PERF: ArrowExtensionArray.fillna when array does not contains any nulls

doc/source/whatsnew/v2.1.0.rst

@@ -102,6 +102,7 @@ Performance improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 - Performance improvement in :meth:`DataFrame.first_valid_index` and :meth:`DataFrame.last_valid_index` for extension array dtypes (:issue:`51549`)
 - Performance improvement in :meth:`DataFrame.clip` and :meth:`Series.clip` (:issue:`51472`)
+- Performance improvement in :meth:`~arrays.ArrowExtensionArray.fillna` when array does not contain nulls (:issue:`51635`)
 - Performance improvement in :func:`read_parquet` on string columns when using ``use_nullable_dtypes=True`` (:issue:`47345`)
 -
 

pandas/core/arrays/arrow/array.py

@@ -631,6 +631,9 @@ def fillna(
     ) -> ArrowExtensionArrayT:
         value, method = validate_fillna_kwargs(value, method)
 
+        if not self._hasna:
+            return self.copy()
+
         if limit is not None:
             return super().fillna(value=value, method=method, limit=limit)
 

pandas/tests/extension/test_arrow.py

@@ -686,8 +686,8 @@ def test_fillna_no_op_returns_copy(self, data):
         result = data.fillna(valid)
         assert result is not data
         self.assert_extension_array_equal(result, data)
-        with tm.assert_produces_warning(PerformanceWarning):
-            result = data.fillna(method="backfill")
+
+        result = data.fillna(method="backfill")
         assert result is not data
         self.assert_extension_array_equal(result, data)
 

pandas/tests/extension/test_string.py

@@ -165,10 +165,7 @@ def test_fillna_no_op_returns_copy(self, data):
         assert result is not data
         self.assert_extension_array_equal(result, data)
 
-        with tm.maybe_produces_warning(
-            PerformanceWarning, data.dtype.storage == "pyarrow"
-        ):
-            result = data.fillna(method="backfill")
+        result = data.fillna(method="backfill")
         assert result is not data
         self.assert_extension_array_equal(result, data)