API/CoW: Return copies for head and tail

doc/source/whatsnew/v2.1.0.rst

@@ -65,6 +65,7 @@ Copy-on-Write improvements
 - The :class:`DataFrame` constructor, when constructing a DataFrame from a dictionary
   of Index objects and specifying ``copy=False``, will now use a lazy copy
   of those Index objects for the columns of the DataFrame (:issue:`52947`)
+- :meth:`DataFrame.head` and :meth:`DataFrame.tail` will now return deep copies (:issue:`54011`)
 - Add lazy copy mechanism to :meth:`DataFrame.eval` (:issue:`53746`)
 
 - Trying to operate inplace on a temporary column selection

pandas/core/generic.py

@@ -5786,6 +5786,8 @@ def head(self, n: int = 5) -> Self:
         4     monkey
         5     parrot
         """
+        if using_copy_on_write():
+            return self.iloc[:n].copy()
         return self.iloc[:n]
 
     @final
@@ -5861,6 +5863,10 @@ def tail(self, n: int = 5) -> Self:
         7   whale
         8   zebra
         """
+        if using_copy_on_write():
+            if n == 0:
+                return self.iloc[0:0].copy()
+            return self.iloc[-n:].copy()
         if n == 0:
             return self.iloc[0:0]
         return self.iloc[-n:]

pandas/tests/copy_view/test_methods.py

@@ -895,16 +895,19 @@ def test_head_tail(method, using_copy_on_write):
     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"))
+        # We are explicitly deviating for CoW here to make an eager copy (avoids
+        # tracking references for very cheap ops)
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+        assert not 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, "b"), get_array(df, "b"))
         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.
+        assert np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
         df2.iloc[0, 0] = 1
     tm.assert_frame_equal(df, df_orig)