CLN: Add comment and clarify if condition in indexing (#37960)

* Implement review comments

* Change test names

Author: phofl

pandas/core/indexing.py

@@ -1821,7 +1821,7 @@ def _setitem_single_block(self, indexer, value, name: str):
                 return
 
             indexer = maybe_convert_ix(*indexer)
-        if isinstance(value, ABCSeries) and name != "iloc" or isinstance(value, dict):
+        if (isinstance(value, ABCSeries) and name != "iloc") or isinstance(value, dict):
             # TODO(EA): ExtensionBlock.setitem this causes issues with
             # setting for extensionarrays that store dicts. Need to decide
             # if it's worth supporting that.
@@ -1859,6 +1859,7 @@ def _setitem_with_indexer_missing(self, indexer, value):
             if index.is_unique:
                 new_indexer = index.get_indexer([new_index[-1]])
                 if (new_indexer != -1).any():
+                    # We get only here with loc, so can hard code
                     return self._setitem_with_indexer(new_indexer, value, "loc")
 
             # this preserves dtype of the value

pandas/tests/indexing/test_iloc.py

@@ -818,15 +818,15 @@ def test_iloc_setitem_bool_indexer(self, klass):
         tm.assert_frame_equal(df, expected)
 
     @pytest.mark.parametrize("indexer", [[1], slice(1, 2)])
-    def test_setitem_iloc_pure_position_based(self, indexer):
+    def test_iloc_setitem_pure_position_based(self, indexer):
         # GH#22046
         df1 = DataFrame({"a2": [11, 12, 13], "b2": [14, 15, 16]})
         df2 = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [7, 8, 9]})
         df2.iloc[:, indexer] = df1.iloc[:, [0]]
         expected = DataFrame({"a": [1, 2, 3], "b": [11, 12, 13], "c": [7, 8, 9]})
         tm.assert_frame_equal(df2, expected)
 
-    def test_setitem_iloc_dictionary_value(self):
+    def test_iloc_setitem_dictionary_value(self):
         # GH#37728
         df = DataFrame({"x": [1, 2], "y": [2, 2]})
         rhs = dict(x=9, y=99)
@@ -1000,7 +1000,7 @@ def test_iloc_getitem_nonunique(self):
         ser = Series([0, 1, 2], index=[0, 1, 0])
         assert ser.iloc[2] == 2
 
-    def test_setitem_iloc_pure_position_based(self):
+    def test_iloc_setitem_pure_position_based(self):
         # GH#22046
         ser1 = Series([1, 2, 3])
         ser2 = Series([4, 5, 6], index=[1, 0, 2])