
_selector_cls = select.poll
_EVENT_READ = select.POLLIN
_EVENT_WRITE = select.POLLOUT
if hasattr(select, 'epoll'):
[docs] class EpollSelector(_PollLikeSelector):
"""Epoll-based selector."""
_selector_cls = select.epoll
_EVENT_READ = select.EPOLLIN
_EVENT_WRITE = select.EPOLLOUT
[docs] def fileno(self):
return self._selector.fileno()
def select(self, timeout=None):
if timeout is None:
timeout = -1
elif timeout <= 0:
timeout = 0
else:
# epoll_wait() has a resolution of 1 millisecond, round away
# from zero to wait *at least* timeout seconds.
timeout = math.ceil(timeout * 1e3) * 1e-3
# epoll_wait() expects `maxevents` to be greater than zero;
# we want to make sure that `select()` can be called when no
# FD is registered.
max_ev = max(len(self._fd_to_key), 1)
ready = []
try:
fd_event_list = self._selector.poll(timeout, max_ev)
except InterruptedError:
return ready
for fd, event in fd_event_list:
events = 0
if event & ~select.EPOLLIN:
events |= EVENT_WRITE
if event & ~select.EPOLLOUT:
events |= EVENT_READ
key = self._key_from_fd(fd)
if key:
ready.append((key, events & key.events))
return ready
def close(self):
self._selector.close()
super().close()
if hasattr(select, 'devpoll'):
class DevpollSelector(_PollLikeSelector):
"""Solaris /dev/poll selector."""
_selector_cls = select.devpoll
_EVENT_READ = select.POLLIN
_EVENT_WRITE = select.POLLOUT
def fileno(self):
return self._selector.fileno()
def close(self):
self._selector.close()
super().close()
if hasattr(select, 'kqueue'):
class KqueueSelector(_BaseSelectorImpl):
"""Kqueue-based selector."""
def __init__(self):
super().__init__()
self._selector = select.kqueue()
def fileno(self):
return self._selector.fileno()
def register(self, fileobj, events, data=None):
key = super().register(fileobj, events, data)
try:
if events & EVENT_READ:
kev = select.kevent(key.fd, select.KQ_FILTER_READ,
select.KQ_EV_ADD)
self._selector.control([kev], 0, 0)
if events & EVENT_WRITE:
kev = select.kevent(key.fd, select.KQ_FILTER_WRITE,
select.KQ_EV_ADD)
self._selector.control([kev], 0, 0)
except:
super().unregister(fileobj)
raise
return key
def unregister(self, fileobj):
key = super().unregister(fileobj)
if key.events & EVENT_READ:
kev = select.kevent(key.fd, select.KQ_FILTER_READ,
select.KQ_EV_DELETE)
try:
self._selector.control([kev], 0, 0)
except OSError:
# This can happen if the FD was closed since it
# was registered.
pass
if key.events & EVENT_WRITE:
kev = select.kevent(key.fd, select.KQ_FILTER_WRITE,
select.KQ_EV_DELETE)
try:
self._selector.control([kev], 0, 0)
except OSError:
# See comment above.
pass
return key
def select(self, timeout=None):
timeout = None if timeout is None else max(timeout, 0)
# If max_ev is 0, kqueue will ignore the timeout. For consistent
# behavior with the other selector classes, we prevent that here
# (using max). See https://bugs.python.org/issue29255
max_ev = max(len(self._fd_to_key), 1)
ready = []
try:
kev_list = self._selector.control(None, max_ev, timeout)
except InterruptedError:
return ready
for kev in kev_list:
fd = kev.ident
flag = kev.filter
events = 0
if flag == select.KQ_FILTER_READ:
events |= EVENT_READ
if flag == select.KQ_FILTER_WRITE:
events |= EVENT_WRITE
key = self._key_from_fd(fd)
if key:
ready.append((key, events & key.events))
return ready
def close(self):
self._selector.close()
super().close()
def _can_use(method):
"""Check if we can use the selector depending upon the
operating system. """
# Implementation based upon https://github.com/sethmlarson/selectors2/blob/master/selectors2.py
selector = getattr(select, method, None)
if selector is None:
# select module does not implement method
return False
# check if the OS and Kernel actually support the method. Call may fail with
# OSError: [Errno 38] Function not implemented
try:
selector_obj = selector()
if method == 'poll':
# check that poll actually works
selector_obj.poll(0)
else:
# close epoll, kqueue, and devpoll fd
selector_obj.close()
return True
except OSError:
return False
# Choose the best implementation, roughly:
# epoll|kqueue|devpoll > poll > select.
# select() also can't accept a FD > FD_SETSIZE (usually around 1024)
if _can_use('kqueue'):
DefaultSelector = KqueueSelector
elif _can_use('epoll'):
DefaultSelector = EpollSelector
elif _can_use('devpoll'):
DefaultSelector = DevpollSelector
elif _can_use('poll'):
DefaultSelector = PollSelector
else:
DefaultSelector = SelectSelector
