bpo-29617: Remove Python 3.3 support from asyncio (GH-232)

Lib/asyncio/futures.py

@@ -27,86 +27,6 @@ _FINISHED = base_futures._FINISHED
 STACK_DEBUG = logging.DEBUG - 1  # heavy-duty debugging
 
 
-class _TracebackLogger:
-    """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.
-    """
-
-    __slots__ = ('loop', 'source_traceback', 'exc', 'tb')
-
-    def __init__(self, future, exc):
-        self.loop = future._loop
-        self.source_traceback = future._source_traceback
-        self.exc = exc
-        self.tb = None
-
-    def activate(self):
-        exc = self.exc
-        if exc is not None:
-            self.exc = None
-            self.tb = traceback.format_exception(exc.__class__, exc,
-                                                 exc.__traceback__)
-
-    def clear(self):
-        self.exc = None
-        self.tb = None
-
-    def __del__(self):
-        if self.tb:
-            msg = 'Future/Task exception was never retrieved\n'
-            if self.source_traceback:
-                src = ''.join(traceback.format_list(self.source_traceback))
-                msg += 'Future/Task created at (most recent call last):\n'
-                msg += '%s\n' % src.rstrip()
-            msg += ''.join(self.tb).rstrip()
-            self.loop.call_exception_handler({'message': msg})
-
-
 class Future:
     """This class is *almost* compatible with concurrent.futures.Future.
 
@@ -164,25 +84,21 @@ class Future:
     def __repr__(self):
         return '<%s %s>' % (self.__class__.__name__, ' '.join(self._repr_info()))
 
-    # On Python 3.3 and older, objects with a destructor part of a reference
-    # cycle are never destroyed. It's not more the case on Python 3.4 thanks
-    # to the PEP 442.
-    if compat.PY34:
-        def __del__(self):
-            if not self._log_traceback:
-                # set_exception() was not called, or result() or exception()
-                # has consumed the exception
-                return
-            exc = self._exception
-            context = {
-                'message': ('%s exception was never retrieved'
-                            % self.__class__.__name__),
-                'exception': exc,
-                'future': self,
-            }
-            if self._source_traceback:
-                context['source_traceback'] = self._source_traceback
-            self._loop.call_exception_handler(context)
+    def __del__(self):
+        if not self._log_traceback:
+            # set_exception() was not called, or result() or exception()
+            # has consumed the exception
+            return
+        exc = self._exception
+        context = {
+            'message': ('%s exception was never retrieved'
+                        % self.__class__.__name__),
+            'exception': exc,
+            'future': self,
+        }
+        if self._source_traceback:
+            context['source_traceback'] = self._source_traceback
+        self._loop.call_exception_handler(context)
 
     def cancel(self):
         """Cancel the future and schedule callbacks.
@@ -317,13 +233,7 @@ class Future:
         self._exception = exception
         self._state = _FINISHED
         self._schedule_callbacks()
-        if compat.PY34:
-            self._log_traceback = True
-        else:
-            self._tb_logger = _TracebackLogger(self, exception)
-            # Arrange for the logger to be activated after all callbacks
-            # have had a chance to call result() or exception().
-            self._loop.call_soon(self._tb_logger.activate)
+        self._log_traceback = True
 
     def __iter__(self):
         if not self.done():