third_party/2/concurrent/futures is for the Python 2 backport of the
concurrent.futures package from Python 3. In Python 3, all exceptions
are derived from BaseException, but Python 2 also supports exceptions
that are old-style objects (which don't derive from BaseException).
Switch from BaseException to Any to allow old-style exceptions.
All old-style objects are instances of types.InstanceType, so an
alternative to Any is Union[BaseException, types.InstanceType]. This
would help avoid accidentally passing a non-BaseException new-style
object to Future.set_exception(). However, only Executors call that
function (usually), and it's not clear that mypy understands old-style
objects and their relationship to types.InstanceType, so Any is
thought to be the more practical choice.
The concurrent.futures.Future class's set_exception() method might be
called with a BaseException that is not an Exception, so change
set_exception()'s parameter type from Exception to BaseException. The
exception set via set_exception() is returned by exception(), so
change exception()'s return type from Exception to BaseException.
Richard Hansen