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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 sha1sum(filename):
 """ Calculates sha1 hash of a file """

    sha1 = hashlib.sha1()
    with open(filename, 'rb') as f:
        for chunk in iter(lambda: f.read(128 * sha1.block_size), b''):
            sha1.update(chunk)
    return sha1.hexdigest()