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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 SHA1_file(filepath, extra=b''):
 """ Returns hex digest of SHA1 hash of file at filepath :param str filepath: File to hash :param bytes extra: Extra content added to raw read of file before taking hash :return: hex digest of hash :rtype: str """

    h = hashlib.sha1()
    with io.open(filepath, 'rb') as f:
        for chunk in iter(lambda: f.read(h.block_size), b''):
            h.update(chunk)
    h.update(extra)
    return h.hexdigest()