code: def default_dtype(field=None):
          
          if field is None or field == RealNumbers():
              return np.dtype('float64')
          elif field == ComplexNumbers():
              return np.dtype('complex128')
          else:
              raise ValueError('no default data type defined for field {}'
                               ''.format(field))
docs: Return the default data type of this class for a given field.
  
          Parameters
          ----------
          field : `Field`, optional
              Set of numbers to be represented by a data type.
              Currently supported : `RealNumbers`, `ComplexNumbers`
              The default ``None`` means `RealNumbers`
  
          Returns
          -------
          dtype : `numpy.dtype`
              Numpy data type specifier. The returned defaults are:
  
                  ``RealNumbers()`` : ``np.dtype('float64')``
  
                  ``ComplexNumbers()`` : ``np.dtype('complex128')``