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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 _hash_the_file(hasher, filename):
 """Helper function for creating hash functions. See implementation of :func:`dtoolcore.filehasher.shasum` for more usage details. """

    BUF_SIZE = 65536
    with open(filename, 'rb') as f:
        buf = f.read(BUF_SIZE)
        while len(buf) > 0:
            hasher.update(buf)
            buf = f.read(BUF_SIZE)
    return hasher