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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 _create_alpha(self, data, fill_value=None):
 """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. """

        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