Does a single update operation.

def update(database, table, key):
    """Does a single update operation."""
    field = random.randrange(10)
    value = ''.join(random.choice(string.printable) for i in range(100))
    with database.batch() as batch:
        batch.update(table=table, columns=('id', 'field%d' % field),
                     values=[(key, value)])