code: def _create_alpha(self, data, fill_value=None):
          
          not_alpha = [b for b in data.coords['bands'].values if b != 'A']
          null_mask = data.sel(bands=not_alpha)
          if np.issubdtype(data.dtype, np.integer) and fill_value is not None:
              null_mask = null_mask != fill_value
          else:
              null_mask = null_mask.notnull()
          # if any of the bands are valid, we don't want transparency
          null_mask = null_mask.any(dim='bands')
          null_mask = null_mask.expand_dims('bands')
          null_mask['bands'] = ['A']
          # match data dtype
          return null_mask
docs: Create an alpha band DataArray object.
  
          If `fill_value` is provided and input data is an integer type
          then it is used to determine invalid "null" pixels instead of
          xarray's `isnull` and `notnull` methods.
  
          The returned array is 1 where data is valid, 0 where invalid.