clarify comments in test_fillna_readonly

Author: jorisvandenbossche

pandas/tests/extension/base/missing.py

@@ -126,7 +126,7 @@ def test_fillna_readonly(self, data_missing):
         data = data_missing.copy()
         data._readonly = True
 
-        # by default copy=True, then this works fine
+        # by default fillna(copy=True), then this works fine
         result = data.fillna(data_missing[1])
         assert result[0] == data_missing[1]
         tm.assert_extension_array_equal(data, data_missing)

pandas/tests/extension/test_arrow.py

@@ -691,13 +691,13 @@ def test_fillna_readonly(self, data_missing):
         data = data_missing.copy()
         data._readonly = True
 
-        # by default copy=True, then this works fine
+        # by default fillna(copy=True), then this works fine
         result = data.fillna(data_missing[1])
         assert result[0] == data_missing[1]
         tm.assert_extension_array_equal(data, data_missing)
 
-        # copy=False is generally not honored by ArrowExtensionArray, always
-        # returns new data -> same result as above
+        # fillna(copy=False) is generally not honored by Arrow-backed array,
+        # but always returns new data -> same result as above
         result = data.fillna(data_missing[1])
         assert result[0] == data_missing[1]
         tm.assert_extension_array_equal(data, data_missing)

pandas/tests/extension/test_sparse.py

@@ -238,17 +238,19 @@ def test_fillna_no_op_returns_copy(self, data, request):
         super().test_fillna_no_op_returns_copy(data)
 
     def test_fillna_readonly(self, data_missing):
-        # copy=False keyword is not ignored by SparseArray.fillna
+        # copy keyword is ignored by SparseArray.fillna
+        # -> copy=True vs False doesn't make a difference
         data = data_missing.copy()
         data._readonly = True
 
-        # by default copy=True, then this works fine
         result = data.fillna(data_missing[1])
         assert result[0] == data_missing[1]
+        tm.assert_extension_array_equal(data, data_missing)
 
-        # copy=False is ignored -> so same result as above
+        # fillna(copy=False) is ignored -> so same result as above
         result = data.fillna(data_missing[1], copy=False)
         assert result[0] == data_missing[1]
+        tm.assert_extension_array_equal(data, data_missing)
 
     @pytest.mark.xfail(reason="Unsupported")
     def test_fillna_series(self, data_missing):

pandas/tests/extension/test_string.py

@@ -173,13 +173,13 @@ def test_fillna_readonly(self, data_missing):
         data = data_missing.copy()
         data._readonly = True
 
-        # by default copy=True, then this works fine
+        # by default fillna(copy=True), then this works fine
         result = data.fillna(data_missing[1])
         assert result[0] == data_missing[1]
         tm.assert_extension_array_equal(data, data_missing)
 
-        # copy=False is generally not honored by Arrow-backed array, always
-        # returns new data -> same result as above
+        # fillna(copy=False) is generally not honored by Arrow-backed array,
+        # but always returns new data -> same result as above
         if data.dtype.storage == "pyarrow":
             result = data.fillna(data_missing[1])
             assert result[0] == data_missing[1]