Use pytest conventions consistently (#6337)

* Do not derive from unittest.TestCase (not needed for pytest)

* assertRaises -> pytest.raises

* Simplify test_empty_dmatrix with test parametrization

* setUpClass -> setup_class, tearDownClass -> teardown_class

* Don't import unittest; import pytest

* Use plain assert

* Use parametrized tests in more places

* Fix test_gpu_with_sklearn.py

* Put back run_empty_dmatrix_reg / run_empty_dmatrix_cls

* Fix test_eta_decay_gpu_hist

* Add parametrized tests for monotone constraints

* Fix test names

* Remove test parametrization

* Revise test_slice to be not flaky

tests/python/test_basic.py

@@ -2,7 +2,7 @@
 import numpy as np
 import os
 import xgboost as xgb
-import unittest
+import pytest
 import json
 from pathlib import Path
 import tempfile
@@ -12,7 +12,7 @@ dpath = 'demo/data/'
 rng = np.random.RandomState(1994)
 
 
-class TestBasic(unittest.TestCase):
+class TestBasic:
     def test_compat(self):
         from xgboost.compat import lazy_isinstance
         a = np.array([1, 2, 3])
@@ -119,30 +119,28 @@ class TestBasic(unittest.TestCase):
 
         # number of feature importances should == number of features
         dump1 = bst.get_dump()
-        self.assertEqual(len(dump1), 1, "Expected only 1 tree to be dumped.")
-        self.assertEqual(len(dump1[0].splitlines()), 3,
-                         "Expected 1 root and 2 leaves - 3 lines in dump.")
+        assert len(dump1) == 1, 'Expected only 1 tree to be dumped.'
+        len(dump1[0].splitlines()) == 3, 'Expected 1 root and 2 leaves - 3 lines in dump.'
 
         dump2 = bst.get_dump(with_stats=True)
-        self.assertEqual(dump2[0].count('\n'), 3,
-                         "Expected 1 root and 2 leaves - 3 lines in dump.")
-        self.assertGreater(dump2[0].find('\n'), dump1[0].find('\n'),
-                           "Expected more info when with_stats=True is given.")
+        assert dump2[0].count('\n') == 3, 'Expected 1 root and 2 leaves - 3 lines in dump.'
+        assert (dump2[0].find('\n') > dump1[0].find('\n'),
+                'Expected more info when with_stats=True is given.')
 
         dump3 = bst.get_dump(dump_format="json")
         dump3j = json.loads(dump3[0])
-        self.assertEqual(dump3j["nodeid"], 0, "Expected the root node on top.")
+        assert dump3j['nodeid'] == 0, 'Expected the root node on top.'
 
         dump4 = bst.get_dump(dump_format="json", with_stats=True)
         dump4j = json.loads(dump4[0])
-        self.assertIn("gain", dump4j, "Expected 'gain' to be dumped in JSON.")
+        assert 'gain' in dump4j, "Expected 'gain' to be dumped in JSON."
 
     def test_load_file_invalid(self):
-        self.assertRaises(xgb.core.XGBoostError, xgb.Booster,
-                          model_file='incorrect_path')
+        with pytest.raises(xgb.core.XGBoostError):
+            xgb.Booster(model_file='incorrect_path')
 
-        self.assertRaises(xgb.core.XGBoostError, xgb.Booster,
-                          model_file=u'不正なパス')
+        with pytest.raises(xgb.core.XGBoostError):
+            xgb.Booster(model_file=u'不正なパス')
 
     def test_dmatrix_numpy_init_omp(self):
 
@@ -226,7 +224,7 @@ class TestBasic(unittest.TestCase):
         assert output == solution
 
 
-class TestBasicPathLike(unittest.TestCase):
+class TestBasicPathLike:
     """Unit tests using pathlib.Path for file interaction."""
 
     def test_DMatrix_init_from_path(self):
@@ -253,8 +251,8 @@ class TestBasicPathLike(unittest.TestCase):
 
     def test_Booster_init_invalid_path(self):
         """An invalid model_file path should raise XGBoostError."""
-        self.assertRaises(xgb.core.XGBoostError, xgb.Booster,
-                          model_file=Path("invalidpath"))
+        with pytest.raises(xgb.core.XGBoostError):
+            xgb.Booster(model_file=Path("invalidpath"))
 
 
     def test_Booster_save_and_load(self):