fix(python): Reject Expr objects in replace sequence arguments

Co-authored-by: Claude <noreply@anthropic.com>

Author: wtn

py-polars/src/polars/expr/expr.py

@@ -11076,8 +11076,20 @@ def replace(
             old = list(old.keys())
         else:
             if isinstance(old, Sequence) and not isinstance(old, (str, pl.Series)):
+                if any(isinstance(v, Expr) for v in old):
+                    msg = (
+                        "passing expressions to `old` is not supported when `old` "
+                        "is a sequence; use a scalar expression or literal values"
+                    )
+                    raise TypeError(msg)
                 old = pl.Series(old)
             if isinstance(new, Sequence) and not isinstance(new, (str, pl.Series)):
+                if any(isinstance(v, Expr) for v in new):
+                    msg = (
+                        "passing expressions to `new` is not supported when `new` "
+                        "is a sequence; use a scalar expression or literal values"
+                    )
+                    raise TypeError(msg)
                 new = pl.Series(new)
 
         old_pyexpr = parse_into_expression(old, str_as_lit=True)  # type: ignore[arg-type]
@@ -11270,6 +11282,21 @@ def replace_strict(
                 raise TypeError(msg)
             new = list(old.values())
             old = list(old.keys())
+        else:
+            if isinstance(old, Sequence) and not isinstance(old, (str, pl.Series)):
+                if any(isinstance(v, Expr) for v in old):
+                    msg = (
+                        "passing expressions to `old` is not supported when `old` "
+                        "is a sequence; use a scalar expression or literal values"
+                    )
+                    raise TypeError(msg)
+            if isinstance(new, Sequence) and not isinstance(new, (str, pl.Series)):
+                if any(isinstance(v, Expr) for v in new):
+                    msg = (
+                        "passing expressions to `new` is not supported when `new` "
+                        "is a sequence; use a scalar expression or literal values"
+                    )
+                    raise TypeError(msg)
 
         old_pyexpr = parse_into_expression(old, str_as_lit=True)  # type: ignore[arg-type]
         new_pyexpr = parse_into_expression(new, str_as_lit=True)  # type: ignore[arg-type]

py-polars/tests/unit/operations/test_replace.py

@@ -289,3 +289,23 @@ def test_replace_single_argument_not_mapping() -> None:
         match="`new` argument is required if `old` argument is not a Mapping type",
     ):
         df.select(pl.col("a").replace("b"))
+
+
+def test_replace_expr_in_sequence_raises() -> None:
+    s = pl.Series([1, 2, 3])
+    with pytest.raises(
+        TypeError,
+        match="passing expressions to `old` is not supported when `old` is a sequence",
+    ):
+        s.replace([pl.lit(1)], [2])
+
+    with pytest.raises(
+        TypeError,
+        match="passing expressions to `new` is not supported when `new` is a sequence",
+    ):
+        s.replace([1], [pl.lit(2)])
+
+    # Scalar expressions are still allowed
+    result = s.replace(pl.lit(1), pl.lit(10))
+    expected = pl.Series([10, 2, 3])
+    assert_series_equal(result, expected)

py-polars/tests/unit/operations/test_replace_strict.py

@@ -419,3 +419,23 @@ def test_replace_strict_nested_mapping_22554() -> None:
         ),
         pl.Series([[42], [13], [37]]),
     )
+
+
+def test_replace_strict_expr_in_sequence_raises() -> None:
+    s = pl.Series([1, 2, 3])
+    with pytest.raises(
+        TypeError,
+        match="passing expressions to `old` is not supported when `old` is a sequence",
+    ):
+        s.replace_strict([pl.lit(1)], [2])
+
+    with pytest.raises(
+        TypeError,
+        match="passing expressions to `new` is not supported when `new` is a sequence",
+    ):
+        s.replace_strict([1], [pl.lit(2)])
+
+    # Scalar expressions are still allowed
+    result = s.replace_strict(pl.lit(1), pl.lit(10), default=pl.lit(None))
+    expected = pl.Series([10, None, None], dtype=pl.Int32)
+    assert_series_equal(result, expected)