TST/CoW: copy-on-write tests for df.head and df.tail

pandas/tests/copy_view/test_methods.py

@@ -250,3 +250,33 @@ def test_set_index(using_copy_on_write):
     df2.iloc[0, 1] = 0
     assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
     tm.assert_frame_equal(df, df_orig)
+
+
+@pytest.mark.parametrize(
+    "method",
+    [
+        lambda df: df.head(),
+        lambda df: df.head(2),
+        lambda df: df.tail(),
+        lambda df: df.tail(3),
+    ],
+)
+def test_head_tail(method, using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+    df_orig = df.copy()
+    df2 = method(df)
+    df2._mgr._verify_integrity()
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+        assert np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
+
+    # modify df2 to trigger CoW for that block
+    df2.iloc[0, 0] = 0
+    assert np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        # without CoW enabled, head and tail return views. Mutating df2 also mutates df.
+        df2.iloc[0, 0] = 1
+    tm.assert_frame_equal(df, df_orig)