Block dimensions for this dataset's data or None if it's not a dask
        array.

def chunks(self):
        """Block dimensions for this dataset's data or None if it's not a dask
        array.
        """
        chunks = {}
        for v in self.variables.values():
            if v.chunks is not None:
                for dim, c in zip(v.dims, v.chunks):
                    if dim in chunks and c != chunks[dim]:
                        raise ValueError('inconsistent chunks')
                    chunks[dim] = c
        return Frozen(SortedKeysDict(chunks))