BUG: np array indexer modifed in iloc

doc/source/whatsnew/v0.24.0.txt

@@ -363,8 +363,7 @@ Indexing
 - ``DataFrame.__getitem__`` now accepts dictionaries and dictionary keys as list-likes of labels, consistently with ``Series.__getitem__`` (:issue:`21294`)
 - Fixed ``DataFrame[np.nan]`` when columns are non-unique (:issue:`21428`)
 - Bug when indexing :class:`DatetimeIndex` with nanosecond resolution dates and timezones (:issue:`11679`)
-
--
+- Bug where indexing with a Numpy array containing negative values would mutate the indexer (:issue:`21867`)
 
 Missing
 ^^^^^^^

pandas/core/indexing.py

@@ -2596,6 +2596,7 @@ def maybe_convert_indices(indices, n):
 
     mask = indices < 0
     if mask.any():
+        indices = indices.copy()
         indices[mask] += n
 
     mask = (indices >= n) | (indices < 0)

pandas/tests/indexing/test_iloc.py

@@ -126,6 +126,23 @@ def test_iloc_getitem_neg_int(self):
                           typs=['labels', 'mixed', 'ts', 'floats', 'empty'],
                           fails=IndexError)
 
+    def test_iloc_array_not_mutating_negative_indices(self):
+
+        # GH 21867
+        array_with_neg_numbers = np.array([1, 2, -1])
+        array_copy = array_with_neg_numbers.copy()
+        df = pd.DataFrame({
+            'A': [100, 101, 102],
+            'B': [103, 104, 105],
+            'C': [106, 107, 108]},
+            index=[1, 2, 3])
+        df.iloc[array_with_neg_numbers]
+        tm.assert_numpy_array_equal(array_with_neg_numbers, array_copy)
+
+        df.iloc[:, array_with_neg_numbers]
+        tm.assert_numpy_array_equal(array_with_neg_numbers, array_copy)
+
+
     def test_iloc_getitem_list_int(self):
 
         # list of ints