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r� Y n X e j ZZ dS )z.A Future class similar to the one in PEP 3148.�CancelledError� TimeoutError�InvalidStateError�Future� wrap_future�isfuture� N� )� base_futures)�compat)�eventsc @ s4 e Zd ZdZdZdd� Zdd � Zd
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Helper to log a traceback upon destruction if not cleared.
This solves a nasty problem with Futures and Tasks that have an
exception set: if nobody asks for the exception, the exception is
never logged. This violates the Zen of Python: 'Errors should
never pass silently. Unless explicitly silenced.'
However, we don't want to log the exception as soon as
set_exception() is called: if the calling code is written
properly, it will get the exception and handle it properly. But
we *do* want to log it if result() or exception() was never called
-- otherwise developers waste a lot of time wondering why their
buggy code fails silently.
An earlier attempt added a __del__() method to the Future class
itself, but this backfired because the presence of __del__()
prevents garbage collection from breaking cycles. A way out of
this catch-22 is to avoid having a __del__() method on the Future
class itself, but instead to have a reference to a helper object
with a __del__() method that logs the traceback, where we ensure
that the helper object doesn't participate in cycles, and only the
Future has a reference to it.
The helper object is added when set_exception() is called. When
the Future is collected, and the helper is present, the helper
object is also collected, and its __del__() method will log the
traceback. When the Future's result() or exception() method is
called (and a helper object is present), it removes the helper
object, after calling its clear() method to prevent it from
logging.
One downside is that we do a fair amount of work to extract the
traceback from the exception, even when it is never logged. It
would seem cheaper to just store the exception object, but that
references the traceback, which references stack frames, which may
reference the Future, which references the _TracebackLogger, and
then the _TracebackLogger would be included in a cycle, which is
what we're trying to avoid! As an optimization, we don't
immediately format the exception; we only do the work when
activate() is called, which call is delayed until after all the
Future's callbacks have run. Since usually a Future has at least
one callback (typically set by 'yield from') and usually that
callback extracts the callback, thereby removing the need to
format the exception.
PS. I don't claim credit for this solution. I first heard of it
in a discussion about closing files when they are collected.
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d� Zdd� Zdd� Zdd� Zdd� Zdd� Zdd� Zdd � Z d d � Z d d � Z ej r�e Z dS )!r a, This class is *almost* compatible with concurrent.futures.Future.
Differences:
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raise an exception when the future isn't done yet.
- Callbacks registered with add_done_callback() are always called
via the event loop's call_soon().
- This class is not compatible with the wait() and as_completed()
methods in the concurrent.futures package.
(In Python 3.4 or later we may be able to unify the implementations.)
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zFuture.__del__c C s& d| _ | jtkrdS t| _| j� dS )z�Cancel the future and schedule callbacks.
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zFuture.exceptionc C s* | j tkr| jj|| � n | jj|� dS )z�Add a callback to be run when the future becomes done.
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zFuture.add_done_callbackc s< � fdd�| j D �}t| j �t|� }|r8|| j dd�< |S )z}Remove all instances of a callback from the "call when done" list.
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zFuture.remove_done_callbackc C s4 | j tkr tdj| j | ���|| _t| _ | j� dS )z�Mark the future done and set its result.
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