Logging, StreamHandler and standard streams

by

The following script, log1.py:

import logging, sys

class SingleLevelFilter(logging.Filter):
    def __init__(self, passlevel, reject):
        self.passlevel = passlevel
        self.reject = reject

    def filter(self, record):
        if self.reject:
            return (record.levelno != self.passlevel)
        else:
            return (record.levelno == self.passlevel)

h1 = logging.StreamHandler(sys.stdout)
f1 = SingleLevelFilter(logging.INFO, False)
h1.addFilter(f1)
rootLogger = logging.getLogger()
rootLogger.addHandler(h1)
h2 = logging.StreamHandler(sys.stderr)
f2 = SingleLevelFilter(logging.INFO, True)
h2.addFilter(f2)
rootLogger.addHandler(h2)
logger = logging.getLogger("my.logger")
logger.setLevel(logging.DEBUG)
logger.debug("A DEBUG message")
logger.info("An INFO message")
logger.warning("A WARNING message")
logger.error("An ERROR message")
logger.critical("A CRITICAL message")

when run, produces the following results.

C:\temp>log1.py
A DEBUG message
An INFO message
A WARNING message
An ERROR message
A CRITICAL message

As you’d expect, since on a terminal sys.stdout and sys.stderr are the same. Now, let’s redirect stdout to a file, tmp:

C:\temp>log1.py >tmp
A DEBUG message
A WARNING message
An ERROR message
A CRITICAL message

So the INFO message has not been printed to the terminal – but the messages directed to sys.stderr have been printed. Let’s look at what’s in tmp:

C:\temp>type tmp
An INFO message

So that approach appears to do what you want.

More Related Contents:

  1. String formatting: % vs. .format vs. f-string literal
  2. How should I log while using multiprocessing in Python?
  3. Where is a complete example of logging.config.dictConfig?
  4. How to disable logging on the standard error stream?
  5. Read from a log file as it’s being written using python
  6. What is the point of setLevel in a python logging handler?
  7. TypeError: only length-1 arrays can be converted to Python scalars while trying to exponentially fit data
  8. How to add a custom loglevel to Python’s logging facility
  9. Logging to two files with different settings
  10. using Python logger class to generate multiple logs for different log levels
  11. How to easily print ascii-art text? [closed]
  12. Logging within pytest tests
  13. Python, want logging with log rotation and compression
  14. Get Output From the logging Module in IPython Notebook
  15. logging with filters
  16. real time subprocess.Popen via stdout and PIPE
  17. How can I disable logging while running unit tests in Python Django?
  18. Log exception with traceback in Python
  19. Can Python’s logging format be modified depending on the message log level?
  20. How to configure logging to syslog in Python?
  21. Lazy logger message string evaluation
  22. How do I log from my Python Spark script
  23. Suppress newline in Python logging module
  24. Naming Python loggers
  25. Run subprocess and print output to logging
  26. Does python logging flush every log?
  27. How to log Keras loss output to a file
  28. How to change filehandle with Python logging on the fly with different classes and imports
  29. Can structured logging be done with Pythons standard library?
  30. Python Logging – Disable logging from imported modules

Leave a Comment