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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 checksum(path):
 """Calculcate checksum for a file."""

    hasher = hashlib.sha1()
    with open(path, 'rb') as stream:
        buf = stream.read(BLOCKSIZE)
        while len(buf) > 0:
            hasher.update(buf)
            buf = stream.read(BLOCKSIZE)
    return hasher.hexdigest()