code: def get_field(self, field):
          
          idx = self._keys.index(field)
          return self._data[idx]
docs: Return the dataset corresponds to the provided key.
  
          Example::
              a = np.ones((2,2))
              b = np.zeros((2,2))
              np.savez('data.npz', a=a, b=b)
              dataset = NumpyDataset('data.npz')
              data_a = dataset.get_field('a')
              data_b = dataset.get_field('b')
  
          Parameters
          ----------
          field : str
              The name of the field to retrieve.