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[doc] Improve logging cookbook example. (GH-98481) #98481

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Oct 20, 2022
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[doc] Improve logging cookbook example. (GH-98481) #98481

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[doc] Improve logging cookbook example.
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@vsajip
vsajip committed Oct 20, 2022
commit ca77b592fb14696ab266be7df603e05acbc19783
65 changes: 65 additions & 0 deletions Doc/howto/logging-cookbook.rst
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Expand Up @@ -3712,6 +3712,71 @@ Of course, the examples above show output according to the format used by
:func:`~logging.basicConfig`, but you can use a different formatter when you
configure logging.

Note that with the above scheme, you are somewhat at the mercy of buffering and
the sequence of write calls which you are intercepting. For example, with the
definition of ``LoggerWriter`` above, if you have the snippet

.. code-block:: python

sys.stderr = LoggerWriter(logger, logging.WARNING)
1 / 0

then running the script results in

.. code-block:: text

WARNING:demo:Traceback (most recent call last):

WARNING:demo: File "/home/runner/cookbook-loggerwriter/test.py", line 53, in <module>

WARNING:demo:
WARNING:demo:main()
WARNING:demo: File "/home/runner/cookbook-loggerwriter/test.py", line 49, in main

WARNING:demo:
WARNING:demo:1 / 0
WARNING:demo:ZeroDivisionError
WARNING:demo::
WARNING:demo:division by zero

As you can see, this output isn't ideal. That's because the underlying code
which writes to ``sys.stderr`` makes mutiple writes, each of which results in a
separate logged line (for example, the last three lines above). To get around
this problem, you need to buffer things and only output log lines when newlines
are seen. Let's use a slghtly better implementation of ``LoggerWriter``:

.. code-block:: python

class BufferingLoggerWriter(LoggerWriter):
def __init__(self, logger, level):
super().__init__(logger, level)
self.buffer = ''

def write(self, message):
if '\n' not in message:
self.buffer += message
else:
parts = message.split('\n')
if self.buffer:
s = self.buffer + parts.pop(0)
self.logger.log(self.level, s)
self.buffer = parts.pop()
for part in parts:
self.logger.log(self.level, part)

This just buffers up stuff until a newline is seen, and then logs complete
lines. With this approach, you get better output:

.. code-block:: text

WARNING:demo:Traceback (most recent call last):
WARNING:demo: File "/home/runner/cookbook-loggerwriter/main.py", line 55, in <module>
WARNING:demo: main()
WARNING:demo: File "/home/runner/cookbook-loggerwriter/main.py", line 52, in main
WARNING:demo: 1/0
WARNING:demo:ZeroDivisionError: division by zero


.. patterns-to-avoid:

Patterns to avoid
Expand Down