Preserving logging streams through a daemon.DaemonContext switch (was: daemon.DaemonContext)

Ben Finney ben+python at
Fri Apr 9 01:46:23 CEST 2010

Vinay Sajip <vinay_sajip at> writes:

> On Apr 8, 1:58 pm, Rebelo <puntabl... at> wrote:
> > Vinay Sajip wrote:
> > > My guess is - files_preserve needs to be passed a file handle and
> > > not alogginghandler.

This is correct. As the ‘DaemonContext.__init__’ docstring says::

 |  `files_preserve`
 |      :Default: ``None``
 |      List of files that should *not* be closed when starting the
 |      daemon. If ``None``, all open file descriptors will be closed.
 |      Elements of the list are file descriptors (as returned by a file
 |      object's `fileno()` method) or Python `file` objects. Each
 |      specifies a file that is not to be closed during daemon start.

> > do you know where can i find good documnetation for python-daemon?

The docstrings and PEP 3143 are currently the best documentation for
‘python-daemon’; both describe the ‘files_preserve’ option as above.


As expressed in the referenced message, I would welcome someone writing
better documentation and contributing patches.

> It may lead you to more information. The thread shows that Sean
> DiZazzo got logging working with the package.

It's important to note (as you did, thanks Vinay) that none of the
‘logging.handlers’ are file descriptors. Nor are they file objects.

If we can get a Python ‘file’ object, we can use its ‘fileno()’ method
to get the file descriptor.

So how do we get the file object for a logging handler? The ‘logging’
module documentation doesn't mention any way to get at the stream object
for the handlers.

But looking at the source code for ‘StreamHandler’ on my system,
‘/usr/lib/python2.5/logging/’, shows a ‘stream’ attribute
that is bound to the stream object. It's not marked private (i.e. it's
not named with a leading underscore), so one presumes it is part of the
public API.

> I think you just have to pass the file object used by the handler
> ( in the files_preserve array.

Not quite. As the docs specify, you need to pass the *file descriptors*
for the files you want preserved. For regular Python file objects, the
‘file.fileno()’ method returns the file descriptor:

    lh = logging.handlers.TimedRotatingFileHandler(
        # …
    log_stream_descriptor =

    daemon_context.files_preserve = [log_stream_descriptor]

 \              “Only the educated are free.” —Epictetus, _Discourses_ |
  `\                                                                   |
_o__)                                                                  |
Ben Finney

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