Add test that diagonal concat with mismatching schemas raises (#16006)

Arguably this should be determined during query optimization by polars, but for now it is raised late during compute, so we must validate on our side.

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

Approvers:
  - Thomas Li (https://github.com/lithomas1)

URL: #16006

python/cudf_polars/cudf_polars/dsl/ir.py

@@ -933,10 +933,10 @@ class Union(IR):
     """Optional slice to apply after concatenation."""
 
     def __post_init__(self) -> None:
-        """Validated preconditions."""
+        """Validate preconditions."""
         schema = self.dfs[0].schema
         if not all(s.schema == schema for s in self.dfs[1:]):
-            raise ValueError("Schema mismatch")
+            raise NotImplementedError("Schema mismatch")
 
     def evaluate(self, *, cache: MutableMapping[int, DataFrame]) -> DataFrame:
         """Evaluate and return a dataframe."""

python/cudf_polars/tests/test_union.py

@@ -2,8 +2,11 @@
 # SPDX-License-Identifier: Apache-2.0
 from __future__ import annotations
 
+import pytest
+
 import polars as pl
 
+from cudf_polars import translate_ir
 from cudf_polars.testing.asserts import assert_gpu_result_equal
 
 
@@ -19,6 +22,19 @@ def test_union():
     assert_gpu_result_equal(query)
 
 
+def test_union_schema_mismatch_raises():
+    ldf = pl.DataFrame(
+        {
+            "a": [1, 2, 3, 4, 5, 6, 7],
+            "b": [1, 1, 1, 1, 1, 1, 1],
+        }
+    ).lazy()
+    ldf2 = ldf.select(pl.col("a").cast(pl.Float32))
+    query = pl.concat([ldf, ldf2], how="diagonal")
+    with pytest.raises(NotImplementedError):
+        _ = translate_ir(query._ldf.visit())
+
+
 def test_concat_vertical():
     ldf = pl.LazyFrame(
         {