import logging
logging.basicConfig(level=logging.DEBUG)


class Foo(object):
    def __init__(self):
        self.bar = "hello"

    def __getitem__(self, key):
        return getattr(self, key)


f = Foo()
# Outputs "DEBUG:root:hello"
logging.debug("%(bar)s" % f)
# Raises "TypeErorr: format requires a mapping"
logging.debug("%(bar)s", f)