[python] add type hints to logging functions in basic.py (#4527)

* [python] add type hints to logging functions in basic.py

* add hints on wrapper

python-package/lightgbm/basic.py

@@ -12,7 +12,7 @@
 from os.path import getsize
 from pathlib import Path
 from tempfile import NamedTemporaryFile
-from typing import Any, Dict, Iterable, List, Optional, Set, Tuple, Union
+from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Tuple, Union
 
 import numpy as np
 import scipy.sparse
@@ -34,17 +34,17 @@ def _get_sample_count(total_nrow: int, params: str):
 
 
 class _DummyLogger:
-    def info(self, msg):
+    def info(self, msg: str) -> None:
         print(msg)
 
-    def warning(self, msg):
+    def warning(self, msg: str) -> None:
         warnings.warn(msg, stacklevel=3)
 
 
-_LOGGER = _DummyLogger()
+_LOGGER: Union[_DummyLogger, Logger] = _DummyLogger()
 
 
-def register_logger(logger):
+def register_logger(logger: Logger) -> None:
     """Register custom logger.
 
     Parameters
@@ -58,12 +58,12 @@ def register_logger(logger):
     _LOGGER = logger
 
 
-def _normalize_native_string(func):
+def _normalize_native_string(func: Callable[[str], None]) -> Callable[[str], None]:
     """Join log messages from native library which come by chunks."""
-    msg_normalized = []
+    msg_normalized: List[str] = []
 
     @wraps(func)
-    def wrapper(msg):
+    def wrapper(msg: str) -> None:
         nonlocal msg_normalized
         if msg.strip() == '':
             msg = ''.join(msg_normalized)
@@ -75,20 +75,20 @@ def wrapper(msg):
     return wrapper
 
 
-def _log_info(msg):
+def _log_info(msg: str) -> None:
     _LOGGER.info(msg)
 
 
-def _log_warning(msg):
+def _log_warning(msg: str) -> None:
     _LOGGER.warning(msg)
 
 
 @_normalize_native_string
-def _log_native(msg):
+def _log_native(msg: str) -> None:
     _LOGGER.info(msg)
 
 
-def _log_callback(msg):
+def _log_callback(msg: bytes) -> None:
     """Redirect logs from native library into Python."""
     _log_native(str(msg.decode('utf-8')))