fix(python): ChainedWhen should not inherit Expr (#17142)

Co-authored-by: Stijn de Gooijer <stijndegooijer@gmail.com>

py-polars/polars/expr/whenthen.py

@@ -99,7 +99,7 @@ def otherwise(self, statement: IntoExpr) -> Expr:
         return wrap_expr(self._then.otherwise(statement_pyexpr))
 
 
-class ChainedWhen(Expr):
+class ChainedWhen:
     """
     Utility class for the `when-then-otherwise` expression.
 

py-polars/tests/unit/functions/test_when_then.py

@@ -622,3 +622,11 @@ def test_when_then_supertype_15975_comment() -> None:
     )
 
     assert q.collect()["val"].to_list() == [1.0, 1.5, 16.0]
+
+
+def test_chained_when_no_subclass_17142() -> None:
+    # https://github.com/pola-rs/polars/pull/17142
+    when = pl.when(True).then(1).when(True)
+
+    assert not isinstance(when, pl.Expr)
+    assert "<polars.expr.whenthen.ChainedWhen object at" in str(when)