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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 sents(self, filename=None):
 """ Returns the file, line by line. Use test_file if no filename specified. """

        filename = filename if filename else self.test_file
        with io.open(filename, 'r') as fin:
            for line in fin:
                yield line.strip()