[docs] Update logging cookbook with recipe for using a logger like an output… (GH...

diff --git a/Doc/howto/logging-cookbook.rst b/Doc/howto/logging-cookbook.rst
index 5b079744df12ed4c448be62a3cb73f46a2d121ef..ff7ba0789608ff7c01385b9c518e1e20a25c3d48 100644 (file)
--- a/Doc/howto/logging-cookbook.rst
+++ b/Doc/howto/logging-cookbook.rst
@@ -3428,6 +3428,82 @@ the above handler, you'd pass structured data using something like this::
     i = 1
     logger.debug('Message %d', i, extra=extra)
 
+How to treat a logger like an output stream
+-------------------------------------------
+
+Sometimes, you need to interface to a third-party API which expects a file-like
+object to write to, but you want to direct the API's output to a logger. You
+can do this using a class which wraps a logger with a file-like API.
+Here's a short script illustrating such a class:
+
+.. code-block:: python
+
+    import logging
+
+    class LoggerWriter:
+        def __init__(self, logger, level):
+            self.logger = logger
+            self.level = level
+
+        def write(self, message):
+            if message != '\n':  # avoid printing bare newlines, if you like
+                self.logger.log(self.level, message)
+
+        def flush(self):
+            # doesn't actually do anything, but might be expected of a file-like
+            # object - so optional depending on your situation
+            pass
+
+        def close(self):
+            # doesn't actually do anything, but might be expected of a file-like
+            # object - so optional depending on your situation. You might want
+            # to set a flag so that later calls to write raise an exception
+            pass
+
+    def main():
+        logging.basicConfig(level=logging.DEBUG)
+        logger = logging.getLogger('demo')
+        info_fp = LoggerWriter(logger, logging.INFO)
+        debug_fp = LoggerWriter(logger, logging.DEBUG)
+        print('An INFO message', file=info_fp)
+        print('A DEBUG message', file=debug_fp)
+
+    if __name__ == "__main__":
+        main()
+
+When this script is run, it prints
+
+.. code-block:: text
+
+    INFO:demo:An INFO message
+    DEBUG:demo:A DEBUG message
+
+You could also use ``LoggerWriter`` to redirect ``sys.stdout`` and
+``sys.stderr`` by doing something like this:
+
+.. code-block:: python
+
+    import sys
+
+    sys.stdout = LoggerWriter(logger, logging.INFO)
+    sys.stderr = LoggerWriter(logger, logging.WARNING)
+
+You should do this *after* configuring logging for your needs. In the above
+example, the :func:`~logging.basicConfig` call does this (using the
+``sys.stderr`` value *before* it is overwritten by a ``LoggerWriter``
+instance). Then, you'd get this kind of result:
+
+.. code-block:: pycon
+
+    >>> print('Foo')
+    INFO:demo:Foo
+    >>> print('Bar', file=sys.stderr)
+    WARNING:demo:Bar
+    >>>
+
+Of course, these above examples show output according to the format used by
+:func:`~logging.basicConfig`, but you can use a different formatter when you
+configure logging.
 
 .. patterns-to-avoid: