From 040131b7710bd35a7a5ec97fe50cb6bc744379e1 Mon Sep 17 00:00:00 2001
From: seljaks <33955366+seljaks@users.noreply.github.com>
Date: Tue, 29 Nov 2022 23:15:35 +0100
Subject: [PATCH 1/3] TST: CoW tests for df.head and df.tail GH49473

---
 pandas/tests/copy_view/test_methods.py | 30 ++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index 0c68f6a866eec..eb4d55ee0530e 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/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(using_copy_on_write, method):
+    df = DataFrame({"a": [1, 1, 1], "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)

From 0c5384ddfb93cd8ffc66d75d22dc9638e164ee2c Mon Sep 17 00:00:00 2001
From: seljaks <33955366+seljaks@users.noreply.github.com>
Date: Tue, 29 Nov 2022 23:24:29 +0100
Subject: [PATCH 2/3] changed arg order

---
 pandas/tests/copy_view/test_methods.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index eb4d55ee0530e..ad0a5799f9595 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -261,7 +261,7 @@ def test_set_index(using_copy_on_write):
         lambda df: df.tail(3),
     ],
 )
-def test_head_tail(using_copy_on_write, method):
+def test_head_tail(method, using_copy_on_write):
     df = DataFrame({"a": [1, 1, 1], "b": [0.1, 0.2, 0.3]})
     df_orig = df.copy()
     df2 = method(df)

From 8451a316152aec61de02b1994edff5f507a2913e Mon Sep 17 00:00:00 2001
From: seljaks <33955366+seljaks@users.noreply.github.com>
Date: Wed, 30 Nov 2022 13:43:44 +0100
Subject: [PATCH 3/3] made df values consistent

---
 pandas/tests/copy_view/test_methods.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index ad0a5799f9595..bf65f153b10dd 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -262,7 +262,7 @@ def test_set_index(using_copy_on_write):
     ],
 )
 def test_head_tail(method, using_copy_on_write):
-    df = DataFrame({"a": [1, 1, 1], "b": [0.1, 0.2, 0.3]})
+    df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
     df_orig = df.copy()
     df2 = method(df)
     df2._mgr._verify_integrity()