From fc69f3b3f44e4a937b8333309191c047f12d6f8b Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Sun, 15 Jan 2023 01:52:14 +0100
Subject: [PATCH] ENH: Add lazy copy to shift

---
 pandas/core/generic.py                 |  2 +-
 pandas/tests/copy_view/test_methods.py | 71 ++++++++++++++++++++++++++
 2 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 4ef91f6de5e27..9bcf7793201cd 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -9908,7 +9908,7 @@ def shift(
         2020-01-08    45    48    52
         """
         if periods == 0:
-            return self.copy()
+            return self.copy(deep=None)
 
         if freq is None:
             # when freq is None, data is shifted, index is not
diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index 5c24a180c4d6d..87903f20f5232 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -244,6 +244,77 @@ def test_filter(using_copy_on_write, filter_kwargs):
     tm.assert_frame_equal(df, df_orig)
 
 
+def test_shift_no_op(using_copy_on_write):
+    df = DataFrame(
+        [[1, 2], [3, 4], [5, 6]],
+        index=date_range("2020-01-01", "2020-01-03"),
+        columns=["a", "b"],
+    )
+    df_orig = df.copy()
+    df2 = df.shift(periods=0)
+
+    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"))
+
+    df.iloc[0, 0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+    tm.assert_frame_equal(df2, df_orig)
+
+
+def test_shift_index(using_copy_on_write):
+    df = DataFrame(
+        [[1, 2], [3, 4], [5, 6]],
+        index=date_range("2020-01-01", "2020-01-03"),
+        columns=["a", "b"],
+    )
+    df2 = df.shift(periods=1, axis=0)
+
+    assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+
+def test_shift_rows_freq(using_copy_on_write):
+    df = DataFrame(
+        [[1, 2], [3, 4], [5, 6]],
+        index=date_range("2020-01-01", "2020-01-03"),
+        columns=["a", "b"],
+    )
+    df_orig = df.copy()
+    df_orig.index = date_range("2020-01-02", "2020-01-04")
+    df2 = df.shift(periods=1, freq="1D")
+
+    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"))
+
+    df.iloc[0, 0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+    tm.assert_frame_equal(df2, df_orig)
+
+
+def test_shift_columns(using_copy_on_write):
+    df = DataFrame(
+        [[1, 2], [3, 4], [5, 6]], columns=date_range("2020-01-01", "2020-01-02")
+    )
+    df2 = df.shift(periods=1, axis=1)
+
+    assert np.shares_memory(get_array(df2, "2020-01-02"), get_array(df, "2020-01-01"))
+    df.iloc[0, 1] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(
+            get_array(df2, "2020-01-02"), get_array(df, "2020-01-01")
+        )
+    expected = DataFrame(
+        [[np.nan, 1], [np.nan, 3], [np.nan, 5]],
+        columns=date_range("2020-01-01", "2020-01-02"),
+    )
+    tm.assert_frame_equal(df2, expected)
+
+
 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()