From bf16a0c17e55e9b546de7e1b338d46919d0eb279 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Thu, 29 Dec 2022 00:11:08 +0100
Subject: [PATCH 1/3] ENH: Add lazy copy for truncate

---
 pandas/core/generic.py                 |  6 +++---
 pandas/tests/copy_view/test_methods.py | 24 ++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index c893e9ce3d9a9..ac3ca7e5401e4 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -9940,7 +9940,7 @@ def truncate(
         before=None,
         after=None,
         axis: Axis | None = None,
-        copy: bool_t = True,
+        copy: bool_t | None = None,
     ) -> NDFrameT:
         """
         Truncate a Series or DataFrame before and after some index value.
@@ -10091,8 +10091,8 @@ def truncate(
         if isinstance(ax, MultiIndex):
             setattr(result, self._get_axis_name(axis), ax.truncate(before, after))
 
-        if copy:
-            result = result.copy()
+        if copy or copy is None:
+            result = result.copy(deep=copy)
 
         return result
 
diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index 878f1d8089d33..3c427657bf207 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -369,6 +369,30 @@ def test_head_tail(method, using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
+@pytest.mark.parametrize(
+    "kwargs",
+    [
+        {"before": "a", "after": "b", "axis": 1},
+        {"before": 0, "after": 1, "axis": 0},
+    ],
+)
+def test_truncate(using_copy_on_write, kwargs):
+    df = DataFrame({"a": [1, 2, 3], "b": 1, "c": 2})
+    df_orig = df.copy()
+    df2 = df.truncate(**kwargs)
+    df2._mgr._verify_integrity()
+
+    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"))
+
+    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)
+
+
 @pytest.mark.parametrize("method", ["assign", "drop_duplicates"])
 def test_assign_drop_duplicates(using_copy_on_write, method):
     df = DataFrame({"a": [1, 2, 3]})

From 8976062e6c7bc717cc08dbb7b1f32530d2acc2ba Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Thu, 29 Dec 2022 21:11:08 +0100
Subject: [PATCH 2/3] Remove test

---
 pandas/tests/frame/methods/test_truncate.py | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/pandas/tests/frame/methods/test_truncate.py b/pandas/tests/frame/methods/test_truncate.py
index bfee3edc085d8..21f0664707ebe 100644
--- a/pandas/tests/frame/methods/test_truncate.py
+++ b/pandas/tests/frame/methods/test_truncate.py
@@ -66,12 +66,6 @@ def test_truncate(self, datetime_frame, frame_or_series):
                 before=ts.index[-1] - ts.index.freq, after=ts.index[0] + ts.index.freq
             )
 
-    def test_truncate_copy(self, datetime_frame):
-        index = datetime_frame.index
-        truncated = datetime_frame.truncate(index[5], index[10])
-        truncated.values[:] = 5.0
-        assert not (datetime_frame.values[5:11] == 5).any()
-
     def test_truncate_nonsortedindex(self, frame_or_series):
         # GH#17935
 

From b2c467c25fa15172c2de3034b68a2d659bd67b93 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Tue, 3 Jan 2023 12:05:31 +0100
Subject: [PATCH 3/3] Adjust condition

---
 pandas/core/generic.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 49eac6d565bf1..f6dfc7d3b076a 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -159,6 +159,7 @@
     SingleArrayManager,
 )
 from pandas.core.internals.construction import mgr_to_mgr
+from pandas.core.internals.managers import _using_copy_on_write
 from pandas.core.missing import (
     clean_fill_method,
     clean_reindex_fill_method,
@@ -10098,7 +10099,7 @@ def truncate(
         if isinstance(ax, MultiIndex):
             setattr(result, self._get_axis_name(axis), ax.truncate(before, after))
 
-        if copy or copy is None:
+        if copy or (copy is None and not _using_copy_on_write()):
             result = result.copy(deep=copy)
 
         return result