Fix empty column selector (#1159)

* Add a test to trigger the bug

Signed-off-by: Pierre Bartet <[email protected]>

* Fix _parse.py

Signed-off-by: Pierre Bartet <[email protected]>

* Fix formatting

Signed-off-by: Pierre Bartet <[email protected]>

---------

Signed-off-by: Pierre Bartet <[email protected]>
Co-authored-by: Pierre Bartet <[email protected]>

Author: Pierre-Bartet

skl2onnx/_parse.py

@@ -362,6 +362,11 @@ def _parse_sklearn_column_transformer(scope, model, inputs, custom_parsers=None)
         elif isinstance(column_indices, (int, str)):
             column_indices = [column_indices]
         names = get_column_indices(column_indices, inputs, multiple=True)
+
+        # Skip transforms which apply to no columns at all
+        if len(names) == 0:
+            continue
+
         transform_inputs = []
         for onnx_var, onnx_is in names.items():
             tr_inputs = _fetch_input_slice(scope, [inputs[onnx_var]], onnx_is)

tests/test_sklearn_pipeline.py

@@ -1335,6 +1335,30 @@ def test_pipeline_make_column_selector(self):
         )
         assert_almost_equal(expected, got[0])
 
+    @unittest.skipIf(TARGET_OPSET < 11, reason="SequenceConstruct not available")
+    @unittest.skipIf(not check_scikit_version(), reason="Scikit 0.21 too old")
+    @ignore_warnings(category=(FutureWarning, UserWarning))
+    def test_pipeline_empty_make_column_selector(self):
+        X = pandas.DataFrame({"city": ["London", "London", "Paris", "Sallisaw"]})
+
+        ct = make_column_transformer(
+            (StandardScaler(), make_column_selector(dtype_include=numpy.number)),
+            (OneHotEncoder(), make_column_selector(dtype_include=object)),
+        )
+        expected = ct.fit_transform(X)
+        onx = to_onnx(ct, X, target_opset=TARGET_OPSET)
+        sess = InferenceSession(
+            onx.SerializeToString(), providers=["CPUExecutionProvider"]
+        )
+        names = [i.name for i in sess.get_inputs()]
+        got = sess.run(
+            None,
+            {
+                names[0]: X[names[0]].values.reshape((-1, 1)),
+            },
+        )
+        assert_almost_equal(expected, got[0])
+
     @unittest.skipIf(not check_scikit_version(), reason="Scikit 0.21 too old")
     def test_feature_selector_no_converter(self):
         class ColumnSelector(TransformerMixin, BaseEstimator):