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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 size(self):
 """The number of instances in the data. If the underlying data source changes, it may be outdated. """

    import tensorflow as tf

    if self._size is None:
      self._size = 0
      options = tf.python_io.TFRecordOptions(tf.python_io.TFRecordCompressionType.GZIP)
      for tfexample_file in self.files:
        self._size += sum(1 for x
                          in tf.python_io.tf_record_iterator(tfexample_file, options=options))

    return self._size