[dask] Accept other inputs for prediction. (#5428)

* Returns a series when input is dataframe.

* Merge assert client.

python-package/xgboost/dask.py

@@ -103,6 +103,9 @@ def concat(value):
 
 def _xgb_get_client(client):
     '''Simple wrapper around testing None.'''
+    if not isinstance(client, (type(get_client()), type(None))):
+        raise TypeError(
+            _expect([type(get_client()), type(None)], type(client)))
     ret = get_client() if client is None else client
     return ret
 
@@ -112,12 +115,6 @@ def _get_client_workers(client):
     return workers
 
 
-def _assert_client(client):
-    if not isinstance(client, (type(get_client()), type(None))):
-        raise TypeError(
-            _expect([type(get_client()), type(None)], type(client)))
-
-
 class DaskDMatrix:
     # pylint: disable=missing-docstring, too-many-instance-attributes
     '''DMatrix holding on references to Dask DataFrame or Dask Array.  Constructing
@@ -155,7 +152,7 @@ class DaskDMatrix:
                  feature_names=None,
                  feature_types=None):
         _assert_dask_support()
-        _assert_client(client)
+        client = _xgb_get_client(client)
 
         self.feature_names = feature_names
         self.feature_types = feature_types
@@ -177,7 +174,6 @@ class DaskDMatrix:
         self.has_label = label is not None
         self.has_weights = weight is not None
 
-        client = _xgb_get_client(client)
         client.sync(self.map_local_data, client, data, label, weight)
 
     async def map_local_data(self, client, data, label=None, weights=None):
@@ -391,13 +387,12 @@ def train(client, params, dtrain, *args, evals=(), **kwargs):
 
     '''
     _assert_dask_support()
-    _assert_client(client)
+    client = _xgb_get_client(client)
     if 'evals_result' in kwargs.keys():
         raise ValueError(
             'evals_result is not supported in dask interface.',
             'The evaluation history is returned as result of training.')
 
-    client = _xgb_get_client(client)
     workers = list(_get_client_workers(client).keys())
 
     rabit_args = _get_rabit_args(workers, client)
@@ -452,7 +447,7 @@ def train(client, params, dtrain, *args, evals=(), **kwargs):
     return list(filter(lambda ret: ret is not None, results))[0]
 
 
-def predict(client, model, data, *args):
+def predict(client, model, data, *args, missing=numpy.nan):
     '''Run prediction with a trained booster.
 
     .. note::
@@ -466,32 +461,55 @@ def predict(client, model, data, *args):
         returned from dask if it's set to None.
     model: A Booster or a dictionary returned by `xgboost.dask.train`.
         The trained model.
-    data: DaskDMatrix
+    data: DaskDMatrix/dask.dataframe.DataFrame/dask.array.Array
         Input data used for prediction.
+    missing: float
+        Used when input data is not DaskDMatrix.  Specify the value
+        considered as missing.
 
     Returns
     -------
-    prediction: dask.array.Array
+    prediction: dask.array.Array/dask.dataframe.Series
 
     '''
     _assert_dask_support()
-    _assert_client(client)
+    client = _xgb_get_client(client)
     if isinstance(model, Booster):
         booster = model
     elif isinstance(model, dict):
         booster = model['booster']
     else:
         raise TypeError(_expect([Booster, dict], type(model)))
+    if not isinstance(data, (DaskDMatrix, da.Array, dd.DataFrame)):
+        raise TypeError(_expect([DaskDMatrix, da.Array, dd.DataFrame],
+                                type(data)))
 
-    if not isinstance(data, DaskDMatrix):
-        raise TypeError(_expect([DaskDMatrix], type(data)))
+    def mapped_predict(partition, is_df):
+        worker = distributed_get_worker()
+        m = DMatrix(partition, missing=missing, nthread=worker.nthreads)
+        predt = booster.predict(m, *args, validate_features=False)
+        if is_df:
+            predt = DataFrame(predt, columns=['prediction'])
+        return predt
 
+    if isinstance(data, da.Array):
+        predictions = client.submit(
+            da.map_blocks,
+            mapped_predict, data, False, drop_axis=1,
+            dtype=numpy.float32
+        ).result()
+        return predictions
+    if isinstance(data, dd.DataFrame):
+        import dask
+        predictions = client.submit(
+            dd.map_partitions,
+            mapped_predict, data, True,
+            meta=dask.dataframe.utils.make_meta({'prediction': 'f4'})
+        ).result()
+        return predictions.iloc[:, 0]
+
+    # Prediction on dask DMatrix.
     worker_map = data.worker_map
-    client = _xgb_get_client(client)
-
-    missing = data.missing
-    feature_names = data.feature_names
-    feature_types = data.feature_types
 
     def dispatched_predict(worker_id):
         '''Perform prediction on each worker.'''
@@ -502,9 +520,9 @@ def predict(client, model, data, *args):
         booster.set_param({'nthread': worker.nthreads})
         for part, order in list_of_parts:
             local_x = DMatrix(part,
-                              feature_names=feature_names,
-                              feature_types=feature_types,
-                              missing=missing,
+                              feature_names=data.feature_names,
+                              feature_types=data.feature_types,
+                              missing=data.missing,
                               nthread=worker.nthreads)
             predt = booster.predict(data=local_x,
                                     validate_features=local_x.num_row() != 0,
@@ -520,8 +538,7 @@ def predict(client, model, data, *args):
         list_of_parts = data.get_worker_x_ordered(worker)
         shapes = []
         for part, order in list_of_parts:
-            s = part.shape
-            shapes.append((s, order))
+            shapes.append((part.shape, order))
         return shapes
 
     def map_function(func):

tests/python/test_with_dask.py

@@ -57,7 +57,13 @@ def test_from_dask_dataframe():
                 xgb.dask.train(
                     client, {}, dtrain, num_boost_round=2, evals_result={})
             # force prediction to be computed
-            prediction = prediction.compute()
+            from_dmatrix = prediction.compute()
+
+            prediction = xgb.dask.predict(client, model=booster, data=X)
+            from_df = prediction.compute()
+
+            assert isinstance(prediction, dd.Series)
+            assert np.all(from_dmatrix == from_df.to_numpy())
 
 
 def test_from_dask_array():
@@ -84,6 +90,12 @@ def test_from_dask_array():
             config = json.loads(booster.save_config())
             assert int(config['learner']['generic_param']['nthread']) == 5
 
+            from_arr = xgb.dask.predict(
+                client, model=booster, data=X)
+
+            assert isinstance(from_arr, da.Array)
+            assert np.all(single_node_predt == from_arr.compute())
+
 
 def test_dask_regressor():
     with LocalCluster(n_workers=5) as cluster: