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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 _encode_filename_to_filesystem(f):
    '''Get a unicode filename and return bytestring, encoded to file system default.

    If the argument already is a bytestring, return as is'''
    log.debug('_encode_filename_to_filesystem(%s)', repr(f))
    if isinstance(f, str):
        return f
    try:
        return f.encode(sys.getfilesystemencoding())
    except UnicodeEncodeError:
        raise