[dask] remove unused private _client attribute (#3904)

* Update test_dask.py

* Update dask.py

* Update .vsts-ci.yml

* Revert "Update .vsts-ci.yml"

This reverts commit 98422be.

python-package/lightgbm/dask.py

@@ -468,11 +468,9 @@ def client_(self) -> Client:
     def _lgb_getstate(self) -> Dict[Any, Any]:
         """Remove un-picklable attributes before serialization."""
         client = self.__dict__.pop("client", None)
-        self.__dict__.pop("_client", None)
         self._other_params.pop("client", None)
         out = deepcopy(self.__dict__)
-        out.update({"_client": None, "client": None})
-        self._client = client
+        out.update({"client": None})
         self.client = client
         return out
 
@@ -521,8 +519,7 @@ def _copy_extra_params(source: Union["_DaskLGBMModel", LGBMModel], dest: Union["
         attributes = source.__dict__
         extra_param_names = set(attributes.keys()).difference(params.keys())
         for name in extra_param_names:
-            if name != "_client":
-                setattr(dest, name, attributes[name])
+            setattr(dest, name, attributes[name])
 
 
 class DaskLGBMClassifier(LGBMClassifier, _DaskLGBMModel):
@@ -554,7 +551,6 @@ def __init__(
         **kwargs: Any
     ):
         """Docstring is inherited from the lightgbm.LGBMClassifier.__init__."""
-        self._client = client
         self.client = client
         super().__init__(
             boosting_type=boosting_type,
@@ -672,7 +668,6 @@ def __init__(
         **kwargs: Any
     ):
         """Docstring is inherited from the lightgbm.LGBMRegressor.__init__."""
-        self._client = client
         self.client = client
         super().__init__(
             boosting_type=boosting_type,
@@ -779,7 +774,6 @@ def __init__(
         **kwargs: Any
     ):
         """Docstring is inherited from the lightgbm.LGBMRanker.__init__."""
-        self._client = client
         self.client = client
         super().__init__(
             boosting_type=boosting_type,

tests/python_package_test/test_dask.py

@@ -2,7 +2,6 @@
 """Tests for lightgbm.dask module"""
 
 import inspect
-import joblib
 import pickle
 import socket
 from itertools import groupby
@@ -19,6 +18,7 @@
 import cloudpickle
 import dask.array as da
 import dask.dataframe as dd
+import joblib
 import numpy as np
 import pandas as pd
 from scipy.stats import spearmanr
@@ -488,56 +488,47 @@ def test_training_works_if_client_not_provided_or_set_after_construction(task, l
 
     # should be able to use the class without specifying a client
     dask_model = model_factory(**params)
-    assert dask_model._client is None
     assert dask_model.client is None
     with pytest.raises(lgb.compat.LGBMNotFittedError, match='Cannot access property client_ before calling fit'):
         dask_model.client_
 
     dask_model.fit(dX, dy, group=dg)
     assert dask_model.fitted_
-    assert dask_model._client is None
     assert dask_model.client is None
     assert dask_model.client_ == client
 
     preds = dask_model.predict(dX)
     assert isinstance(preds, da.Array)
     assert dask_model.fitted_
-    assert dask_model._client is None
     assert dask_model.client is None
     assert dask_model.client_ == client
 
     local_model = dask_model.to_local()
     with pytest.raises(AttributeError):
-        local_model._client
         local_model.client
         local_model.client_
 
     # should be able to set client after construction
     dask_model = model_factory(**params)
     dask_model.set_params(client=client)
-    assert dask_model._client == client
     assert dask_model.client == client
 
     with pytest.raises(lgb.compat.LGBMNotFittedError, match='Cannot access property client_ before calling fit'):
         dask_model.client_
 
     dask_model.fit(dX, dy, group=dg)
     assert dask_model.fitted_
-    assert dask_model._client == client
     assert dask_model.client == client
     assert dask_model.client_ == client
 
     preds = dask_model.predict(dX)
     assert isinstance(preds, da.Array)
     assert dask_model.fitted_
-    assert dask_model._client == client
     assert dask_model.client == client
     assert dask_model.client_ == client
 
     local_model = dask_model.to_local()
-    assert getattr(local_model, "_client", None) is None
     with pytest.raises(AttributeError):
-        local_model._client
         local_model.client
         local_model.client_
 
@@ -606,10 +597,8 @@ def test_model_and_local_version_are_picklable_whether_or_not_client_set_explici
                     dask_model = model_factory(**params)
                     local_model = dask_model.to_local()
                     if set_client:
-                        assert dask_model._client == client1
                         assert dask_model.client == client1
                     else:
-                        assert dask_model._client is None
                         assert dask_model.client is None
 
                     with pytest.raises(lgb.compat.LGBMNotFittedError, match='Cannot access property client_ before calling fit'):
@@ -640,14 +629,11 @@ def test_model_and_local_version_are_picklable_whether_or_not_client_set_explici
                         serializer=serializer
                     )
 
-                    assert model_from_disk._client is None
                     assert model_from_disk.client is None
 
                     if set_client:
-                        assert dask_model._client == client1
                         assert dask_model.client == client1
                     else:
-                        assert dask_model._client is None
                         assert dask_model.client is None
 
                     with pytest.raises(lgb.compat.LGBMNotFittedError, match='Cannot access property client_ before calling fit'):
@@ -674,7 +660,6 @@ def test_model_and_local_version_are_picklable_whether_or_not_client_set_explici
 
                     assert "client" not in local_model.get_params()
                     with pytest.raises(AttributeError):
-                        local_model._client
                         local_model.client
                         local_model.client_
 
@@ -701,17 +686,14 @@ def test_model_and_local_version_are_picklable_whether_or_not_client_set_explici
                     )
 
                     if set_client:
-                        assert dask_model._client == client1
                         assert dask_model.client == client1
                         assert dask_model.client_ == client1
                     else:
-                        assert dask_model._client is None
                         assert dask_model.client is None
                         assert dask_model.client_ == default_client()
                         assert dask_model.client_ == client2
 
                     assert isinstance(fitted_model_from_disk, model_factory)
-                    assert fitted_model_from_disk._client is None
                     assert fitted_model_from_disk.client is None
                     assert fitted_model_from_disk.client_ == default_client()
                     assert fitted_model_from_disk.client_ == client2