TST: CoW with df.isetitem() (pandas-dev#50692)

Author: lithomas1

pandas/tests/copy_view/test_methods.py

@@ -780,3 +780,25 @@ def test_squeeze(using_copy_on_write):
         # Without CoW the original will be modified
         assert np.shares_memory(series.values, get_array(df, "a"))
         assert df.loc[0, "a"] == 0
+
+
+def test_isetitem(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [7, 8, 9]})
+    df_orig = df.copy()
+    df2 = df.copy(deep=None)  # Trigger a CoW
+    df2.isetitem(1, np.array([-1, -2, -3]))  # This is inplace
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+        assert np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+    else:
+        assert not np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    df2.loc[0, "a"] = 0
+    tm.assert_frame_equal(df, df_orig)  # Original is unchanged
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+    else:
+        assert not np.shares_memory(get_array(df, "c"), get_array(df2, "c"))