From c6769488b3a615092b60ba351a60c07b95ec85df Mon Sep 17 00:00:00 2001
From: Jiaming Yuan <jm.yuan@outlook.com>
Date: Thu, 28 Oct 2021 20:47:04 +0800
Subject: [PATCH] Typehint for subset of core API. (#7348)

---
 python-package/xgboost/callback.py           | 197 +++++++++-------
 python-package/xgboost/core.py               | 226 ++++++++++---------
 python-package/xgboost/dask.py               | 105 ++++-----
 python-package/xgboost/sklearn.py            | 171 +++++---------
 python-package/xgboost/tracker.py            |   2 +-
 python-package/xgboost/training.py           | 107 +++++----
 tests/python/test_interaction_constraints.py |   4 +-
 7 files changed, 400 insertions(+), 412 deletions(-)

diff --git a/python-package/xgboost/callback.py b/python-package/xgboost/callback.py
index 7552db79d..53a1c4e9a 100644
--- a/python-package/xgboost/callback.py
+++ b/python-package/xgboost/callback.py
@@ -6,17 +6,18 @@ from abc import ABC
 import collections
 import os
 import pickle
-from typing import Callable, List, Optional, Union, Dict, Tuple
+from typing import Callable, List, Optional, Union, Dict, Tuple, TypeVar, cast
+from typing import Sequence
 import numpy
 
 from . import rabit
-from .core import Booster, XGBoostError
+from .core import Booster, DMatrix, XGBoostError
 from .compat import STRING_TYPES
 
 
-# The new implementation of callback functions.
-# Breaking:
-# - reset learning rate no longer accepts total boosting rounds
+_Score = Union[float, Tuple[float, float]]
+_ScoreList = Union[List[float], List[Tuple[float, float]]]
+
 
 # pylint: disable=unused-argument
 class TrainingCallback(ABC):
@@ -26,9 +27,9 @@ class TrainingCallback(ABC):
 
     '''
 
-    EvalsLog = Dict[str, Dict[str, Union[List[float], List[Tuple[float, float]]]]]
+    EvalsLog = Dict[str, Dict[str, _ScoreList]]
 
-    def __init__(self):
+    def __init__(self) -> None:
         pass
 
     def before_training(self, model):
@@ -48,18 +49,18 @@ class TrainingCallback(ABC):
         return False
 
 
-def _aggcv(rlist):
-    # pylint: disable=invalid-name
+def _aggcv(rlist: List[str]) -> List[Tuple[str, float, float]]:
+    # pylint: disable=invalid-name, too-many-locals
     """Aggregate cross-validation results.
 
     """
-    cvmap = {}
+    cvmap: Dict[Tuple[int, str], List[float]] = {}
     idx = rlist[0].split()[0]
     for line in rlist:
-        arr = line.split()
+        arr: List[str] = line.split()
         assert idx == arr[0]
         for metric_idx, it in enumerate(arr[1:]):
-            if not isinstance(it, STRING_TYPES):
+            if not isinstance(it, str):
                 it = it.decode()
             k, v = it.split(':')
             if (metric_idx, k) not in cvmap:
@@ -67,16 +68,20 @@ def _aggcv(rlist):
             cvmap[(metric_idx, k)].append(float(v))
     msg = idx
     results = []
-    for (metric_idx, k), v in sorted(cvmap.items(), key=lambda x: x[0][0]):
-        v = numpy.array(v)
+    for (_, name), s in sorted(cvmap.items(), key=lambda x: x[0][0]):
+        as_arr = numpy.array(s)
         if not isinstance(msg, STRING_TYPES):
             msg = msg.decode()
-        mean, std = numpy.mean(v), numpy.std(v)
-        results.extend([(k, mean, std)])
+        mean, std = numpy.mean(as_arr), numpy.std(as_arr)
+        results.extend([(name, mean, std)])
     return results
 
 
-def _allreduce_metric(score):
+# allreduce type
+_ART = TypeVar("_ART")
+
+
+def _allreduce_metric(score: _ART) -> _ART:
     '''Helper function for computing customized metric in distributed
     environment.  Not strictly correct as many functions don't use mean value
     as final result.
@@ -89,13 +94,13 @@ def _allreduce_metric(score):
     if isinstance(score, tuple):  # has mean and stdv
         raise ValueError(
             'xgboost.cv function should not be used in distributed environment.')
-    score = numpy.array([score])
-    score = rabit.allreduce(score, rabit.Op.SUM) / world
-    return score[0]
+    arr = numpy.array([score])
+    arr = rabit.allreduce(arr, rabit.Op.SUM) / world
+    return arr[0]
 
 
 class CallbackContainer:
-    '''A special callback for invoking a list of other callbacks.
+    '''A special internal callback for invoking a list of other callbacks.
 
     .. versionadded:: 1.3.0
 
@@ -105,7 +110,7 @@ class CallbackContainer:
 
     def __init__(
         self,
-        callbacks: List[TrainingCallback],
+        callbacks: Sequence[TrainingCallback],
         metric: Callable = None,
         output_margin: bool = True,
         is_cv: bool = False
@@ -146,33 +151,50 @@ class CallbackContainer:
                 assert isinstance(model, Booster), msg
         return model
 
-    def before_iteration(self, model, epoch, dtrain, evals) -> bool:
+    def before_iteration(
+        self, model, epoch: int, dtrain: DMatrix, evals: List[Tuple[DMatrix, str]]
+    ) -> bool:
         '''Function called before training iteration.'''
         return any(c.before_iteration(model, epoch, self.history)
                    for c in self.callbacks)
 
-    def _update_history(self, score, epoch):
+    def _update_history(
+        self,
+        score: Union[List[Tuple[str, float]], List[Tuple[str, float, float]]],
+        epoch: int
+    ) -> None:
         for d in score:
-            name, s = d[0], float(d[1])
+            name: str = d[0]
+            s: float = d[1]
             if self.is_cv:
-                std = float(d[2])
-                s = (s, std)
+                std = float(cast(Tuple[str, float, float], d)[2])
+                x: _Score = (s, std)
+            else:
+                x = s
             splited_names = name.split('-')
             data_name = splited_names[0]
             metric_name = '-'.join(splited_names[1:])
-            s = _allreduce_metric(s)
-            if data_name in self.history:
-                data_history = self.history[data_name]
-                if metric_name in data_history:
-                    data_history[metric_name].append(s)
-                else:
-                    data_history[metric_name] = [s]
-            else:
+            x = _allreduce_metric(x)
+            if data_name not in self.history:
                 self.history[data_name] = collections.OrderedDict()
-                self.history[data_name][metric_name] = [s]
-        return False
+            data_history = self.history[data_name]
+            if metric_name not in data_history:
+                data_history[metric_name] = cast(_ScoreList, [])
+            metric_history = data_history[metric_name]
+            if self.is_cv:
+                cast(List[Tuple[float, float]], metric_history).append(
+                    cast(Tuple[float, float], x)
+                )
+            else:
+                cast(List[float], metric_history).append(cast(float, x))
 
-    def after_iteration(self, model, epoch, dtrain, evals) -> bool:
+    def after_iteration(
+        self,
+        model,
+        epoch: int,
+        dtrain: DMatrix,
+        evals: Optional[List[Tuple[DMatrix, str]]],
+    ) -> bool:
         '''Function called after training iteration.'''
         if self.is_cv:
             scores = model.eval(epoch, self.metric, self._output_margin)
@@ -183,18 +205,20 @@ class CallbackContainer:
             evals = [] if evals is None else evals
             for _, name in evals:
                 assert name.find('-') == -1, 'Dataset name should not contain `-`'
-            score = model.eval_set(evals, epoch, self.metric, self._output_margin)
-            score = score.split()[1:]  # into datasets
+            score: str = model.eval_set(evals, epoch, self.metric, self._output_margin)
+            splited = score.split()[1:]  # into datasets
             # split up `test-error:0.1234`
-            score = [tuple(s.split(':')) for s in score]
-            self._update_history(score, epoch)
+            metric_score_str = [tuple(s.split(':')) for s in splited]
+            # convert to float
+            metric_score = [(n, float(s)) for n, s in metric_score_str]
+            self._update_history(metric_score, epoch)
         ret = any(c.after_iteration(model, epoch, self.history)
                   for c in self.callbacks)
         return ret
 
 
 class LearningRateScheduler(TrainingCallback):
-    '''Callback function for scheduling learning rate.
+    """Callback function for scheduling learning rate.
 
     .. versionadded:: 1.3.0
 
@@ -207,18 +231,24 @@ class LearningRateScheduler(TrainingCallback):
         should be a sequence like list or tuple with the same size of boosting
         rounds.
 
-    '''
-    def __init__(self, learning_rates) -> None:
-        assert callable(learning_rates) or \
-            isinstance(learning_rates, collections.abc.Sequence)
+    """
+
+    def __init__(
+        self, learning_rates: Union[Callable[[int], float], Sequence[float]]
+    ) -> None:
+        assert callable(learning_rates) or isinstance(
+            learning_rates, collections.abc.Sequence
+        )
         if callable(learning_rates):
             self.learning_rates = learning_rates
         else:
-            self.learning_rates = lambda epoch: learning_rates[epoch]
+            self.learning_rates = lambda epoch: cast(Sequence, learning_rates)[epoch]
         super().__init__()
 
-    def after_iteration(self, model, epoch, evals_log) -> bool:
-        model.set_param('learning_rate', self.learning_rates(epoch))
+    def after_iteration(
+        self, model, epoch: int, evals_log: TrainingCallback.EvalsLog
+    ) -> bool:
+        model.set_param("learning_rate", self.learning_rates(epoch))
         return False
 
 
@@ -230,17 +260,17 @@ class EarlyStopping(TrainingCallback):
 
     Parameters
     ----------
-    rounds
+    rounds :
         Early stopping rounds.
-    metric_name
+    metric_name :
         Name of metric that is used for early stopping.
-    data_name
+    data_name :
         Name of dataset that is used for early stopping.
-    maximize
+    maximize :
         Whether to maximize evaluation metric.  None means auto (discouraged).
-    save_best
+    save_best :
         Whether training should return the best model or the last model.
-    min_delta
+    min_delta :
         Minimum absolute change in score to be qualified as an improvement.
 
         .. versionadded:: 1.5.0
@@ -279,8 +309,6 @@ class EarlyStopping(TrainingCallback):
         if self._min_delta < 0:
             raise ValueError("min_delta must be greater or equal to 0.")
 
-        self.improve_op = None
-
         self.current_rounds: int = 0
         self.best_scores: dict = {}
         self.starting_round: int = 0
@@ -290,16 +318,18 @@ class EarlyStopping(TrainingCallback):
         self.starting_round = model.num_boosted_rounds()
         return model
 
-    def _update_rounds(self, score, name, metric, model, epoch) -> bool:
-        def get_s(x):
+    def _update_rounds(
+        self, score: _Score, name: str, metric: str, model, epoch: int
+    ) -> bool:
+        def get_s(x: _Score) -> float:
             """get score if it's cross validation history."""
             return x[0] if isinstance(x, tuple) else x
 
-        def maximize(new, best):
+        def maximize(new: _Score, best: _Score) -> bool:
             """New score should be greater than the old one."""
             return numpy.greater(get_s(new) - self._min_delta, get_s(best))
 
-        def minimize(new, best):
+        def minimize(new: _Score, best: _Score) -> bool:
             """New score should be smaller than the old one."""
             return numpy.greater(get_s(best) - self._min_delta, get_s(new))
 
@@ -314,25 +344,25 @@ class EarlyStopping(TrainingCallback):
                 self.maximize = False
 
         if self.maximize:
-            self.improve_op = maximize
+            improve_op = maximize
         else:
-            self.improve_op = minimize
+            improve_op = minimize
 
-        assert self.improve_op
+        assert improve_op
 
         if not self.stopping_history:  # First round
             self.current_rounds = 0
             self.stopping_history[name] = {}
-            self.stopping_history[name][metric] = [score]
+            self.stopping_history[name][metric] = cast(_ScoreList, [score])
             self.best_scores[name] = {}
             self.best_scores[name][metric] = [score]
             model.set_attr(best_score=str(score), best_iteration=str(epoch))
-        elif not self.improve_op(score, self.best_scores[name][metric][-1]):
+        elif not improve_op(score, self.best_scores[name][metric][-1]):
             # Not improved
-            self.stopping_history[name][metric].append(score)
+            self.stopping_history[name][metric].append(score)  # type: ignore
             self.current_rounds += 1
         else:  # Improved
-            self.stopping_history[name][metric].append(score)
+            self.stopping_history[name][metric].append(score)  # type: ignore
             self.best_scores[name][metric].append(score)
             record = self.stopping_history[name][metric][-1]
             model.set_attr(best_score=str(record), best_iteration=str(epoch))
@@ -390,16 +420,16 @@ class EvaluationMonitor(TrainingCallback):
     Parameters
     ----------
 
-    metric : callable
+    metric :
         Extra user defined metric.
-    rank : int
+    rank :
         Which worker should be used for printing the result.
-    period : int
+    period :
         How many epoches between printing.
-    show_stdv : bool
+    show_stdv :
         Used in cv to show standard deviation.  Users should not specify it.
     '''
-    def __init__(self, rank=0, period=1, show_stdv=False) -> None:
+    def __init__(self, rank: int = 0, period: int = 1, show_stdv: bool = False) -> None:
         self.printer_rank = rank
         self.show_stdv = show_stdv
         self.period = period
@@ -457,22 +487,27 @@ class TrainingCheckPoint(TrainingCallback):
     Parameters
     ----------
 
-    directory : os.PathLike
+    directory :
         Output model directory.
-    name : str
+    name :
         pattern of output model file.  Models will be saved as name_0.json, name_1.json,
         name_2.json ....
-    as_pickle : boolean
+    as_pickle :
         When set to Ture, all training parameters will be saved in pickle format, instead
         of saving only the model.
-    iterations : int
+    iterations :
         Interval of checkpointing.  Checkpointing is slow so setting a larger number can
         reduce performance hit.
 
     '''
-    def __init__(self, directory: os.PathLike, name: str = 'model',
-                 as_pickle=False, iterations: int = 100):
-        self._path = directory
+    def __init__(
+        self,
+        directory: Union[str, os.PathLike],
+        name: str = 'model',
+        as_pickle: bool = False,
+        iterations: int = 100
+    ) -> None:
+        self._path = os.fspath(directory)
         self._name = name
         self._as_pickle = as_pickle
         self._iterations = iterations
diff --git a/python-package/xgboost/core.py b/python-package/xgboost/core.py
index e53fdb21d..594ed74bb 100644
--- a/python-package/xgboost/core.py
+++ b/python-package/xgboost/core.py
@@ -6,7 +6,7 @@
 from collections.abc import Mapping
 from typing import List, Optional, Any, Union, Dict, TypeVar
 # pylint: enable=no-name-in-module,import-error
-from typing import Callable, Tuple, cast
+from typing import Callable, Tuple, cast, Sequence
 import ctypes
 import os
 import re
@@ -31,20 +31,6 @@ class XGBoostError(ValueError):
     """Error thrown by xgboost trainer."""
 
 
-class EarlyStopException(Exception):
-    """Exception to signal early stopping.
-
-    Parameters
-    ----------
-    best_iteration : int
-        The best iteration stopped.
-    """
-
-    def __init__(self, best_iteration):
-        super().__init__()
-        self.best_iteration = best_iteration
-
-
 def from_pystr_to_cstr(data: Union[str, List[str]]):
     """Convert a Python str or list of Python str to C pointer
 
@@ -132,18 +118,19 @@ def _log_callback(msg: bytes) -> None:
     print(py_str(msg))
 
 
-def _get_log_callback_func():
+def _get_log_callback_func() -> Callable:
     """Wrap log_callback() method in ctypes callback type"""
     # pylint: disable=invalid-name
     CALLBACK = ctypes.CFUNCTYPE(None, ctypes.c_char_p)
     return CALLBACK(_log_callback)
 
 
-def _load_lib():
+def _load_lib() -> ctypes.CDLL:
     """Load xgboost Library."""
     lib_paths = find_lib_path()
     if not lib_paths:
-        return None
+        # This happens only when building document.
+        return None             # type: ignore
     try:
         pathBackup = os.environ['PATH'].split(os.pathsep)
     except KeyError:
@@ -190,7 +177,7 @@ Error message(s): {os_error_list}
 _LIB = _load_lib()
 
 
-def _check_call(ret):
+def _check_call(ret: int) -> None:
     """Check the return value of C API call
 
     This function will raise exception when error occurs.
@@ -234,7 +221,7 @@ def _cuda_array_interface(data) -> bytes:
     return interface_str
 
 
-def ctypes2numpy(cptr, length, dtype):
+def ctypes2numpy(cptr, length, dtype) -> np.ndarray:
     """Convert a ctypes pointer array to a numpy array."""
     ctype = _numpy2ctypes_type(dtype)
     if not isinstance(cptr, ctypes.POINTER(ctype)):
@@ -271,7 +258,7 @@ def ctypes2cupy(cptr, length, dtype):
     return arr
 
 
-def ctypes2buffer(cptr, length):
+def ctypes2buffer(cptr, length) -> bytearray:
     """Convert ctypes pointer to buffer type."""
     if not isinstance(cptr, ctypes.POINTER(ctypes.c_char)):
         raise RuntimeError('expected char pointer')
@@ -428,7 +415,7 @@ class DataIter:  # pylint: disable=too-many-instance-attributes
         Parameters
         ----------
 
-        data_handle:
+        input_data:
             A function with same data fields like `data`, `label` with
             `xgboost.DMatrix`.
 
@@ -627,7 +614,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         if feature_types is not None:
             self.feature_types = feature_types
 
-    def _init_from_iter(self, iterator: DataIter, enable_categorical: bool):
+    def _init_from_iter(self, iterator: DataIter, enable_categorical: bool) -> None:
         it = iterator
         args = {
             "missing": self.missing,
@@ -654,7 +641,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         _check_call(ret)
         self.handle = handle
 
-    def __del__(self):
+    def __del__(self) -> None:
         if hasattr(self, "handle") and self.handle:
             _check_call(_LIB.XGDMatrixFree(self.handle))
             self.handle = None
@@ -699,7 +686,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
             dispatch_meta_backend(matrix=self, data=feature_weights,
                                   name='feature_weights')
 
-    def get_float_info(self, field):
+    def get_float_info(self, field: str) -> np.ndarray:
         """Get float property from the DMatrix.
 
         Parameters
@@ -720,7 +707,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
                                                ctypes.byref(ret)))
         return ctypes2numpy(ret, length.value, np.float32)
 
-    def get_uint_info(self, field):
+    def get_uint_info(self, field: str) -> np.ndarray:
         """Get unsigned integer property from the DMatrix.
 
         Parameters
@@ -741,7 +728,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
                                               ctypes.byref(ret)))
         return ctypes2numpy(ret, length.value, np.uint32)
 
-    def set_float_info(self, field, data):
+    def set_float_info(self, field: str, data) -> None:
         """Set float type property into the DMatrix.
 
         Parameters
@@ -755,7 +742,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         from .data import dispatch_meta_backend
         dispatch_meta_backend(self, data, field, 'float')
 
-    def set_float_info_npy2d(self, field, data):
+    def set_float_info_npy2d(self, field: str, data) -> None:
         """Set float type property into the DMatrix
            for numpy 2d array input
 
@@ -770,7 +757,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         from .data import dispatch_meta_backend
         dispatch_meta_backend(self, data, field, 'float')
 
-    def set_uint_info(self, field, data):
+    def set_uint_info(self, field: str, data) -> None:
         """Set uint type property into the DMatrix.
 
         Parameters
@@ -784,7 +771,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         from .data import dispatch_meta_backend
         dispatch_meta_backend(self, data, field, 'uint32')
 
-    def save_binary(self, fname, silent=True):
+    def save_binary(self, fname, silent=True) -> None:
         """Save DMatrix to an XGBoost buffer.  Saved binary can be later loaded
         by providing the path to :py:func:`xgboost.DMatrix` as input.
 
@@ -800,7 +787,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
                                              c_str(fname),
                                              ctypes.c_int(silent)))
 
-    def set_label(self, label):
+    def set_label(self, label) -> None:
         """Set label of dmatrix
 
         Parameters
@@ -811,7 +798,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         from .data import dispatch_meta_backend
         dispatch_meta_backend(self, label, 'label', 'float')
 
-    def set_weight(self, weight):
+    def set_weight(self, weight) -> None:
         """Set weight of each instance.
 
         Parameters
@@ -830,7 +817,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         from .data import dispatch_meta_backend
         dispatch_meta_backend(self, weight, 'weight', 'float')
 
-    def set_base_margin(self, margin):
+    def set_base_margin(self, margin) -> None:
         """Set base margin of booster to start from.
 
         This can be used to specify a prediction value of existing model to be
@@ -847,7 +834,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         from .data import dispatch_meta_backend
         dispatch_meta_backend(self, margin, 'base_margin', 'float')
 
-    def set_group(self, group):
+    def set_group(self, group) -> None:
         """Set group size of DMatrix (used for ranking).
 
         Parameters
@@ -858,7 +845,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         from .data import dispatch_meta_backend
         dispatch_meta_backend(self, group, 'group', 'uint32')
 
-    def get_label(self):
+    def get_label(self) -> np.ndarray:
         """Get the label of the DMatrix.
 
         Returns
@@ -867,7 +854,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         """
         return self.get_float_info('label')
 
-    def get_weight(self):
+    def get_weight(self) -> np.ndarray:
         """Get the weight of the DMatrix.
 
         Returns
@@ -876,7 +863,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         """
         return self.get_float_info('weight')
 
-    def get_base_margin(self):
+    def get_base_margin(self) -> np.ndarray:
         """Get the base margin of the DMatrix.
 
         Returns
@@ -885,7 +872,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
         """
         return self.get_float_info('base_margin')
 
-    def num_row(self):
+    def num_row(self) -> int:
         """Get the number of rows in the DMatrix.
 
         Returns
@@ -897,7 +884,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
                                          ctypes.byref(ret)))
         return ret.value
 
-    def num_col(self):
+    def num_col(self) -> int:
         """Get the number of columns (features) in the DMatrix.
 
         Returns
@@ -1191,7 +1178,7 @@ class DeviceQuantileDMatrix(DMatrix):
             enable_categorical=enable_categorical,
         )
 
-    def _init(self, data, enable_categorical, **meta):
+    def _init(self, data, enable_categorical: bool, **meta) -> None:
         from .data import (
             _is_dlpack,
             _transform_dlpack,
@@ -1265,7 +1252,7 @@ def _get_booster_layer_trees(model: "Booster") -> Tuple[int, int]:
     return num_parallel_tree, num_groups
 
 
-class Booster(object):
+class Booster:
     # pylint: disable=too-many-public-methods
     """A Booster of XGBoost.
 
@@ -1273,7 +1260,12 @@ class Booster(object):
     training, prediction and evaluation.
     """
 
-    def __init__(self, params=None, cache=(), model_file=None):
+    def __init__(
+        self,
+        params: Optional[Dict] = None,
+        cache: Optional[Sequence[DMatrix]] = None,
+        model_file: Optional[Union["Booster", bytearray, os.PathLike, str]] = None
+    ) -> None:
         # pylint: disable=invalid-name
         """
         Parameters
@@ -1285,12 +1277,13 @@ class Booster(object):
         model_file : string/os.PathLike/Booster/bytearray
             Path to the model file if it's string or PathLike.
         """
+        cache = cache if cache is not None else []
         for d in cache:
             if not isinstance(d, DMatrix):
                 raise TypeError(f'invalid cache item: {type(d).__name__}', cache)
 
         dmats = c_array(ctypes.c_void_p, [d.handle for d in cache])
-        self.handle = ctypes.c_void_p()
+        self.handle: Optional[ctypes.c_void_p] = ctypes.c_void_p()
         _check_call(_LIB.XGBoosterCreate(dmats, c_bst_ulong(len(cache)),
                                          ctypes.byref(self.handle)))
         for d in cache:
@@ -1405,12 +1398,12 @@ class Booster(object):
 
         return params
 
-    def __del__(self):
+    def __del__(self) -> None:
         if hasattr(self, 'handle') and self.handle is not None:
             _check_call(_LIB.XGBoosterFree(self.handle))
             self.handle = None
 
-    def __getstate__(self):
+    def __getstate__(self) -> Dict:
         # can't pickle ctypes pointers, put model content in bytearray
         this = self.__dict__.copy()
         handle = this['handle']
@@ -1424,7 +1417,7 @@ class Booster(object):
             this["handle"] = buf
         return this
 
-    def __setstate__(self, state):
+    def __setstate__(self, state: Dict) -> None:
         # reconstruct handle from raw data
         handle = state['handle']
         if handle is not None:
@@ -1440,7 +1433,7 @@ class Booster(object):
             state['handle'] = handle
         self.__dict__.update(state)
 
-    def __getitem__(self, val):
+    def __getitem__(self, val: Union[int, tuple, slice]) -> "Booster":
         if isinstance(val, int):
             val = slice(val, val+1)
         if isinstance(val, tuple):
@@ -1461,13 +1454,14 @@ class Booster(object):
 
         step = val.step if val.step is not None else 1
 
-        start = ctypes.c_int(start)
-        stop = ctypes.c_int(stop)
-        step = ctypes.c_int(step)
+        c_start = ctypes.c_int(start)
+        c_stop = ctypes.c_int(stop)
+        c_step = ctypes.c_int(step)
 
         sliced_handle = ctypes.c_void_p()
-        status = _LIB.XGBoosterSlice(self.handle, start, stop, step,
-                                     ctypes.byref(sliced_handle))
+        status = _LIB.XGBoosterSlice(
+            self.handle, c_start, c_stop, c_step, ctypes.byref(sliced_handle)
+        )
         if status == -2:
             raise IndexError('Layer index out of range')
         _check_call(status)
@@ -1477,7 +1471,7 @@ class Booster(object):
         sliced.handle = sliced_handle
         return sliced
 
-    def save_config(self):
+    def save_config(self) -> str:
         '''Output internal parameter configuration of Booster as a JSON
         string.
 
@@ -1489,10 +1483,11 @@ class Booster(object):
             self.handle,
             ctypes.byref(length),
             ctypes.byref(json_string)))
-        json_string = json_string.value.decode()  # pylint: disable=no-member
-        return json_string
+        assert json_string.value is not None
+        result = json_string.value.decode()  # pylint: disable=no-member
+        return result
 
-    def load_config(self, config):
+    def load_config(self, config: str) -> None:
         '''Load configuration returned by `save_config`.
 
         .. versionadded:: 1.0.0
@@ -1502,14 +1497,14 @@ class Booster(object):
             self.handle,
             c_str(config)))
 
-    def __copy__(self):
+    def __copy__(self) -> "Booster":
         return self.__deepcopy__(None)
 
-    def __deepcopy__(self, _):
+    def __deepcopy__(self, _) -> "Booster":
         '''Return a copy of booster.'''
         return Booster(model_file=self)
 
-    def copy(self):
+    def copy(self) -> "Booster":
         """Copy the booster object.
 
         Returns
@@ -1519,7 +1514,7 @@ class Booster(object):
         """
         return self.__copy__()
 
-    def attr(self, key):
+    def attr(self, key: str) -> Optional[str]:
         """Get attribute string from the Booster.
 
         Parameters
@@ -1540,7 +1535,7 @@ class Booster(object):
             return py_str(ret.value)
         return None
 
-    def attributes(self):
+    def attributes(self) -> Dict[str, str]:
         """Get attributes stored in the Booster as a dictionary.
 
         Returns
@@ -1572,7 +1567,7 @@ class Booster(object):
             _check_call(_LIB.XGBoosterSetAttr(
                 self.handle, c_str(key), value))
 
-    def _get_feature_info(self, field: str):
+    def _get_feature_info(self, field: str) -> Optional[List[str]]:
         length = c_bst_ulong()
         sarr = ctypes.POINTER(ctypes.c_char_p)()
         if not hasattr(self, "handle") or self.handle is None:
@@ -1585,22 +1580,6 @@ class Booster(object):
         feature_info = from_cstr_to_pystr(sarr, length)
         return feature_info if feature_info else None
 
-    @property
-    def feature_types(self) -> Optional[List[str]]:
-        """Feature types for this booster.  Can be directly set by input data or by
-        assignment.
-
-        """
-        return self._get_feature_info("feature_type")
-
-    @property
-    def feature_names(self) -> Optional[List[str]]:
-        """Feature names for this booster.  Can be directly set by input data or by
-        assignment.
-
-        """
-        return self._get_feature_info("feature_name")
-
     def _set_feature_info(self, features: Optional[List[str]], field: str) -> None:
         if features is not None:
             assert isinstance(features, list)
@@ -1618,14 +1597,30 @@ class Booster(object):
                 )
             )
 
-    @feature_names.setter
-    def feature_names(self, features: Optional[List[str]]) -> None:
-        self._set_feature_info(features, "feature_name")
+    @property
+    def feature_types(self) -> Optional[List[str]]:
+        """Feature types for this booster.  Can be directly set by input data or by
+        assignment.
+
+        """
+        return self._get_feature_info("feature_type")
 
     @feature_types.setter
     def feature_types(self, features: Optional[List[str]]) -> None:
         self._set_feature_info(features, "feature_type")
 
+    @property
+    def feature_names(self) -> Optional[List[str]]:
+        """Feature names for this booster.  Can be directly set by input data or by
+        assignment.
+
+        """
+        return self._get_feature_info("feature_name")
+
+    @feature_names.setter
+    def feature_names(self, features: Optional[List[str]]) -> None:
+        self._set_feature_info(features, "feature_name")
+
     def set_param(self, params, value=None):
         """Set parameters into the Booster.
 
@@ -1645,7 +1640,9 @@ class Booster(object):
                 _check_call(_LIB.XGBoosterSetParam(self.handle, c_str(key),
                                                    c_str(str(val))))
 
-    def update(self, dtrain, iteration, fobj=None):
+    def update(
+        self, dtrain: DMatrix, iteration: int, fobj: Optional[Objective] = None
+    ) -> None:
         """Update for one iteration, with objective function calculated
         internally.  This function should not be called directly by users.
 
@@ -1672,18 +1669,18 @@ class Booster(object):
             grad, hess = fobj(pred, dtrain)
             self.boost(dtrain, grad, hess)
 
-    def boost(self, dtrain, grad, hess):
+    def boost(self, dtrain: DMatrix, grad: np.ndarray, hess: np.ndarray) -> None:
         """Boost the booster for one iteration, with customized gradient
         statistics.  Like :py:func:`xgboost.Booster.update`, this
         function should not be called directly by users.
 
         Parameters
         ----------
-        dtrain : DMatrix
+        dtrain :
             The training DMatrix.
-        grad : list
+        grad :
             The first order of gradient.
-        hess : list
+        hess :
             The second order of gradient.
 
         """
@@ -1700,17 +1697,23 @@ class Booster(object):
                                                c_array(ctypes.c_float, hess),
                                                c_bst_ulong(len(grad))))
 
-    def eval_set(self, evals, iteration=0, feval=None, output_margin=True):
+    def eval_set(
+        self,
+        evals: Sequence[Tuple[DMatrix, str]],
+        iteration: int = 0,
+        feval: Optional[Metric] = None,
+        output_margin: bool = True
+    ) -> str:
         # pylint: disable=invalid-name
         """Evaluate a set of data.
 
         Parameters
         ----------
-        evals : list of tuples (DMatrix, string)
+        evals :
             List of items to be evaluated.
-        iteration : int
+        iteration :
             Current iteration.
-        feval : function
+        feval :
             Custom evaluation function.
 
         Returns
@@ -1738,6 +1741,7 @@ class Booster(object):
                 ctypes.byref(msg),
             )
         )
+        assert msg.value is not None
         res = msg.value.decode()  # pylint: disable=no-member
         if feval is not None:
             for dmat, evname in evals:
@@ -1754,18 +1758,18 @@ class Booster(object):
                     res += "\t%s-%s:%f" % (evname, name, val)
         return res
 
-    def eval(self, data, name='eval', iteration=0):
+    def eval(self, data: DMatrix, name: str = 'eval', iteration: int = 0) -> str:
         """Evaluate the model on mat.
 
         Parameters
         ----------
-        data : DMatrix
+        data :
             The dmatrix storing the input.
 
-        name : str, optional
+        name :
             The name of the dataset.
 
-        iteration : int, optional
+        iteration :
             The current iteration number.
 
         Returns
@@ -2101,7 +2105,7 @@ class Booster(object):
             "Data type:" + str(type(data)) + " not supported by inplace prediction."
         )
 
-    def save_model(self, fname: Union[str, os.PathLike]):
+    def save_model(self, fname: Union[str, os.PathLike]) -> None:
         """Save the model to a file.
 
         The model is saved in an XGBoost internal format which is universal among the
@@ -2124,7 +2128,7 @@ class Booster(object):
         else:
             raise TypeError("fname must be a string or os PathLike")
 
-    def save_raw(self):
+    def save_raw(self) -> bytearray:
         """Save the model to a in memory buffer representation instead of file.
 
         Returns
@@ -2232,18 +2236,23 @@ class Booster(object):
         if need_close:
             fout.close()
 
-    def get_dump(self, fmap='', with_stats=False, dump_format="text"):
+    def get_dump(
+        self,
+        fmap: Union[str, os.PathLike] = "",
+        with_stats: bool = False,
+        dump_format: str = "text"
+    ) -> List[str]:
         """Returns the model dump as a list of strings.  Unlike `save_model`, the
         output format is primarily used for visualization or interpretation,
         hence it's more human readable but cannot be loaded back to XGBoost.
 
         Parameters
         ----------
-        fmap : string or os.PathLike, optional
+        fmap :
             Name of the file containing feature map names.
-        with_stats : bool, optional
+        with_stats :
             Controls whether the split statistics are output.
-        dump_format : string, optional
+        dump_format :
             Format of model dump. Can be 'text', 'json' or 'dot'.
 
         """
@@ -2259,7 +2268,9 @@ class Booster(object):
         res = from_cstr_to_pystr(sarr, length)
         return res
 
-    def get_fscore(self, fmap=''):
+    def get_fscore(
+        self, fmap: Union[str, os.PathLike] = ""
+    ) -> Dict[str, Union[float, List[float]]]:
         """Get feature importance of each feature.
 
         .. note:: Zero-importance features will not be included
@@ -2269,7 +2280,7 @@ class Booster(object):
 
         Parameters
         ----------
-        fmap: str or os.PathLike (optional)
+        fmap :
            The name of feature map file
         """
 
@@ -2299,9 +2310,9 @@ class Booster(object):
 
         Parameters
         ----------
-        fmap: str or os.PathLike (optional)
+        fmap:
            The name of feature map file.
-        importance_type: str, default 'weight'
+        importance_type:
             One of the importance types defined above.
 
         Returns
@@ -2343,7 +2354,8 @@ class Booster(object):
                 results[feat] = float(score)
         return results
 
-    def trees_to_dataframe(self, fmap=''):  # pylint: disable=too-many-statements
+    # pylint: disable=too-many-statements
+    def trees_to_dataframe(self, fmap: Union[str, os.PathLike] = '') -> DataFrame:
         """Parse a boosted tree model text dump into a pandas DataFrame structure.
 
         This feature is only defined when the decision tree model is chosen as base
@@ -2370,7 +2382,7 @@ class Booster(object):
         node_ids = []
         fids = []
         splits = []
-        categories = []
+        categories: List[Optional[float]] = []
         y_directs = []
         n_directs = []
         missings = []
@@ -2444,7 +2456,7 @@ class Booster(object):
         # pylint: disable=no-member
         return df.sort(['Tree', 'Node']).reset_index(drop=True)
 
-    def _validate_features(self, data: DMatrix):
+    def _validate_features(self, data: DMatrix) -> None:
         """
         Validate Booster and data's feature_names are identical.
         Set feature_names and feature_types from DMatrix
diff --git a/python-package/xgboost/dask.py b/python-package/xgboost/dask.py
index e96a21a12..cb103b194 100644
--- a/python-package/xgboost/dask.py
+++ b/python-package/xgboost/dask.py
@@ -17,12 +17,13 @@ https://github.com/dask/dask-xgboost
 """
 import platform
 import logging
+import collections
 from contextlib import contextmanager
 from collections import defaultdict
-from collections.abc import Sequence
 from threading import Thread
 from functools import partial, update_wrapper
 from typing import TYPE_CHECKING, List, Tuple, Callable, Optional, Any, Union, Dict, Set
+from typing import Sequence
 from typing import Awaitable, Generator, TypeVar
 
 import numpy
@@ -524,9 +525,9 @@ class DaskPartitionIter(DataIter):  # pylint: disable=R0902
         self._feature_names = feature_names
         self._feature_types = feature_types
 
-        assert isinstance(self._data, Sequence)
+        assert isinstance(self._data, collections.abc.Sequence)
 
-        types = (Sequence, type(None))
+        types = (collections.abc.Sequence, type(None))
         assert isinstance(self._labels, types)
         assert isinstance(self._weights, types)
         assert isinstance(self._base_margin, types)
@@ -817,7 +818,7 @@ async def _get_rabit_args(n_workers: int, client: "distributed.Client") -> List[
 
 def _get_workers_from_data(
     dtrain: DaskDMatrix,
-    evals: Optional[List[Tuple[DaskDMatrix, str]]]
+    evals: Optional[Sequence[Tuple[DaskDMatrix, str]]]
 ) -> List[str]:
     X_worker_map: Set[str] = set(dtrain.worker_map.keys())
     if evals:
@@ -837,13 +838,13 @@ async def _train_async(
     params: Dict[str, Any],
     dtrain: DaskDMatrix,
     num_boost_round: int,
-    evals: Optional[List[Tuple[DaskDMatrix, str]]],
+    evals: Optional[Sequence[Tuple[DaskDMatrix, str]]],
     obj: Optional[Objective],
     feval: Optional[Metric],
     early_stopping_rounds: Optional[int],
     verbose_eval: Union[int, bool],
     xgb_model: Optional[Booster],
-    callbacks: Optional[List[TrainingCallback]],
+    callbacks: Optional[Sequence[TrainingCallback]],
     custom_metric: Optional[Metric],
 ) -> Optional[TrainReturnT]:
     workers = _get_workers_from_data(dtrain, evals)
@@ -951,13 +952,13 @@ def train(                      # pylint: disable=unused-argument
     dtrain: DaskDMatrix,
     num_boost_round: int = 10,
     *,
-    evals: Optional[List[Tuple[DaskDMatrix, str]]] = None,
+    evals: Optional[Sequence[Tuple[DaskDMatrix, str]]] = None,
     obj: Optional[Objective] = None,
     feval: Optional[Metric] = None,
     early_stopping_rounds: Optional[int] = None,
     xgb_model: Optional[Booster] = None,
     verbose_eval: Union[int, bool] = True,
-    callbacks: Optional[List[TrainingCallback]] = None,
+    callbacks: Optional[Sequence[TrainingCallback]] = None,
     custom_metric: Optional[Metric] = None,
 ) -> Any:
     """Train XGBoost model.
@@ -1648,15 +1649,15 @@ class DaskXGBRegressor(DaskScikitLearnBase, XGBRegressorBase):
         y: _DaskCollection,
         sample_weight: Optional[_DaskCollection],
         base_margin: Optional[_DaskCollection],
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]],
-        eval_metric: Optional[Union[str, List[str], Metric]],
-        sample_weight_eval_set: Optional[List[_DaskCollection]],
-        base_margin_eval_set: Optional[List[_DaskCollection]],
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]],
+        eval_metric: Optional[Union[str, Sequence[str], Metric]],
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]],
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]],
         early_stopping_rounds: Optional[int],
         verbose: bool,
         xgb_model: Optional[Union[Booster, XGBModel]],
         feature_weights: Optional[_DaskCollection],
-        callbacks: Optional[List[TrainingCallback]],
+        callbacks: Optional[Sequence[TrainingCallback]],
     ) -> _DaskCollection:
         params = self.get_xgb_params()
         dtrain, evals = await _async_wrap_evaluation_matrices(
@@ -1714,15 +1715,15 @@ class DaskXGBRegressor(DaskScikitLearnBase, XGBRegressorBase):
         *,
         sample_weight: Optional[_DaskCollection] = None,
         base_margin: Optional[_DaskCollection] = None,
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Callable]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: bool = True,
         xgb_model: Optional[Union[Booster, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[_DaskCollection]] = None,
-        base_margin_eval_set: Optional[List[_DaskCollection]] = None,
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]] = None,
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]] = None,
         feature_weights: Optional[_DaskCollection] = None,
-        callbacks: Optional[List[TrainingCallback]] = None,
+        callbacks: Optional[Sequence[TrainingCallback]] = None,
     ) -> "DaskXGBRegressor":
         _assert_dask_support()
         args = {k: v for k, v in locals().items() if k not in ("self", "__class__")}
@@ -1738,15 +1739,15 @@ class DaskXGBClassifier(DaskScikitLearnBase, XGBClassifierBase):
         self, X: _DaskCollection, y: _DaskCollection,
         sample_weight: Optional[_DaskCollection],
         base_margin: Optional[_DaskCollection],
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]],
-        eval_metric: Optional[Union[str, List[str], Metric]],
-        sample_weight_eval_set: Optional[List[_DaskCollection]],
-        base_margin_eval_set: Optional[List[_DaskCollection]],
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]],
+        eval_metric: Optional[Union[str, Sequence[str], Metric]],
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]],
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]],
         early_stopping_rounds: Optional[int],
         verbose: bool,
         xgb_model: Optional[Union[Booster, XGBModel]],
         feature_weights: Optional[_DaskCollection],
-        callbacks: Optional[List[TrainingCallback]]
+        callbacks: Optional[Sequence[TrainingCallback]]
     ) -> "DaskXGBClassifier":
         params = self.get_xgb_params()
         dtrain, evals = await _async_wrap_evaluation_matrices(
@@ -1818,15 +1819,15 @@ class DaskXGBClassifier(DaskScikitLearnBase, XGBClassifierBase):
         *,
         sample_weight: Optional[_DaskCollection] = None,
         base_margin: Optional[_DaskCollection] = None,
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Callable]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: bool = True,
         xgb_model: Optional[Union[Booster, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[_DaskCollection]] = None,
-        base_margin_eval_set: Optional[List[_DaskCollection]] = None,
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]] = None,
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]] = None,
         feature_weights: Optional[_DaskCollection] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "DaskXGBClassifier":
         _assert_dask_support()
         args = {k: v for k, v in locals().items() if k not in ("self", "__class__")}
@@ -1935,17 +1936,17 @@ class DaskXGBRanker(DaskScikitLearnBase, XGBRankerMixIn):
         qid: Optional[_DaskCollection],
         sample_weight: Optional[_DaskCollection],
         base_margin: Optional[_DaskCollection],
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]],
-        sample_weight_eval_set: Optional[List[_DaskCollection]],
-        base_margin_eval_set: Optional[List[_DaskCollection]],
-        eval_group: Optional[List[_DaskCollection]],
-        eval_qid: Optional[List[_DaskCollection]],
-        eval_metric: Optional[Union[str, List[str], Metric]],
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]],
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]],
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]],
+        eval_group: Optional[Sequence[_DaskCollection]],
+        eval_qid: Optional[Sequence[_DaskCollection]],
+        eval_metric: Optional[Union[str, Sequence[str], Metric]],
         early_stopping_rounds: Optional[int],
         verbose: bool,
         xgb_model: Optional[Union[XGBModel, Booster]],
         feature_weights: Optional[_DaskCollection],
-        callbacks: Optional[List[TrainingCallback]],
+        callbacks: Optional[Sequence[TrainingCallback]],
     ) -> "DaskXGBRanker":
         msg = "Use `qid` instead of `group` on dask interface."
         if not (group is None and eval_group is None):
@@ -2010,17 +2011,17 @@ class DaskXGBRanker(DaskScikitLearnBase, XGBRankerMixIn):
         qid: Optional[_DaskCollection] = None,
         sample_weight: Optional[_DaskCollection] = None,
         base_margin: Optional[_DaskCollection] = None,
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]] = None,
-        eval_group: Optional[List[_DaskCollection]] = None,
-        eval_qid: Optional[List[_DaskCollection]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]] = None,
+        eval_group: Optional[Sequence[_DaskCollection]] = None,
+        eval_qid: Optional[Sequence[_DaskCollection]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Callable]] = None,
         early_stopping_rounds: int = None,
         verbose: bool = False,
         xgb_model: Optional[Union[XGBModel, Booster]] = None,
-        sample_weight_eval_set: Optional[List[_DaskCollection]] = None,
-        base_margin_eval_set: Optional[List[_DaskCollection]] = None,
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]] = None,
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]] = None,
         feature_weights: Optional[_DaskCollection] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "DaskXGBRanker":
         _assert_dask_support()
         args = {k: v for k, v in locals().items() if k not in ("self", "__class__")}
@@ -2077,15 +2078,15 @@ class DaskXGBRFRegressor(DaskXGBRegressor):
         *,
         sample_weight: Optional[_DaskCollection] = None,
         base_margin: Optional[_DaskCollection] = None,
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Callable]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: bool = True,
         xgb_model: Optional[Union[Booster, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[_DaskCollection]] = None,
-        base_margin_eval_set: Optional[List[_DaskCollection]] = None,
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]] = None,
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]] = None,
         feature_weights: Optional[_DaskCollection] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "DaskXGBRFRegressor":
         _assert_dask_support()
         args = {k: v for k, v in locals().items() if k not in ("self", "__class__")}
@@ -2141,15 +2142,15 @@ class DaskXGBRFClassifier(DaskXGBClassifier):
         *,
         sample_weight: Optional[_DaskCollection] = None,
         base_margin: Optional[_DaskCollection] = None,
-        eval_set: Optional[List[Tuple[_DaskCollection, _DaskCollection]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[_DaskCollection, _DaskCollection]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Callable]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: bool = True,
         xgb_model: Optional[Union[Booster, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[_DaskCollection]] = None,
-        base_margin_eval_set: Optional[List[_DaskCollection]] = None,
+        sample_weight_eval_set: Optional[Sequence[_DaskCollection]] = None,
+        base_margin_eval_set: Optional[Sequence[_DaskCollection]] = None,
         feature_weights: Optional[_DaskCollection] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "DaskXGBRFClassifier":
         _assert_dask_support()
         args = {k: v for k, v in locals().items() if k not in ("self", "__class__")}
diff --git a/python-package/xgboost/sklearn.py b/python-package/xgboost/sklearn.py
index dd877cfc0..7313351fd 100644
--- a/python-package/xgboost/sklearn.py
+++ b/python-package/xgboost/sklearn.py
@@ -4,7 +4,8 @@ import copy
 import warnings
 import json
 import os
-from typing import Union, Optional, List, Dict, Callable, Tuple, Any, TypeVar, Type
+from typing import Union, Optional, List, Dict, Callable, Tuple, Any, TypeVar, Type, cast
+from typing import Sequence
 import numpy as np
 
 from .core import Booster, DMatrix, XGBoostError
@@ -36,7 +37,7 @@ class XGBRankerMixIn:  # pylint: disable=too-few-public-methods
 
 def _check_rf_callback(
     early_stopping_rounds: Optional[int],
-    callbacks: Optional[List[TrainingCallback]],
+    callbacks: Optional[Sequence[TrainingCallback]],
 ) -> None:
     if early_stopping_rounds is not None or callbacks is not None:
         raise NotImplementedError(
@@ -343,14 +344,14 @@ def _wrap_evaluation_matrices(
     sample_weight: Optional[Any],
     base_margin: Optional[Any],
     feature_weights: Optional[Any],
-    eval_set: Optional[List[Tuple[Any, Any]]],
-    sample_weight_eval_set: Optional[List[Any]],
-    base_margin_eval_set: Optional[List[Any]],
-    eval_group: Optional[List[Any]],
-    eval_qid: Optional[List[Any]],
+    eval_set: Optional[Sequence[Tuple[Any, Any]]],
+    sample_weight_eval_set: Optional[Sequence[Any]],
+    base_margin_eval_set: Optional[Sequence[Any]],
+    eval_group: Optional[Sequence[Any]],
+    eval_qid: Optional[Sequence[Any]],
     create_dmatrix: Callable,
     enable_categorical: bool,
-) -> Tuple[Any, Optional[List[Tuple[Any, str]]]]:
+) -> Tuple[Any, List[Tuple[Any, str]]]:
     """Convert array_like evaluation matrices into DMatrix.  Perform validation on the way.
 
     """
@@ -368,7 +369,7 @@ def _wrap_evaluation_matrices(
 
     n_validation = 0 if eval_set is None else len(eval_set)
 
-    def validate_or_none(meta: Optional[List], name: str) -> List:
+    def validate_or_none(meta: Optional[Sequence], name: str) -> Sequence:
         if meta is None:
             return [None] * n_validation
         if len(meta) != n_validation:
@@ -464,7 +465,7 @@ class XGBModel(XGBModelBase):
         missing: float = np.nan,
         num_parallel_tree: Optional[int] = None,
         monotone_constraints: Optional[Union[Dict[str, int], str]] = None,
-        interaction_constraints: Optional[Union[str, List[Tuple[str]]]] = None,
+        interaction_constraints: Optional[Union[str, Sequence[Sequence[str]]]] = None,
         importance_type: Optional[str] = None,
         gpu_id: Optional[int] = None,
         validate_parameters: Optional[bool] = None,
@@ -715,7 +716,7 @@ class XGBModel(XGBModelBase):
     def _configure_fit(
         self,
         booster: Optional[Union[Booster, "XGBModel", str]],
-        eval_metric: Optional[Union[Callable, str, List[str]]],
+        eval_metric: Optional[Union[Callable, str, Sequence[str]]],
         params: Dict[str, Any],
         early_stopping_rounds: Optional[int],
     ) -> Tuple[
@@ -788,10 +789,7 @@ class XGBModel(XGBModelBase):
 
     def _set_evaluation_result(self, evals_result: TrainingCallback.EvalsLog) -> None:
         if evals_result:
-            for val in evals_result.items():
-                evals_result_key = list(val[1].keys())[0]
-                evals_result[val[0]][evals_result_key] = val[1][evals_result_key]
-            self.evals_result_ = evals_result
+            self.evals_result_ = cast(Dict[str, Dict[str, List[float]]], evals_result)
 
     @_deprecate_positional_args
     def fit(
@@ -801,15 +799,15 @@ class XGBModel(XGBModelBase):
         *,
         sample_weight: Optional[array_like] = None,
         base_margin: Optional[array_like] = None,
-        eval_set: Optional[List[Tuple[array_like, array_like]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[array_like, array_like]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Metric]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: Optional[bool] = True,
         xgb_model: Optional[Union[Booster, str, "XGBModel"]] = None,
-        sample_weight_eval_set: Optional[List[array_like]] = None,
-        base_margin_eval_set: Optional[List[array_like]] = None,
+        sample_weight_eval_set: Optional[Sequence[array_like]] = None,
+        base_margin_eval_set: Optional[Sequence[array_like]] = None,
         feature_weights: Optional[array_like] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "XGBModel":
         # pylint: disable=invalid-name,attribute-defined-outside-init
         """Fit gradient boosting model.
@@ -1031,7 +1029,7 @@ class XGBModel(XGBModelBase):
             Input features matrix.
 
         iteration_range :
-            See :py:meth:`xgboost.XGBRegressor.predict`.
+            See :py:meth:`predict`.
 
         ntree_limit :
             Deprecated, use ``iteration_range`` instead.
@@ -1055,40 +1053,26 @@ class XGBModel(XGBModelBase):
             iteration_range=iteration_range
         )
 
-    def evals_result(self) -> TrainingCallback.EvalsLog:
+    def evals_result(self) -> Dict[str, Dict[str, List[float]]]:
         """Return the evaluation results.
 
-        If **eval_set** is passed to the `fit` function, you can call
-        ``evals_result()`` to get evaluation results for all passed **eval_sets**.
-        When **eval_metric** is also passed to the `fit` function, the
-        **evals_result** will contain the **eval_metrics** passed to the `fit` function.
+        If **eval_set** is passed to the :py:meth:`fit` function, you can call
+        ``evals_result()`` to get evaluation results for all passed **eval_sets**.  When
+        **eval_metric** is also passed to the :py:meth:`fit` function, the
+        **evals_result** will contain the **eval_metrics** passed to the :py:meth:`fit`
+        function.
 
-        Returns
-        -------
-        evals_result : dictionary
-
-        Example
-        -------
-
-        .. code-block:: python
-
-            param_dist = {'objective':'binary:logistic', 'n_estimators':2}
-
-            clf = xgb.XGBModel(**param_dist)
-
-            clf.fit(X_train, y_train,
-                    eval_set=[(X_train, y_train), (X_test, y_test)],
-                    eval_metric='logloss',
-                    verbose=True)
-
-            evals_result = clf.evals_result()
-
-        The variable **evals_result** will contain:
+        The returned evaluation result is a dictionary:
 
         .. code-block:: python
 
             {'validation_0': {'logloss': ['0.604835', '0.531479']},
              'validation_1': {'logloss': ['0.41965', '0.17686']}}
+
+        Returns
+        -------
+        evals_result
+
         """
         if getattr(self, "evals_result_", None) is not None:
             evals_result = self.evals_result_
@@ -1193,8 +1177,8 @@ class XGBModel(XGBModelBase):
         .. note:: Intercept is defined only for linear learners
 
             Intercept (bias) is only defined when the linear model is chosen as base
-            learner (`booster=gblinear`). It is not defined for other base learner types, such
-            as tree learners (`booster=gbtree`).
+            learner (`booster=gblinear`). It is not defined for other base learner types,
+            such as tree learners (`booster=gbtree`).
 
         Returns
         -------
@@ -1251,15 +1235,15 @@ class XGBClassifier(XGBModel, XGBClassifierBase):
         *,
         sample_weight: Optional[array_like] = None,
         base_margin: Optional[array_like] = None,
-        eval_set: Optional[List[Tuple[array_like, array_like]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[array_like, array_like]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Metric]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: Optional[bool] = True,
         xgb_model: Optional[Union[Booster, str, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[array_like]] = None,
-        base_margin_eval_set: Optional[List[array_like]] = None,
+        sample_weight_eval_set: Optional[Sequence[array_like]] = None,
+        base_margin_eval_set: Optional[Sequence[array_like]] = None,
         feature_weights: Optional[array_like] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "XGBClassifier":
         # pylint: disable = attribute-defined-outside-init,too-many-statements
         evals_result: TrainingCallback.EvalsLog = {}
@@ -1445,51 +1429,6 @@ class XGBClassifier(XGBModel, XGBClassifierBase):
             getattr(self, "n_classes_", None), class_probs, np.vstack
         )
 
-    def evals_result(self) -> TrainingCallback.EvalsLog:
-        """Return the evaluation results.
-
-        If **eval_set** is passed to the `fit` function, you can call
-        ``evals_result()`` to get evaluation results for all passed **eval_sets**.
-
-        When **eval_metric** is also passed as a parameter, the **evals_result** will
-        contain the **eval_metric** passed to the `fit` function.
-
-        Returns
-        -------
-        evals_result : dictionary
-
-        Example
-        -------
-
-        .. code-block:: python
-
-            param_dist = {
-                'objective':'binary:logistic', 'n_estimators':2, eval_metric="logloss"
-            }
-
-            clf = xgb.XGBClassifier(**param_dist)
-
-            clf.fit(X_train, y_train,
-                    eval_set=[(X_train, y_train), (X_test, y_test)],
-                    verbose=True)
-
-            evals_result = clf.evals_result()
-
-        The variable **evals_result** will contain
-
-        .. code-block:: python
-
-            {'validation_0': {'logloss': ['0.604835', '0.531479']},
-            'validation_1': {'logloss': ['0.41965', '0.17686']}}
-
-        """
-        if self.evals_result_:
-            evals_result = self.evals_result_
-        else:
-            raise XGBoostError('No results.')
-
-        return evals_result
-
 
 @xgboost_model_doc(
     "scikit-learn API for XGBoost random forest classification.",
@@ -1533,15 +1472,15 @@ class XGBRFClassifier(XGBClassifier):
         *,
         sample_weight: Optional[array_like] = None,
         base_margin: Optional[array_like] = None,
-        eval_set: Optional[List[Tuple[array_like, array_like]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[array_like, array_like]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Metric]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: Optional[bool] = True,
         xgb_model: Optional[Union[Booster, str, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[array_like]] = None,
-        base_margin_eval_set: Optional[List[array_like]] = None,
+        sample_weight_eval_set: Optional[Sequence[array_like]] = None,
+        base_margin_eval_set: Optional[Sequence[array_like]] = None,
         feature_weights: Optional[array_like] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "XGBRFClassifier":
         args = {k: v for k, v in locals().items() if k not in ("self", "__class__")}
         _check_rf_callback(early_stopping_rounds, callbacks)
@@ -1605,15 +1544,15 @@ class XGBRFRegressor(XGBRegressor):
         *,
         sample_weight: Optional[array_like] = None,
         base_margin: Optional[array_like] = None,
-        eval_set: Optional[List[Tuple[array_like, array_like]]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[array_like, array_like]]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Metric]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: Optional[bool] = True,
         xgb_model: Optional[Union[Booster, str, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[array_like]] = None,
-        base_margin_eval_set: Optional[List[array_like]] = None,
+        sample_weight_eval_set: Optional[Sequence[array_like]] = None,
+        base_margin_eval_set: Optional[Sequence[array_like]] = None,
         feature_weights: Optional[array_like] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "XGBRFRegressor":
         args = {k: v for k, v in locals().items() if k not in ("self", "__class__")}
         _check_rf_callback(early_stopping_rounds, callbacks)
@@ -1682,17 +1621,17 @@ class XGBRanker(XGBModel, XGBRankerMixIn):
         qid: Optional[array_like] = None,
         sample_weight: Optional[array_like] = None,
         base_margin: Optional[array_like] = None,
-        eval_set: Optional[List[Tuple[array_like, array_like]]] = None,
-        eval_group: Optional[List[array_like]] = None,
-        eval_qid: Optional[List[array_like]] = None,
-        eval_metric: Optional[Union[str, List[str], Metric]] = None,
+        eval_set: Optional[Sequence[Tuple[array_like, array_like]]] = None,
+        eval_group: Optional[Sequence[array_like]] = None,
+        eval_qid: Optional[Sequence[array_like]] = None,
+        eval_metric: Optional[Union[str, Sequence[str], Metric]] = None,
         early_stopping_rounds: Optional[int] = None,
         verbose: Optional[bool] = False,
         xgb_model: Optional[Union[Booster, str, XGBModel]] = None,
-        sample_weight_eval_set: Optional[List[array_like]] = None,
-        base_margin_eval_set: Optional[List[array_like]] = None,
+        sample_weight_eval_set: Optional[Sequence[array_like]] = None,
+        base_margin_eval_set: Optional[Sequence[array_like]] = None,
         feature_weights: Optional[array_like] = None,
-        callbacks: Optional[List[TrainingCallback]] = None
+        callbacks: Optional[Sequence[TrainingCallback]] = None
     ) -> "XGBRanker":
         # pylint: disable = attribute-defined-outside-init,arguments-differ
         """Fit gradient boosting ranker
diff --git a/python-package/xgboost/tracker.py b/python-package/xgboost/tracker.py
index 61e3a1a06..06051c574 100644
--- a/python-package/xgboost/tracker.py
+++ b/python-package/xgboost/tracker.py
@@ -13,7 +13,7 @@ from threading import Thread
 import argparse
 import sys
 
-from typing import Dict, List,  Tuple, Union, Optional
+from typing import Dict, List, Tuple, Union, Optional
 
 _RingMap = Dict[int, Tuple[int, int]]
 _TreeMap = Dict[int, List[int]]
diff --git a/python-package/xgboost/training.py b/python-package/xgboost/training.py
index 2b0035a9a..9127b8e6f 100644
--- a/python-package/xgboost/training.py
+++ b/python-package/xgboost/training.py
@@ -3,18 +3,20 @@
 # pylint: disable=too-many-branches, too-many-statements
 """Training Library containing training routines."""
 import copy
-from typing import Optional, List
+import os
 import warnings
+from typing import Optional, Dict, Any, Union, Tuple, cast, Sequence
 
 import numpy as np
-from .core import Booster, XGBoostError, _get_booster_layer_trees
-from .core import _deprecate_positional_args
-from .core import Objective, Metric
+from .core import Booster, DMatrix, XGBoostError, _get_booster_layer_trees
+from .core import Metric, Objective
 from .compat import (SKLEARN_INSTALLED, XGBStratifiedKFold)
 from . import callback
 
 
-def _assert_new_callback(callbacks: Optional[List[callback.TrainingCallback]]) -> None:
+def _assert_new_callback(
+    callbacks: Optional[Sequence[callback.TrainingCallback]]
+) -> None:
     is_new_callback: bool = not callbacks or all(
         isinstance(c, callback.TrainingCallback) for c in callbacks
     )
@@ -44,24 +46,24 @@ def _configure_custom_metric(
 
 
 def _train_internal(
-    params,
-    dtrain,
-    num_boost_round=10,
-    evals=(),
-    obj=None,
-    feval=None,
-    custom_metric=None,
-    xgb_model=None,
-    callbacks=None,
-    evals_result=None,
-    maximize=None,
-    verbose_eval=None,
-    early_stopping_rounds=None,
-):
+    params: Dict[str, Any],
+    dtrain: DMatrix,
+    num_boost_round: int = 10,
+    evals: Optional[Sequence[Tuple[DMatrix, str]]] = None,
+    obj: Optional[Objective] = None,
+    feval: Optional[Metric] = None,
+    custom_metric: Optional[Metric] = None,
+    xgb_model: Optional[Union[str, os.PathLike, Booster, bytearray]] = None,
+    callbacks: Optional[Sequence[callback.TrainingCallback]] = None,
+    evals_result: callback.TrainingCallback.EvalsLog = None,
+    maximize: Optional[bool] = None,
+    verbose_eval: Optional[Union[bool, int]] = True,
+    early_stopping_rounds: Optional[int] = None,
+) -> Booster:
     """internal training function"""
-    callbacks = [] if callbacks is None else copy.copy(callbacks)
+    callbacks = [] if callbacks is None else copy.copy(list(callbacks))
     metric_fn = _configure_custom_metric(feval, custom_metric)
-    evals = list(evals)
+    evals = list(evals) if evals else []
 
     bst = Booster(params, [dtrain] + [d[0] for d in evals])
 
@@ -78,7 +80,7 @@ def _train_internal(
         callbacks.append(
             callback.EarlyStopping(rounds=early_stopping_rounds, maximize=maximize)
         )
-    callbacks = callback.CallbackContainer(
+    cb_container = callback.CallbackContainer(
         callbacks,
         metric=metric_fn,
         # For old `feval` parameter, the behavior is unchanged.  For the new
@@ -87,32 +89,32 @@ def _train_internal(
         output_margin=callable(obj) or metric_fn is feval,
     )
 
-    bst = callbacks.before_training(bst)
+    bst = cb_container.before_training(bst)
 
     for i in range(start_iteration, num_boost_round):
-        if callbacks.before_iteration(bst, i, dtrain, evals):
+        if cb_container.before_iteration(bst, i, dtrain, evals):
             break
         bst.update(dtrain, i, obj)
-        if callbacks.after_iteration(bst, i, dtrain, evals):
+        if cb_container.after_iteration(bst, i, dtrain, evals):
             break
 
-    bst = callbacks.after_training(bst)
+    bst = cb_container.after_training(bst)
 
     if evals_result is not None:
-        evals_result.update(callbacks.history)
+        evals_result.update(cb_container.history)
 
     # These should be moved into callback functions `after_training`, but until old
     # callbacks are removed, the train function is the only place for setting the
     # attributes.
     num_parallel_tree, _ = _get_booster_layer_trees(bst)
     if bst.attr('best_score') is not None:
-        bst.best_score = float(bst.attr('best_score'))
-        bst.best_iteration = int(bst.attr('best_iteration'))
+        bst.best_score = float(cast(str, bst.attr('best_score')))
+        bst.best_iteration = int(cast(str, bst.attr('best_iteration')))
         # num_class is handled internally
         bst.set_attr(
             best_ntree_limit=str((bst.best_iteration + 1) * num_parallel_tree)
         )
-        bst.best_ntree_limit = int(bst.attr("best_ntree_limit"))
+        bst.best_ntree_limit = int(cast(str, bst.attr("best_ntree_limit")))
     else:
         # Due to compatibility with version older than 1.4, these attributes are added
         # to Python object even if early stopping is not used.
@@ -126,35 +128,32 @@ def _train_internal(
     return bst.copy()
 
 
-@_deprecate_positional_args
 def train(
-    params,
-    dtrain,
-    num_boost_round=10,
-    *,
-    evals=(),
+    params: Dict[str, Any],
+    dtrain: DMatrix,
+    num_boost_round: int = 10,
+    evals: Optional[Sequence[Tuple[DMatrix, str]]] = None,
     obj: Optional[Objective] = None,
-    feval=None,
-    maximize=None,
-    early_stopping_rounds=None,
-    evals_result=None,
-    verbose_eval=True,
-    xgb_model=None,
-    callbacks=None,
+    feval: Optional[Metric] = None,
+    maximize: Optional[bool] = None,
+    early_stopping_rounds: Optional[int] = None,
+    evals_result: callback.TrainingCallback.EvalsLog = None,
+    verbose_eval: Optional[Union[bool, int]] = True,
+    xgb_model: Optional[Union[str, os.PathLike, Booster, bytearray]] = None,
+    callbacks: Optional[Sequence[callback.TrainingCallback]] = None,
     custom_metric: Optional[Metric] = None,
-):
-    # pylint: disable=too-many-statements,too-many-branches, attribute-defined-outside-init
+) -> Booster:
     """Train a booster with given parameters.
 
     Parameters
     ----------
-    params : dict
+    params :
         Booster params.
-    dtrain : DMatrix
+    dtrain :
         Data to be trained.
-    num_boost_round: int
+    num_boost_round :
         Number of boosting iterations.
-    evals: list of pairs (DMatrix, string)
+    evals :
         List of validation sets for which metrics will evaluated during training.
         Validation metrics will help us track the performance of the model.
     obj
@@ -166,7 +165,7 @@ def train(
             Use `custom_metric` instead.
     maximize : bool
         Whether to maximize feval.
-    early_stopping_rounds: int
+    early_stopping_rounds :
         Activates early stopping. Validation metric needs to improve at least once in
         every **early_stopping_rounds** round(s) to continue training.
         Requires at least one item in **evals**.
@@ -178,7 +177,7 @@ def train(
         **params**, the last metric will be used for early stopping.
         If early stopping occurs, the model will have two additional fields:
         ``bst.best_score``, ``bst.best_iteration``.
-    evals_result: dict
+    evals_result :
         This dictionary stores the evaluation results of all the items in watchlist.
 
         Example: with a watchlist containing
@@ -191,7 +190,7 @@ def train(
             {'train': {'logloss': ['0.48253', '0.35953']},
              'eval': {'logloss': ['0.480385', '0.357756']}}
 
-    verbose_eval : bool or int
+    verbose_eval :
         Requires at least one item in **evals**.
         If **verbose_eval** is True then the evaluation metric on the validation set is
         printed at each boosting stage.
@@ -200,9 +199,9 @@ def train(
         / the boosting stage found by using **early_stopping_rounds** is also printed.
         Example: with ``verbose_eval=4`` and at least one item in **evals**, an evaluation metric
         is printed every 4 boosting stages, instead of every boosting stage.
-    xgb_model : file name of stored xgb model or 'Booster' instance
+    xgb_model :
         Xgb model to be loaded before training (allows training continuation).
-    callbacks : list of callback functions
+    callbacks :
         List of callback functions that are applied at end of each iteration.
         It is possible to use predefined callbacks by using
         :ref:`Callback API <callback_api>`.
diff --git a/tests/python/test_interaction_constraints.py b/tests/python/test_interaction_constraints.py
index c582614c8..12653b538 100644
--- a/tests/python/test_interaction_constraints.py
+++ b/tests/python/test_interaction_constraints.py
@@ -9,7 +9,9 @@ rng = np.random.RandomState(1994)
 
 
 class TestInteractionConstraints:
-    def run_interaction_constraints(self, tree_method, feature_names=None, interaction_constraints='[[0, 1]]'):
+    def run_interaction_constraints(
+        self, tree_method, feature_names=None, interaction_constraints='[[0, 1]]'
+    ):
         x1 = np.random.normal(loc=1.0, scale=1.0, size=1000)
         x2 = np.random.normal(loc=1.0, scale=1.0, size=1000)
         x3 = np.random.choice([1, 2, 3], size=1000, replace=True)