[dask] Support pred_contrib in Dask predict() methods (fixes #3713) (#3774)

* adding pred_contrib support

* add tests

* linting

* remove raw_score

* add pred kwargs

* faster tests

* Apply suggestions from code review

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* changes to tests

* Update tests/python_package_test/test_dask.py

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

python-package/lightgbm/dask.py

@@ -280,18 +280,30 @@ def _train(client, data, label, params, model_factory, sample_weight=None, group
     return results[0]
 
 
-def _predict_part(part, model, proba, **kwargs):
+def _predict_part(part, model, raw_score, pred_proba, pred_leaf, pred_contrib, **kwargs):
     data = part.values if isinstance(part, pd.DataFrame) else part
 
     if data.shape[0] == 0:
         result = np.array([])
-    elif proba:
-        result = model.predict_proba(data, **kwargs)
+    elif pred_proba:
+        result = model.predict_proba(
+            data,
+            raw_score=raw_score,
+            pred_leaf=pred_leaf,
+            pred_contrib=pred_contrib,
+            **kwargs
+        )
     else:
-        result = model.predict(data, **kwargs)
+        result = model.predict(
+            data,
+            raw_score=raw_score,
+            pred_leaf=pred_leaf,
+            pred_contrib=pred_contrib,
+            **kwargs
+        )
 
     if isinstance(part, pd.DataFrame):
-        if proba:
+        if pred_proba or pred_contrib:
             result = pd.DataFrame(result, index=part.index)
         else:
             result = pd.Series(result, index=part.index, name='predictions')
@@ -299,7 +311,8 @@ def _predict_part(part, model, proba, **kwargs):
     return result
 
 
-def _predict(model, data, proba=False, dtype=np.float32, **kwargs):
+def _predict(model, data, raw_score=False, pred_proba=False, pred_leaf=False, pred_contrib=False,
+             dtype=np.float32, **kwargs):
     """Inner predict routine.
 
     Parameters
@@ -307,20 +320,42 @@ def _predict(model, data, proba=False, dtype=np.float32, **kwargs):
     model : lightgbm.LGBMClassifier, lightgbm.LGBMRegressor, or lightgbm.LGBMRanker class
     data : dask array of shape = [n_samples, n_features]
         Input feature matrix.
-    proba : bool
-        Should method return results of predict_proba (proba == True) or predict (proba == False).
+    pred_proba : bool, optional (default=False)
+        Should method return results of ``predict_proba`` (``pred_proba=True``) or ``predict`` (``pred_proba=False``).
+    pred_leaf : bool, optional (default=False)
+        Whether to predict leaf index.
+    pred_contrib : bool, optional (default=False)
+        Whether to predict feature contributions.
     dtype : np.dtype
         Dtype of the output.
-    kwargs : other parameters passed to predict or predict_proba method
+    kwargs : dict
+        Other parameters passed to ``predict`` or ``predict_proba`` method.
     """
     if isinstance(data, dd._Frame):
-        return data.map_partitions(_predict_part, model=model, proba=proba, **kwargs).values
+        return data.map_partitions(
+            _predict_part,
+            model=model,
+            raw_score=raw_score,
+            pred_proba=pred_proba,
+            pred_leaf=pred_leaf,
+            pred_contrib=pred_contrib,
+            **kwargs
+        ).values
     elif isinstance(data, da.Array):
-        if proba:
+        if pred_proba:
             kwargs['chunks'] = (data.chunks[0], (model.n_classes_,))
         else:
             kwargs['drop_axis'] = 1
-        return data.map_blocks(_predict_part, model=model, proba=proba, dtype=dtype, **kwargs)
+        return data.map_blocks(
+            _predict_part,
+            model=model,
+            raw_score=raw_score,
+            pred_proba=pred_proba,
+            pred_leaf=pred_leaf,
+            pred_contrib=pred_contrib,
+            dtype=dtype,
+            **kwargs
+        )
     else:
         raise TypeError('Data must be either Dask array or dataframe. Got %s.' % str(type(data)))
 
@@ -370,7 +405,7 @@ class DaskLGBMClassifier(_LGBMModel, LGBMClassifier):
 
     def predict_proba(self, X, **kwargs):
         """Docstring is inherited from the lightgbm.LGBMClassifier.predict_proba."""
-        return _predict(self.to_local(), X, proba=True, **kwargs)
+        return _predict(self.to_local(), X, pred_proba=True, **kwargs)
     predict_proba.__doc__ = LGBMClassifier.predict_proba.__doc__
 
     def to_local(self):

tests/python_package_test/test_dask.py

@@ -235,6 +235,55 @@ def test_classifier(output, centers, client, listen_port):
     client.close()
 
 
+@pytest.mark.parametrize('output', data_output)
+@pytest.mark.parametrize('centers', data_centers)
+def test_classifier_pred_contrib(output, centers, client, listen_port):
+    X, y, w, dX, dy, dw = _create_data('classification', output=output, centers=centers)
+
+    dask_classifier = dlgbm.DaskLGBMClassifier(
+        time_out=5,
+        local_listen_port=listen_port,
+        tree_learner='data',
+        n_estimators=10,
+        num_leaves=10
+    )
+    dask_classifier = dask_classifier.fit(dX, dy, sample_weight=dw, client=client)
+    preds_with_contrib = dask_classifier.predict(dX, pred_contrib=True).compute()
+
+    local_classifier = lightgbm.LGBMClassifier(
+        n_estimators=10,
+        num_leaves=10
+    )
+    local_classifier.fit(X, y, sample_weight=w)
+    local_preds_with_contrib = local_classifier.predict(X, pred_contrib=True)
+
+    if output == 'scipy_csr_matrix':
+        preds_with_contrib = np.array(preds_with_contrib.todense())
+
+    # shape depends on whether it is binary or multiclass classification
+    num_features = dask_classifier.n_features_
+    num_classes = dask_classifier.n_classes_
+    if num_classes == 2:
+        expected_num_cols = num_features + 1
+    else:
+        expected_num_cols = (num_features + 1) * num_classes
+
+    # * shape depends on whether it is binary or multiclass classification
+    # * matrix for binary classification is of the form [feature_contrib, base_value],
+    #   for multi-class it's [feat_contrib_class1, base_value_class1, feat_contrib_class2, base_value_class2, etc.]
+    # * contrib outputs for distributed training are different than from local training, so we can just test
+    #   that the output has the right shape and base values are in the right position
+    assert preds_with_contrib.shape[1] == expected_num_cols
+    assert preds_with_contrib.shape == local_preds_with_contrib.shape
+
+    if num_classes == 2:
+        assert len(np.unique(preds_with_contrib[:, num_features]) == 1)
+    else:
+        for i in range(num_classes):
+            base_value_col = num_features * (i + 1) + i
+            assert len(np.unique(preds_with_contrib[:, base_value_col]) == 1)
+
+
 def test_training_does_not_fail_on_port_conflicts(client):
     _, _, _, dX, dy, dw = _create_data('classification', output='array')
 
@@ -315,6 +364,37 @@ def test_regressor(output, client, listen_port):
     client.close()
 
 
+@pytest.mark.parametrize('output', data_output)
+def test_regressor_pred_contrib(output, client, listen_port):
+    X, y, w, dX, dy, dw = _create_data('regression', output=output)
+
+    dask_regressor = dlgbm.DaskLGBMRegressor(
+        time_out=5,
+        local_listen_port=listen_port,
+        tree_learner='data',
+        n_estimators=10,
+        num_leaves=10
+    )
+    dask_regressor = dask_regressor.fit(dX, dy, sample_weight=dw, client=client)
+    preds_with_contrib = dask_regressor.predict(dX, pred_contrib=True).compute()
+
+    local_regressor = lightgbm.LGBMRegressor(
+        n_estimators=10,
+        num_leaves=10
+    )
+    local_regressor.fit(X, y, sample_weight=w)
+    local_preds_with_contrib = local_regressor.predict(X, pred_contrib=True)
+
+    if output == "scipy_csr_matrix":
+        preds_with_contrib = np.array(preds_with_contrib.todense())
+
+    # contrib outputs for distributed training are different than from local training, so we can just test
+    # that the output has the right shape and base values are in the right position
+    num_features = dX.shape[1]
+    assert preds_with_contrib.shape[1] == num_features + 1
+    assert preds_with_contrib.shape == local_preds_with_contrib.shape
+
+
 @pytest.mark.parametrize('output', data_output)
 @pytest.mark.parametrize('alpha', [.1, .5, .9])
 def test_regressor_quantile(output, client, listen_port, alpha):