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from __future__ import print_function
from eventlet import hubs
from eventlet.support import greenlets as greenlet
__all__ = ['Event']
class NOT_USED:
def __repr__(self):
return 'NOT_USED'
NOT_USED = NOT_USED()
class Event(object):
"""An abstraction where an arbitrary number of coroutines
can wait for one event from another.
Events are similar to a Queue that can only hold one item, but differ
in two important ways:
1. calling :meth:`send` never unschedules the current greenthread
2. :meth:`send` can only be called once; create a new event to send again.
They are good for communicating results between coroutines, and
are the basis for how
:meth:`GreenThread.wait() <eventlet.greenthread.GreenThread.wait>`
is implemented.
>>> from eventlet import event
>>> import eventlet
>>> evt = event.Event()
>>> def baz(b):
... evt.send(b + 1)
...
>>> _ = eventlet.spawn_n(baz, 3)
>>> evt.wait()
4
"""
_result = None
_exc = None
def __init__(self):
self._waiters = set()
self.reset()
def __str__(self):
params = (self.__class__.__name__, hex(id(self)),
self._result, self._exc, len(self._waiters))
return '<%s at %s result=%r _exc=%r _waiters[%d]>' % params
def reset(self):
# this is kind of a misfeature and doesn't work perfectly well,
# it's better to create a new event rather than reset an old one
# removing documentation so that we don't get new use cases for it
assert self._result is not NOT_USED, 'Trying to re-reset() a fresh event.'
self._result = NOT_USED
self._exc = None
def ready(self):
""" Return true if the :meth:`wait` call will return immediately.
Used to avoid waiting for things that might take a while to time out.
For example, you can put a bunch of events into a list, and then visit
them all repeatedly, calling :meth:`ready` until one returns ``True``,
and then you can :meth:`wait` on that one."""
return self._result is not NOT_USED
def has_exception(self):
return self._exc is not None
def has_result(self):
return self._result is not NOT_USED and self._exc is None
def poll(self, notready=None):
if self.ready():
return self.wait()
return notready
# QQQ make it return tuple (type, value, tb) instead of raising
# because
# 1) "poll" does not imply raising
# 2) it's better not to screw up caller's sys.exc_info() by default
# (e.g. if caller wants to calls the function in except or finally)
def poll_exception(self, notready=None):
if self.has_exception():
return self.wait()
return notready
def poll_result(self, notready=None):
if self.has_result():
return self.wait()
return notready
def wait(self, timeout=None):
"""Wait until another coroutine calls :meth:`send`.
Returns the value the other coroutine passed to :meth:`send`.
>>> import eventlet
>>> evt = eventlet.Event()
>>> def wait_on():
... retval = evt.wait()
... print("waited for {0}".format(retval))
>>> _ = eventlet.spawn(wait_on)
>>> evt.send('result')
>>> eventlet.sleep(0)
waited for result
Returns immediately if the event has already occurred.
>>> evt.wait()
'result'
When the timeout argument is present and not None, it should be a floating point number
specifying a timeout for the operation in seconds (or fractions thereof).
"""
current = greenlet.getcurrent()
if self._result is NOT_USED:
hub = hubs.get_hub()
self._waiters.add(current)
timer = None
if timeout is not None:
timer = hub.schedule_call_local(timeout, self._do_send, None, None, current)
try:
result = hub.switch()
if timer is not None:
timer.cancel()
return result
finally:
self._waiters.discard(current)
if self._exc is not None:
current.throw(*self._exc)
return self._result
def send(self, result=None, exc=None):
"""Makes arrangements for the waiters to be woken with the
result and then returns immediately to the parent.
>>> from eventlet import event
>>> import eventlet
>>> evt = event.Event()
>>> def waiter():
... print('about to wait')
... result = evt.wait()
... print('waited for {0}'.format(result))
>>> _ = eventlet.spawn(waiter)
>>> eventlet.sleep(0)
about to wait
>>> evt.send('a')
>>> eventlet.sleep(0)
waited for a
It is an error to call :meth:`send` multiple times on the same event.
>>> evt.send('whoops')
Traceback (most recent call last):
...
AssertionError: Trying to re-send() an already-triggered event.
Use :meth:`reset` between :meth:`send` s to reuse an event object.
"""
assert self._result is NOT_USED, 'Trying to re-send() an already-triggered event.'
self._result = result
if exc is not None and not isinstance(exc, tuple):
exc = (exc, )
self._exc = exc
hub = hubs.get_hub()
for waiter in self._waiters:
hub.schedule_call_global(
0, self._do_send, self._result, self._exc, waiter)
def _do_send(self, result, exc, waiter):
if waiter in self._waiters:
if exc is None:
waiter.switch(result)
else:
waiter.throw(*exc)
def send_exception(self, *args):
"""Same as :meth:`send`, but sends an exception to waiters.
The arguments to send_exception are the same as the arguments
to ``raise``. If a single exception object is passed in, it
will be re-raised when :meth:`wait` is called, generating a
new stacktrace.
>>> from eventlet import event
>>> evt = event.Event()
>>> evt.send_exception(RuntimeError())
>>> evt.wait()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "eventlet/event.py", line 120, in wait
current.throw(*self._exc)
RuntimeError
If it's important to preserve the entire original stack trace,
you must pass in the entire :func:`sys.exc_info` tuple.
>>> import sys
>>> evt = event.Event()
>>> try:
... raise RuntimeError()
... except RuntimeError:
... evt.send_exception(*sys.exc_info())
...
>>> evt.wait()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "eventlet/event.py", line 120, in wait
current.throw(*self._exc)
File "<stdin>", line 2, in <module>
RuntimeError
Note that doing so stores a traceback object directly on the
Event object, which may cause reference cycles. See the
:func:`sys.exc_info` documentation.
"""
# the arguments and the same as for greenlet.throw
return self.send(None, args)