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Editorial Team
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Asked: 2026-05-24T06:01:50+00:00

I am using the logger module of python (python version 3.x, but it should

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I am using the logger module of python (python version 3.x, but it should not matter) and I noticed that an error in a format string is reported as such:

Traceback (most recent call last):
  File "/usr/lib/python3.1/logging/__init__.py", line 770, in emit
    msg = self.format(record)
  File "/usr/lib/python3.1/logging/__init__.py", line 650, in format
    return fmt.format(record)
  File "/usr/lib/python3.1/logging/__init__.py", line 438, in format
    record.message = record.getMessage()
  File "/usr/lib/python3.1/logging/__init__.py", line 308, in getMessage
    msg = msg % self.args
TypeError: %d format: a number is required, not str

As you can see, there is no mention of where the actual error (in my code) was. By the way, here’s what was wrong in my code:

logging.debug('This is a string %d', str(foo))

Changing the %d in %s solved the problem.

My question is: how do I get some slightly more useful information from the logging module output? Do I have to write my own logger? Where do I tweak the logger module?

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1 Answer

  1. Editorial Team

    Was just about to post this when Unutbu beat me to it. But here it is anyway:

    You could try subclassing whichever handler you’re using (example below using StreamHandler) and overiding the format method with a function that wraps it in a try: block

    import traceback, logging

class MyStreamHandler(logging.StreamHandler):
    def format(self, record):
        try:
            return logging.StreamHandler.format(self, record)
        except TypeError:
            # Print a stack trace that includes the original log call
            traceback.print_stack() 


if __name__ == "__main__":
    log = logging.getLogger("testlogger")
    handler = MyStreamHandler()

    log.addHandler(handler)

    log.error("Try interpolating an int correctly: %i", 1)

    log.error("Now try passing a string to an int: %d", 'abc')

    log.error("And then a string to a string %s", 'abc')

    gives me:

    Try interpolating an int correctly: 1
  File "logtest2.py", line 19, in ?
    log.error("Now try passing a string to an int: %d", 'abc')
  File "/usr/lib64/python2.4/logging/__init__.py", line 999, in error
    apply(self._log, (ERROR, msg, args), kwargs)
  File "/usr/lib64/python2.4/logging/__init__.py", line 1079, in _log
    self.handle(record)
  File "/usr/lib64/python2.4/logging/__init__.py", line 1089, in handle
    self.callHandlers(record)
  File "/usr/lib64/python2.4/logging/__init__.py", line 1126, in callHandlers
    hdlr.handle(record)
  File "/usr/lib64/python2.4/logging/__init__.py", line 642, in handle
    self.emit(record)
  File "/usr/lib64/python2.4/logging/__init__.py", line 731, in emit
    msg = self.format(record)
  File "logtest2.py", line 8, in format
    traceback.print_stack()
None
And then a string to a string abc

    I wouldn’t leave this in any production code, but it should help you find things like 

    log.error("%d", 'a string')
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