ConcurrentLogHandler is for multiple *processes* writing to the same
file, not multiple threads in a single process. Python logging does not
support multiple processes writing to the same file because there is no
portable IPC locking across all platforms supported by Python.
ConcurrentLogHandler uses portalocker to achieve interprocess
synchronization, and there is no equivalent mechanism which is part of
the Python stdlib. AFAIK portalocker works on Windows and Linux - I'm
not sure about other platforms.

Python logging *does* support multiple threads in a single process
writing to the same file, which is why I asked Robert if it was
definitely a single-process environment he was working in.