Mini Shell
"""Support for tasks, coroutines and the scheduler."""
__all__ = ['Task',
'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
'wait', 'wait_for', 'as_completed', 'sleep', 'async',
'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe',
'timeout',
]
import concurrent.futures
import functools
import inspect
import linecache
import traceback
import warnings
import weakref
from . import compat
from . import coroutines
from . import events
from . import futures
from .coroutines import coroutine
class Task(futures.Future):
"""A coroutine wrapped in a Future."""
# An important invariant maintained while a Task not done:
#
# - Either _fut_waiter is None, and _step() is scheduled;
# - or _fut_waiter is some Future, and _step() is *not* scheduled.
#
# The only transition from the latter to the former is through
# _wakeup(). When _fut_waiter is not None, one of its callbacks
# must be _wakeup().
# Weak set containing all tasks alive.
_all_tasks = weakref.WeakSet()
# Dictionary containing tasks that are currently active in
# all running event loops. {EventLoop: Task}
_current_tasks = {}
# If False, don't log a message if the task is destroyed whereas its
# status is still pending
_log_destroy_pending = True
@classmethod
def current_task(cls, loop=None):
"""Return the currently running task in an event loop or None.
By default the current task for the current event loop is returned.
None is returned when called not in the context of a Task.
"""
if loop is None:
loop = events.get_event_loop()
return cls._current_tasks.get(loop)
@classmethod
def all_tasks(cls, loop=None):
"""Return a set of all tasks for an event loop.
By default all tasks for the current event loop are returned.
"""
if loop is None:
loop = events.get_event_loop()
return {t for t in cls._all_tasks if t._loop is loop}
def __init__(self, coro, *, loop=None):
assert coroutines.iscoroutine(coro), repr(coro)
super().__init__(loop=loop)
if self._source_traceback:
del self._source_traceback[-1]
self._coro = coro
self._fut_waiter = None
self._must_cancel = False
self._loop.call_soon(self._step)
self.__class__._all_tasks.add(self)
# On Python 3.3 or older, objects with a destructor that are part of a
# reference cycle are never destroyed. That's not the case any more on
# Python 3.4 thanks to the PEP 442.
if compat.PY34:
def __del__(self):
if self._state == futures._PENDING and self._log_destroy_pending:
context = {
'task': self,
'message': 'Task was destroyed but it is pending!',
}
if self._source_traceback:
context['source_traceback'] = self._source_traceback
self._loop.call_exception_handler(context)
futures.Future.__del__(self)
def _repr_info(self):
info = super()._repr_info()
if self._must_cancel:
# replace status
info[0] = 'cancelling'
coro = coroutines._format_coroutine(self._coro)
info.insert(1, 'coro=<%s>' % coro)
if self._fut_waiter is not None:
info.insert(2, 'wait_for=%r' % self._fut_waiter)
return info
def get_stack(self, *, limit=None):
"""Return the list of stack frames for this task's coroutine.
If the coroutine is not done, this returns the stack where it is
suspended. If the coroutine has completed successfully or was
cancelled, this returns an empty list. If the coroutine was
terminated by an exception, this returns the list of traceback
frames.
The frames are always ordered from oldest to newest.
The optional limit gives the maximum number of frames to
return; by default all available frames are returned. Its
meaning differs depending on whether a stack or a traceback is
returned: the newest frames of a stack are returned, but the
oldest frames of a traceback are returned. (This matches the
behavior of the traceback module.)
For reasons beyond our control, only one stack frame is
returned for a suspended coroutine.
"""
frames = []
try:
# 'async def' coroutines
f = self._coro.cr_frame
except AttributeError:
f = self._coro.gi_frame
if f is not None:
while f is not None:
if limit is not None:
if limit <= 0:
break
limit -= 1
frames.append(f)
f = f.f_back
frames.reverse()
elif self._exception is not None:
tb = self._exception.__traceback__
while tb is not None:
if limit is not None:
if limit <= 0:
break
limit -= 1
frames.append(tb.tb_frame)
tb = tb.tb_next
return frames
def print_stack(self, *, limit=None, file=None):
"""Print the stack or traceback for this task's coroutine.
This produces output similar to that of the traceback module,
for the frames retrieved by get_stack(). The limit argument
is passed to get_stack(). The file argument is an I/O stream
to which the output is written; by default output is written
to sys.stderr.
"""
extracted_list = []
checked = set()
for f in self.get_stack(limit=limit):
lineno = f.f_lineno
co = f.f_code
filename = co.co_filename
name = co.co_name
if filename not in checked:
checked.add(filename)
linecache.checkcache(filename)
line = linecache.getline(filename, lineno, f.f_globals)
extracted_list.append((filename, lineno, name, line))
exc = self._exception
if not extracted_list:
print('No stack for %r' % self, file=file)
elif exc is not None:
print('Traceback for %r (most recent call last):' % self,
file=file)
else:
print('Stack for %r (most recent call last):' % self,
file=file)
traceback.print_list(extracted_list, file=file)
if exc is not None:
for line in traceback.format_exception_only(exc.__class__, exc):
print(line, file=file, end='')
def cancel(self):
"""Request that this task cancel itself.
This arranges for a CancelledError to be thrown into the
wrapped coroutine on the next cycle through the event loop.
The coroutine then has a chance to clean up or even deny
the request using try/except/finally.
Unlike Future.cancel, this does not guarantee that the
task will be cancelled: the exception might be caught and
acted upon, delaying cancellation of the task or preventing
cancellation completely. The task may also return a value or
raise a different exception.
Immediately after this method is called, Task.cancelled() will
not return True (unless the task was already cancelled). A
task will be marked as cancelled when the wrapped coroutine
terminates with a CancelledError exception (even if cancel()
was not called).
"""
if self.done():
return False
if self._fut_waiter is not None:
if self._fut_waiter.cancel():
# Leave self._fut_waiter; it may be a Task that
# catches and ignores the cancellation so we may have
# to cancel it again later.
return True
# It must be the case that self._step is already scheduled.
self._must_cancel = True
return True
def _step(self, exc=None):
assert not self.done(), \
'_step(): already done: {!r}, {!r}'.format(self, exc)
if self._must_cancel:
if not isinstance(exc, futures.CancelledError):
exc = futures.CancelledError()
self._must_cancel = False
coro = self._coro
self._fut_waiter = None
self.__class__._current_tasks[self._loop] = self
# Call either coro.throw(exc) or coro.send(None).
try:
if exc is None:
# We use the `send` method directly, because coroutines
# don't have `__iter__` and `__next__` methods.
result = coro.send(None)
else:
result = coro.throw(exc)
except StopIteration as exc:
self.set_result(exc.value)
except futures.CancelledError as exc:
super().cancel() # I.e., Future.cancel(self).
except Exception as exc:
self.set_exception(exc)
except BaseException as exc:
self.set_exception(exc)
raise
else:
if isinstance(result, futures.Future):
# Yielded Future must come from Future.__iter__().
if result._loop is not self._loop:
self._loop.call_soon(
self._step,
RuntimeError(
'Task {!r} got Future {!r} attached to a '
'different loop'.format(self, result)))
elif result._blocking:
result._blocking = False
result.add_done_callback(self._wakeup)
self._fut_waiter = result
if self._must_cancel:
if self._fut_waiter.cancel():
self._must_cancel = False
else:
self._loop.call_soon(
self._step,
RuntimeError(
'yield was used instead of yield from '
'in task {!r} with {!r}'.format(self, result)))
elif result is None:
# Bare yield relinquishes control for one event loop iteration.
self._loop.call_soon(self._step)
elif inspect.isgenerator(result):
# Yielding a generator is just wrong.
self._loop.call_soon(
self._step,
RuntimeError(
'yield was used instead of yield from for '
'generator in task {!r} with {}'.format(
self, result)))
else:
# Yielding something else is an error.
self._loop.call_soon(
self._step,
RuntimeError(
'Task got bad yield: {!r}'.format(result)))
finally:
self.__class__._current_tasks.pop(self._loop)
self = None # Needed to break cycles when an exception occurs.
def _wakeup(self, future):
try:
future.result()
except Exception as exc:
# This may also be a cancellation.
self._step(exc)
else:
# Don't pass the value of `future.result()` explicitly,
# as `Future.__iter__` and `Future.__await__` don't need it.
# If we call `_step(value, None)` instead of `_step()`,
# Python eval loop would use `.send(value)` method call,
# instead of `__next__()`, which is slower for futures
# that return non-generator iterators from their `__iter__`.
self._step()
self = None # Needed to break cycles when an exception occurs.
# wait() and as_completed() similar to those in PEP 3148.
FIRST_COMPLETED = concurrent.futures.FIRST_COMPLETED
FIRST_EXCEPTION = concurrent.futures.FIRST_EXCEPTION
ALL_COMPLETED = concurrent.futures.ALL_COMPLETED
@coroutine
def wait(fs, *, loop=None, timeout=None, return_when=ALL_COMPLETED):
"""Wait for the Futures and coroutines given by fs to complete.
The sequence futures must not be empty.
Coroutines will be wrapped in Tasks.
Returns two sets of Future: (done, pending).
Usage:
done, pending = yield from asyncio.wait(fs)
Note: This does not raise TimeoutError! Futures that aren't done
when the timeout occurs are returned in the second set.
"""
if isinstance(fs, futures.Future) or coroutines.iscoroutine(fs):
raise TypeError("expect a list of futures, not %s" % type(fs).__name__)
if not fs:
raise ValueError('Set of coroutines/Futures is empty.')
if return_when not in (FIRST_COMPLETED, FIRST_EXCEPTION, ALL_COMPLETED):
raise ValueError('Invalid return_when value: {}'.format(return_when))
if loop is None:
loop = events.get_event_loop()
fs = {ensure_future(f, loop=loop) for f in set(fs)}
return (yield from _wait(fs, timeout, return_when, loop))
def _release_waiter(waiter, *args):
if not waiter.done():
waiter.set_result(None)
@coroutine
def wait_for(fut, timeout, *, loop=None):
"""Wait for the single Future or coroutine to complete, with timeout.
Coroutine will be wrapped in Task.
Returns result of the Future or coroutine. When a timeout occurs,
it cancels the task and raises TimeoutError. To avoid the task
cancellation, wrap it in shield().
If the wait is cancelled, the task is also cancelled.
This function is a coroutine.
"""
if loop is None:
loop = events.get_event_loop()
if timeout is None:
return (yield from fut)
waiter = futures.Future(loop=loop)
timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
cb = functools.partial(_release_waiter, waiter)
fut = ensure_future(fut, loop=loop)
fut.add_done_callback(cb)
try:
# wait until the future completes or the timeout
try:
yield from waiter
except futures.CancelledError:
fut.remove_done_callback(cb)
fut.cancel()
raise
if fut.done():
return fut.result()
else:
fut.remove_done_callback(cb)
fut.cancel()
raise futures.TimeoutError()
finally:
timeout_handle.cancel()
@coroutine
def _wait(fs, timeout, return_when, loop):
"""Internal helper for wait() and _wait_for().
The fs argument must be a collection of Futures.
"""
assert fs, 'Set of Futures is empty.'
waiter = futures.Future(loop=loop)
timeout_handle = None
if timeout is not None:
timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
counter = len(fs)
def _on_completion(f):
nonlocal counter
counter -= 1
if (counter <= 0 or
return_when == FIRST_COMPLETED or
return_when == FIRST_EXCEPTION and (not f.cancelled() and
f.exception() is not None)):
if timeout_handle is not None:
timeout_handle.cancel()
if not waiter.done():
waiter.set_result(None)
for f in fs:
f.add_done_callback(_on_completion)
try:
yield from waiter
finally:
if timeout_handle is not None:
timeout_handle.cancel()
done, pending = set(), set()
for f in fs:
f.remove_done_callback(_on_completion)
if f.done():
done.add(f)
else:
pending.add(f)
return done, pending
# This is *not* a @coroutine! It is just an iterator (yielding Futures).
def as_completed(fs, *, loop=None, timeout=None):
"""Return an iterator whose values are coroutines.
When waiting for the yielded coroutines you'll get the results (or
exceptions!) of the original Futures (or coroutines), in the order
in which and as soon as they complete.
This differs from PEP 3148; the proper way to use this is:
for f in as_completed(fs):
result = yield from f # The 'yield from' may raise.
# Use result.
If a timeout is specified, the 'yield from' will raise
TimeoutError when the timeout occurs before all Futures are done.
Note: The futures 'f' are not necessarily members of fs.
"""
if isinstance(fs, futures.Future) or coroutines.iscoroutine(fs):
raise TypeError("expect a list of futures, not %s" % type(fs).__name__)
loop = loop if loop is not None else events.get_event_loop()
todo = {ensure_future(f, loop=loop) for f in set(fs)}
from .queues import Queue # Import here to avoid circular import problem.
done = Queue(loop=loop)
timeout_handle = None
def _on_timeout():
for f in todo:
f.remove_done_callback(_on_completion)
done.put_nowait(None) # Queue a dummy value for _wait_for_one().
todo.clear() # Can't do todo.remove(f) in the loop.
def _on_completion(f):
if not todo:
return # _on_timeout() was here first.
todo.remove(f)
done.put_nowait(f)
if not todo and timeout_handle is not None:
timeout_handle.cancel()
@coroutine
def _wait_for_one():
f = yield from done.get()
if f is None:
# Dummy value from _on_timeout().
raise futures.TimeoutError
return f.result() # May raise f.exception().
for f in todo:
f.add_done_callback(_on_completion)
if todo and timeout is not None:
timeout_handle = loop.call_later(timeout, _on_timeout)
for _ in range(len(todo)):
yield _wait_for_one()
@coroutine
def sleep(delay, result=None, *, loop=None):
"""Coroutine that completes after a given time (in seconds)."""
if delay == 0:
yield
return result
future = futures.Future(loop=loop)
h = future._loop.call_later(delay,
futures._set_result_unless_cancelled,
future, result)
try:
return (yield from future)
finally:
h.cancel()
def async(coro_or_future, *, loop=None):
"""Wrap a coroutine in a future.
If the argument is a Future, it is returned directly.
This function is deprecated in 3.5. Use asyncio.ensure_future() instead.
"""
warnings.warn("asyncio.async() function is deprecated, use ensure_future()",
DeprecationWarning)
return ensure_future(coro_or_future, loop=loop)
def ensure_future(coro_or_future, *, loop=None):
"""Wrap a coroutine or an awaitable in a future.
If the argument is a Future, it is returned directly.
"""
if isinstance(coro_or_future, futures.Future):
if loop is not None and loop is not coro_or_future._loop:
raise ValueError('loop argument must agree with Future')
return coro_or_future
elif coroutines.iscoroutine(coro_or_future):
if loop is None:
loop = events.get_event_loop()
task = loop.create_task(coro_or_future)
if task._source_traceback:
del task._source_traceback[-1]
return task
elif compat.PY35 and inspect.isawaitable(coro_or_future):
return ensure_future(_wrap_awaitable(coro_or_future), loop=loop)
else:
raise TypeError('A Future, a coroutine or an awaitable is required')
@coroutine
def _wrap_awaitable(awaitable):
"""Helper for asyncio.ensure_future().
Wraps awaitable (an object with __await__) into a coroutine
that will later be wrapped in a Task by ensure_future().
"""
return (yield from awaitable.__await__())
class _GatheringFuture(futures.Future):
"""Helper for gather().
This overrides cancel() to cancel all the children and act more
like Task.cancel(), which doesn't immediately mark itself as
cancelled.
"""
def __init__(self, children, *, loop=None):
super().__init__(loop=loop)
self._children = children
def cancel(self):
if self.done():
return False
for child in self._children:
child.cancel()
return True
def gather(*coros_or_futures, loop=None, return_exceptions=False):
"""Return a future aggregating results from the given coroutines
or futures.
All futures must share the same event loop. If all the tasks are
done successfully, the returned future's result is the list of
results (in the order of the original sequence, not necessarily
the order of results arrival). If *return_exceptions* is True,
exceptions in the tasks are treated the same as successful
results, and gathered in the result list; otherwise, the first
raised exception will be immediately propagated to the returned
future.
Cancellation: if the outer Future is cancelled, all children (that
have not completed yet) are also cancelled. If any child is
cancelled, this is treated as if it raised CancelledError --
the outer Future is *not* cancelled in this case. (This is to
prevent the cancellation of one child to cause other children to
be cancelled.)
"""
if not coros_or_futures:
outer = futures.Future(loop=loop)
outer.set_result([])
return outer
arg_to_fut = {}
for arg in set(coros_or_futures):
if not isinstance(arg, futures.Future):
fut = ensure_future(arg, loop=loop)
if loop is None:
loop = fut._loop
# The caller cannot control this future, the "destroy pending task"
# warning should not be emitted.
fut._log_destroy_pending = False
else:
fut = arg
if loop is None:
loop = fut._loop
elif fut._loop is not loop:
raise ValueError("futures are tied to different event loops")
arg_to_fut[arg] = fut
children = [arg_to_fut[arg] for arg in coros_or_futures]
nchildren = len(children)
outer = _GatheringFuture(children, loop=loop)
nfinished = 0
results = [None] * nchildren
def _done_callback(i, fut):
nonlocal nfinished
if outer.done():
if not fut.cancelled():
# Mark exception retrieved.
fut.exception()
return
if fut.cancelled():
res = futures.CancelledError()
if not return_exceptions:
outer.set_exception(res)
return
elif fut._exception is not None:
res = fut.exception() # Mark exception retrieved.
if not return_exceptions:
outer.set_exception(res)
return
else:
res = fut._result
results[i] = res
nfinished += 1
if nfinished == nchildren:
outer.set_result(results)
for i, fut in enumerate(children):
fut.add_done_callback(functools.partial(_done_callback, i))
return outer
def shield(arg, *, loop=None):
"""Wait for a future, shielding it from cancellation.
The statement
res = yield from shield(something())
is exactly equivalent to the statement
res = yield from something()
*except* that if the coroutine containing it is cancelled, the
task running in something() is not cancelled. From the POV of
something(), the cancellation did not happen. But its caller is
still cancelled, so the yield-from expression still raises
CancelledError. Note: If something() is cancelled by other means
this will still cancel shield().
If you want to completely ignore cancellation (not recommended)
you can combine shield() with a try/except clause, as follows:
try:
res = yield from shield(something())
except CancelledError:
res = None
"""
inner = ensure_future(arg, loop=loop)
if inner.done():
# Shortcut.
return inner
loop = inner._loop
outer = futures.Future(loop=loop)
def _done_callback(inner):
if outer.cancelled():
if not inner.cancelled():
# Mark inner's result as retrieved.
inner.exception()
return
if inner.cancelled():
outer.cancel()
else:
exc = inner.exception()
if exc is not None:
outer.set_exception(exc)
else:
outer.set_result(inner.result())
inner.add_done_callback(_done_callback)
return outer
def run_coroutine_threadsafe(coro, loop):
"""Submit a coroutine object to a given event loop.
Return a concurrent.futures.Future to access the result.
"""
if not coroutines.iscoroutine(coro):
raise TypeError('A coroutine object is required')
future = concurrent.futures.Future()
def callback():
try:
futures._chain_future(ensure_future(coro, loop=loop), future)
except Exception as exc:
if future.set_running_or_notify_cancel():
future.set_exception(exc)
raise
loop.call_soon_threadsafe(callback)
return future
def timeout(timeout, *, loop=None):
"""A factory which produce a context manager with timeout.
Useful in cases when you want to apply timeout logic around block
of code or in cases when asyncio.wait_for is not suitable.
For example:
>>> with asyncio.timeout(0.001):
... yield from coro()
timeout: timeout value in seconds
loop: asyncio compatible event loop
"""
if loop is None:
loop = events.get_event_loop()
return _Timeout(timeout, loop=loop)
class _Timeout:
def __init__(self, timeout, *, loop):
self._timeout = timeout
self._loop = loop
self._task = None
self._cancelled = False
self._cancel_handler = None
def __enter__(self):
self._task = Task.current_task(loop=self._loop)
if self._task is None:
raise RuntimeError('Timeout context manager should be used '
'inside a task')
self._cancel_handler = self._loop.call_later(
self._timeout, self._cancel_task)
return self
def __exit__(self, exc_type, exc_val, exc_tb):
if exc_type is futures.CancelledError and self._cancelled:
self._cancel_handler = None
self._task = None
raise futures.TimeoutError
self._cancel_handler.cancel()
self._cancel_handler = None
self._task = None
def _cancel_task(self):
self._cancelled = self._task.cancel()
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