fix: Fix pickling polars.col on Python versions <3.11 (#21333)

py-polars/polars/functions/col.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import contextlib
+import sys
 from collections.abc import Iterable
 from datetime import datetime, timedelta
 from typing import TYPE_CHECKING
@@ -21,6 +22,9 @@
     from polars._typing import PolarsDataType, PythonDataType
     from polars.expr.expr import Expr
 
+    if not sys.version_info >= (3, 11):
+        from typing import Any
+
 __all__ = ["col"]
 
 
@@ -362,5 +366,13 @@ def __getattr__(self, name: str) -> Expr:
 
         return _create_col(name)
 
+    if not sys.version_info >= (3, 11):
+
+        def __getstate__(self) -> Any:
+            return self.__dict__
+
+        def __setstate__(self, state: Any) -> None:
+            self.__dict__ = state
+
 
 col: Col = Col()

py-polars/tests/unit/test_serde.py

@@ -225,3 +225,9 @@ def test_serde_empty_df_lazy_frame() -> None:
     f.write(lf.serialize())
     f.seek(0)
     assert pl.LazyFrame.deserialize(f).collect().shape == (0, 0)
+
+
+def test_pickle_class_objects_21021() -> None:
+    assert isinstance(pickle.loads(pickle.dumps(pl.col))("A"), pl.Expr)
+    assert isinstance(pickle.loads(pickle.dumps(pl.DataFrame))(), pl.DataFrame)
+    assert isinstance(pickle.loads(pickle.dumps(pl.LazyFrame))(), pl.LazyFrame)