BUG: np array indexer modifed in iloc

pandas/core/indexing.py

@@ -1,4 +1,5 @@
 # pylint: disable=W0223
+from copy import deepcopy
 import textwrap
 import warnings
 import numpy as np
@@ -2586,6 +2587,7 @@ def maybe_convert_indices(indices, n):
     IndexError : one of the converted indices either exceeded the number
         of elements (specified by `n`) OR was still negative.
     """
+    indices = deepcopy(indices)
 
     if isinstance(indices, list):
         indices = np.array(indices)
@@ -2596,6 +2598,8 @@ def maybe_convert_indices(indices, n):
 
     mask = indices < 0
     if mask.any():
+        # indices is np.array, so we don't use deepcopy
+        indices = np.copy(indices)
         indices[mask] += n
 
     mask = (indices >= n) | (indices < 0)

pandas/tests/indexing/test_iloc.py

@@ -126,6 +126,24 @@ def test_iloc_getitem_neg_int(self):
                           typs=['labels', 'mixed', 'ts', 'floats', 'empty'],
                           fails=IndexError)
 
+    def test_iloc_array_not_mutating_negative_indices(self):
+
+        # Test that iloc does not modify index array
+        # if the array contains negative integers
+        array_with_neg_numbers = np.array([1, 2, -1])
+        array_copy = np.copy(array_with_neg_numbers)
+        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]
+        assert np.all(array_with_neg_numbers == array_copy)
+
+        df.iloc[:, array_with_neg_numbers]
+        assert np.all(array_with_neg_numbers == array_copy)
+
+
     def test_iloc_getitem_list_int(self):
 
         # list of ints