DM-15361: Enable logging to Python streams.

[ktlim]

This allows log output to go to Jupyter notebook cells.

[andy-slac]

This is not just for logging only - it takes anything written to a file descriptor and sends it somewhere? Can't we implement special appender for log4cxx to write directly to Python object?

[ktlim]

In a notebook environment, nothing else should be writing to the file descriptor. I thought this would be a quick, Python-only way to start getting the desired functionality. Writing a log4cxx appender seemed more difficult.

[andy-slac]

Implementing log4cxx appender should not be hard, the only non-trivial question is how to configure log4cxx, but that can be done through some special utility method. Anyways, I'm OK with this initial implementation. Maybe if we can provide a more abstract API for enabling redirection then we can re-implement it later in a different way without need to change user code, e.g:

def enable_notebook_logging():
    """Enable notebook output for log4cxx messages."""
    # current implementation
    global redirect
    redirect = LogRedirect()

def disable_notebook_logging():
    """Stop notebook output for log4cxx messages."""
    # current implementation
    global redirect
    redirect.finish()

[andy-slac]

I don't like singletons but this should not be allowed to be instantiated more than once.