def combine_first(self, other):
new_index = self.index.union(other.index)<EOL>this = self.reindex(new_index, copy=False)<EOL>other = other.reindex(new_index, copy=False)<EOL>if is_datetimelike(this) and not is_datetimelike(other):<EOL><INDENT>other = to_datetime(other)<EOL><DEDENT>return this.where(notna(this), other)<EOL>
Combine Series values, choosing the calling Series's values first.

Parameters
----------
other : Series
    The value(s) to be combined with the `Series`.

Returns
-------
Series
    The result of combining the Series with the other object.

See Also
--------
Series.combine : Perform elementwise operation on two Series
    using a given function.

Notes
-----
Result index will be the union of the two indexes.

Examples
--------
>>> s1 = pd.Series([1, np.nan])
>>> s2 = pd.Series([3, 4])
>>> s1.combine_first(s2)
0    1.0
1    4.0
dtype: float64