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The BufferedReader class is documented as being thread safe: <https://docs.python.org/dev/library/io.html#multi-threading>. Some experimentation suggests that checking the "raw.closed" property is not actually serialized, but that raw.readinto() calls are serialized. I don’t think this is a big problem in practice, so I think BZ2File would remain as thread-safe as BufferedReader is.
But Serhiy’s point is definitely valid about the classes breaking the iterator protocol. FWIW I originally made this patch to satisfy my personal curiosity about why wrapping a second BufferedReader made things so much faster. Now I accept it is partly due to the overhead of the extra LZMAFile.readline() to BufferedReader.readline() delegation. Maybe it is not even worth optimizing around this kind of overhead, so I would even be happy to drop this patch and close the issue.