diff --git a/Doc/howto/logging-cookbook.rst b/Doc/howto/logging-cookbook.rst index eac34aaab3a..ae101e3cdf2 100644 --- a/Doc/howto/logging-cookbook.rst +++ b/Doc/howto/logging-cookbook.rst @@ -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 + + 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 + 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