From 8e60d77b483d471b747524450161b7f88578fa70 Mon Sep 17 00:00:00 2001
From: Thomas Li <47963215+lithomas1@users.noreply.github.com>
Date: Thu, 5 Jan 2023 09:26:28 -0800
Subject: [PATCH 1/2] TST: Copy on Write for filter

---
 pandas/tests/copy_view/test_methods.py | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index 0ff359061fd67..33e30639ceba8 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -174,6 +174,27 @@ def test_select_dtypes(using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
+@pytest.mark.parametrize(
+    "filter_kwargs", [{"items": ["a"]}, {"like": "a"}, {"regex": "a"}]
+)
+def test_filter(using_copy_on_write, filter_kwargs):
+    # Case: selecting columns using `filter()` returns a new dataframe
+    # + afterwards modifying the result
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [0.1, 0.2, 0.3]})
+    df_orig = df.copy()
+    df2 = df.filter(**filter_kwargs)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    # mutating df2 triggers a copy-on-write for that column/block
+    df2.iloc[0, 0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
 def test_pop(using_copy_on_write):
     df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [0.1, 0.2, 0.3]})
     df_orig = df.copy()

From 378b61f8a78b0e5bd23b06b45fd972b2ef1b4acc Mon Sep 17 00:00:00 2001
From: Thomas Li <47963215+lithomas1@users.noreply.github.com>
Date: Thu, 5 Jan 2023 10:30:16 -0800
Subject: [PATCH 2/2] Move iloc inside CoW case

---
 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 33e30639ceba8..926d54ebdd6cd 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -189,8 +189,8 @@ def test_filter(using_copy_on_write, filter_kwargs):
         assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
 
     # mutating df2 triggers a copy-on-write for that column/block
-    df2.iloc[0, 0] = 0
     if using_copy_on_write:
+        df2.iloc[0, 0] = 0
         assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
     tm.assert_frame_equal(df, df_orig)