Raise early on unhandled PythonScan node (#15992)

Add test of the behaviour.

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #15992

python/cudf_polars/cudf_polars/dsl/ir.py

@@ -165,6 +165,10 @@ class PythonScan(IR):
     predicate: expr.NamedExpr | None
     """Filter to apply to the constructed dataframe before returning it."""
 
+    def __post_init__(self):
+        """Validate preconditions."""
+        raise NotImplementedError("PythonScan not implemented")
+
 
 @dataclasses.dataclass(slots=True)
 class Scan(IR):

python/cudf_polars/tests/test_python_scan.py

@@ -0,0 +1,20 @@
+# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+# SPDX-License-Identifier: Apache-2.0
+from __future__ import annotations
+
+import pytest
+
+import polars as pl
+
+from cudf_polars import translate_ir
+
+
+def test_python_scan():
+    def source(with_columns, predicate, nrows):
+        return pl.DataFrame({"a": pl.Series([1, 2, 3], dtype=pl.Int8())})
+
+    q = pl.LazyFrame._scan_python_function({"a": pl.Int8}, source, pyarrow=False)
+    with pytest.raises(NotImplementedError):
+        _ = translate_ir(q._ldf.visit())
+
+    assert q.collect().equals(source(None, None, None))