Backport PR #40592: REGR: where not copying on no-op

doc/source/whatsnew/v1.2.4.rst

@@ -17,6 +17,7 @@ Fixed regressions
 
 - Fixed regression in :meth:`DataFrame.sum` when ``min_count`` greater than the :class:`DataFrame` shape was passed resulted in a ``ValueError`` (:issue:`39738`)
 - Fixed regression in :meth:`DataFrame.to_json` raising ``AttributeError`` when run on PyPy (:issue:`39837`)
+- Fixed regression in :meth:`DataFrame.where` not returning a copy in the case of an all True condition (:issue:`39595`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/internals/blocks.py

@@ -1458,7 +1458,7 @@ def where(
             raise ValueError("where must have a condition that is ndarray like")
 
         if cond.ravel("K").all():
-            result = values
+            result = values.copy()
         else:
             # see if we can operate on the entire block, or need item-by-item
             # or if we are a single block (ndim == 1)

pandas/tests/frame/indexing/test_where.py

@@ -653,3 +653,20 @@ def test_where_categorical_filtering(self):
         expected.loc[0, :] = np.nan
 
         tm.assert_equal(result, expected)
+
+
+def test_where_copies_with_noop(frame_or_series):
+    # GH-39595
+    result = frame_or_series([1, 2, 3, 4])
+    expected = result.copy()
+    col = result[0] if frame_or_series is DataFrame else result
+
+    where_res = result.where(col < 5)
+    where_res *= 2
+
+    tm.assert_equal(result, expected)
+
+    where_res = result.where(col > 5, [1, 2, 3, 4])
+    where_res *= 2
+
+    tm.assert_equal(result, expected)