fix for infering dtype in failing s.combine on extention array

Author: vladserkoff

pandas/core/series.py

@@ -2634,21 +2634,21 @@ def combine(self, other, func, fill_value=None):
                 rv = other.get(idx, fill_value)
                 with np.errstate(all='ignore'):
                     new_values.append(func(lv, rv))
-            new_dtype = type(func(lv, rv))
         else:
             # Assume that other is a scalar, so apply the function for
             # each element in the Series
             new_index = self.index
             with np.errstate(all='ignore'):
                 new_values = [func(lv, other) for lv in self._values]
             new_name = self.name
-            new_dtype = type(func(self._values[0], other))
 
         if is_categorical_dtype(self.values):
             pass
         elif is_extension_array_dtype(self.values):
             # The function can return something of any type, so check
             # if the type is compatible with the calling EA.
+            non_na_values = [v for v in new_values if notna(v)]
+            new_dtype, _ = infer_dtype_from(non_na_values, pandas_dtype=True)
             try:
                 new_values = self._values._from_sequence(new_values, dtype=new_dtype)
             except Exception:

pandas/tests/extension/base/methods.py

@@ -164,14 +164,14 @@ def test_combine_add(self, data_repeated):
                 orig_data1._from_sequence([a + b for (a, b) in
                                            zip(list(orig_data1),
                                                list(orig_data2))],
-                                               dtype=s1.dtype))
+                                          dtype=s1.dtype))
         self.assert_series_equal(result, expected)
 
         val = s1.iloc[0]
         result = s1.combine(val, lambda x1, x2: x1 + x2)
         expected = pd.Series(
             orig_data1._from_sequence([a + val for a in list(orig_data1)],
-            dtype=s1.dtype))
+                                      dtype=s1.dtype))
         self.assert_series_equal(result, expected)
 
     def test_combine_first(self, data):