def combine_first(self, other):
if isinstance(other, SparseSeries):<EOL><INDENT>other = other.to_dense()<EOL><DEDENT>dense_combined = self.to_dense().combine_first(other)<EOL>return dense_combined.to_sparse(fill_value=self.fill_value)<EOL>
Combine Series values, choosing the calling Series's values
first. Result index will be the union of the two indexes

Parameters
----------
other : Series

Returns
-------
y : Series