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In the logging module's config.py, see the _create_formatters(cp) method used by the fileConfig() method. Note that it pulls "format" and "datefmt" and submits these in the formatter constructor:
f = c(fs, dfs)
However, the Formatter constructor has a third argument for formatting style:
def __init__(self, fmt=None, datefmt=None, style='%')
Since the argument is not passed, ConfigParser-format logging configs must use %-style logging format masks. We'd prefer to use curlies.
Note that the code for the dictionary configurator does this correctly:
 fmt = config.get('format', None)
 dfmt = config.get('datefmt', None)
 style = config.get('style', '%')
 result = logging.Formatter(fmt, dfmt, style)