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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _is_valid_extension(extension):
 """Checks if the file extension is blacklisted in valid_extensions. :param str extension: a file extension to check :returns: flag indicating if the extension is valid based on list of valid extensions. :rtype: bool """

    if not cfg.CONF.valid_extensions:
        return True

    valid = False
    for cext in cfg.CONF.valid_extensions:
        if not cext.startswith('.'):
            cext = '.%s' % cext
        if extension == cext:
            valid = True
            break

    return valid