Python: Logging TypeError: not all arguments converted during string formatting

Question

Here is a quick reproduction of the issue:

>>> import logging
>>> logging.getLogger().setLevel(logging.INFO)
>>> from datetime import date
>>> date = date.today()
>>> logging.info('date={}', date)
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/logging/__init__.py", line 846, in emit
    msg = self.format(record)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/logging/__init__.py", line 723, in format
    return fmt.format(record)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/logging/__init__.py", line 464, in format
    record.message = record.getMessage()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/logging/__init__.py", line 328, in getMessage
    msg = msg % self.args
TypeError: not all arguments converted during string formatting
Logged from file <stdin>, line 1
>>> 

How do I make it work?

---

_{This is effectively a special case of Why do I get "TypeError: not all arguments converted during string formatting" trying to substitute a placeholder like {0} using %? - but because the actual formatting step happens outside the user's code, different workarounds are necessary.}

Answer 1

You could do the formatting yourself:

logging.info('date={}'.format(date))

As was pointed out by Martijn Pieters, this will always run the string formatting, while using the logging module would cause the formatting to only be performed if the message is actually logged.