ENH / CoW: Add lazy copy to eval (#53746)

Author: phofl

doc/source/user_guide/copy_on_write.rst

@@ -211,6 +211,7 @@ following methods:
   - :meth:`DataFrame.astype` / :meth:`Series.astype`
   - :meth:`DataFrame.convert_dtypes` / :meth:`Series.convert_dtypes`
   - :meth:`DataFrame.join`
+  - :meth:`DataFrame.eval`
   - :func:`concat`
   - :func:`merge`
 

doc/source/whatsnew/v2.1.0.rst

@@ -25,6 +25,7 @@ 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`)
+- Add lazy copy mechanism to :meth:`DataFrame.eval` (:issue:`53746`)
 
 .. _whatsnew_210.enhancements.enhancement2:
 

pandas/core/computation/eval.py

@@ -373,7 +373,11 @@ def eval(
             # if returning a copy, copy only on the first assignment
             if not inplace and first_expr:
                 try:
-                    target = env.target.copy()
+                    target = env.target
+                    if isinstance(target, NDFrame):
+                        target = target.copy(deep=None)
+                    else:
+                        target = target.copy()
                 except AttributeError as err:
                     raise ValueError("Cannot return a copy of the target") from err
             else:
@@ -389,7 +393,9 @@ def eval(
                     if inplace and isinstance(target, NDFrame):
                         target.loc[:, assigner] = ret
                     else:
-                        target[assigner] = ret
+                        target[  # pyright: ignore[reportGeneralTypeIssues]
+                            assigner
+                        ] = ret
             except (TypeError, IndexError) as err:
                 raise ValueError("Cannot assign expression output to target") from err
 

pandas/tests/copy_view/test_methods.py

@@ -1850,3 +1850,30 @@ def test_insert_series(using_copy_on_write):
 
     df.iloc[0, 1] = 100
     tm.assert_series_equal(ser, ser_orig)
+
+
+def test_eval(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": 1})
+    df_orig = df.copy()
+
+    result = df.eval("c = a+b")
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+    else:
+        assert not np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+
+    result.iloc[0, 0] = 100
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_eval_inplace(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": 1})
+    df_orig = df.copy()
+    df_view = df[:]
+
+    df.eval("c = a+b", inplace=True)
+    assert np.shares_memory(get_array(df, "a"), get_array(df_view, "a"))
+
+    df.iloc[0, 0] = 100
+    if using_copy_on_write:
+        tm.assert_frame_equal(df_view, df_orig)