fail on fallback

python/cudf/cudf/pandas/fast_slow_proxy.py

@@ -881,6 +881,12 @@ def _assert_fast_slow_eq(left, right):
         assert_eq(left, right)
 
 
+class CudfFallbackError(Exception):
+    """Raised when fallback occurs"""
+
+    pass
+
+
 def _fast_slow_function_call(
     func: Callable,
     /,
@@ -942,10 +948,9 @@ def _fast_slow_function_call(
                             f"The exception was {e}."
                         )
     except Exception as err:
-        if _env_get_bool("CUDF_PANDAS_WARN_ON_FALLBACK", False):
-            warnings.warn(
-                "Falling back to the slow path. " f"The exception was {err}.",
-                UserWarning,
+        if _env_get_bool("CUDF_PANDAS_FAIL_ON_FALLBACK", False):
+            raise CudfFallbackError(
+                f"The operation failed with cuDF, the reason was {type(err)}: {err}."
             )
         with nvtx.annotate(
             "EXECUTE_SLOW",

python/cudf/cudf_pandas_tests/test_cudf_pandas_fallback.py

@@ -16,8 +16,8 @@
 
 
 @pytest.fixture(autouse=True)
-def warn_on_fallback(monkeypatch):
-    monkeypatch.setenv("CUDF_PANDAS_WARN_ON_FALLBACK", "True")
+def fail_on_fallback(monkeypatch):
+    monkeypatch.setenv("CUDF_PANDAS_FAIL_ON_FALLBACK", "True")
 
 
 @pytest.fixture
@@ -33,9 +33,6 @@ def series(dataframe):
     return (pdf["a"], df["a"])
 
 
-@pytest.mark.filterwarnings(
-    "error::UserWarning:.*Falling back to the slow path.*"
-)
 @pytest.mark.parametrize(
     "op",
     [
@@ -56,9 +53,7 @@ def test_no_fallback_in_reduction_ops(series, op):
     tm.assert_almost_equal(res, expect)
 
 
-@pytest.mark.filterwarnings(
-    "error::UserWarning:.*Falling back to the slow path.*"
-)
+@pytest.mark.xfail(strict=True, reason="Fallback expected")
 @pytest.mark.parametrize(
     "op",
     [