It seems like the closing attribute is not *required* to fix bpo-30931, asyncore race condition:
http://bugs.python.org/issue30931#msg299162

Another approach doesn't rely on close() "implementation", but more on close() "expected behaviour" (remove the dispatcher file descriptor from map). IMHO this approach would reduce the risk of backward incompatibility.

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I'm still in favor of rejecting this feature request, *but* I would prefer to first wait until we agree how to fix bpo-30931 race condition.