why can't filter 'DeprecationWarning' that is generated by warnings

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import logging
import warnings

class stringFilter(logging.Filter):
    def filter(self, record):
        if record.msg.find('DeprecationWarning') == -1:
            return True
        return False

logging.basicConfig(level=logging.INFO)
logging.captureWarnings(True)
warnings_logger = logging.getLogger("py.warnings")
warnings_logger.setLevel(logging.INFO)
warnings_logger.addFilter(stringFilter())

warnings.warn('DeprecationWarning')