[python] allow to register any custom logger (fixes #4783) (#4880)

* [python] allow to register any custom logger

* allow customizable logging method name; add unit test

* [python] allow to register any custom logger

* allow customizable logging method name; add unit test

* update tests

* fix lint error

* remove unused method

* fix docstring style
Co-authored-by: gongxudong <gongxudong@kuaishou.com>

python-package/lightgbm/basic.py

@@ -41,21 +41,37 @@ class _DummyLogger:
         warnings.warn(msg, stacklevel=3)
 
 
-_LOGGER: Union[_DummyLogger, Logger] = _DummyLogger()
+_LOGGER: Any = _DummyLogger()
+_INFO_METHOD_NAME = "info"
+_WARNING_METHOD_NAME = "warning"
 
 
-def register_logger(logger: Logger) -> None:
+def register_logger(
+    logger: Any, info_method_name: str = "info", warning_method_name: str = "warning"
+) -> None:
     """Register custom logger.
 
     Parameters
     ----------
-    logger : logging.Logger
+    logger : Any
         Custom logger.
+    info_method_name : str, optional (default="info")
+        Method used to log info messages.
+    warning_method_name : str, optional (default="warning")
+        Method used to log warning messages.
     """
-    if not isinstance(logger, Logger):
-        raise TypeError("Logger should inherit logging.Logger class")
-    global _LOGGER
+    def _has_method(logger: Any, method_name: str) -> bool:
+        return callable(getattr(logger, method_name, None))
+
+    if not _has_method(logger, info_method_name) or not _has_method(logger, warning_method_name):
+        raise TypeError(
+            f"Logger must provide '{info_method_name}' and '{warning_method_name}' method"
+        )
+
+    global _LOGGER, _INFO_METHOD_NAME, _WARNING_METHOD_NAME
     _LOGGER = logger
+    _INFO_METHOD_NAME = info_method_name
+    _WARNING_METHOD_NAME = warning_method_name
 
 
 def _normalize_native_string(func: Callable[[str], None]) -> Callable[[str], None]:
@@ -76,16 +92,16 @@ def _normalize_native_string(func: Callable[[str], None]) -> Callable[[str], Non
 
 
 def _log_info(msg: str) -> None:
-    _LOGGER.info(msg)
+    getattr(_LOGGER, _INFO_METHOD_NAME)(msg)
 
 
 def _log_warning(msg: str) -> None:
-    _LOGGER.warning(msg)
+    getattr(_LOGGER, _WARNING_METHOD_NAME)(msg)
 
 
 @_normalize_native_string
 def _log_native(msg: str) -> None:
-    _LOGGER.info(msg)
+    getattr(_LOGGER, _INFO_METHOD_NAME)(msg)
 
 
 def _log_callback(msg: bytes) -> None:

tests/python_package_test/test_utilities.py

@@ -2,6 +2,7 @@
 import logging
 
 import numpy as np
+import pytest
 
 import lightgbm as lgb
 
@@ -97,3 +98,70 @@ WARNING | More than one metric available, picking one to plot.
                 actual_log_wo_gpu_stuff.append(line)
 
     assert "\n".join(actual_log_wo_gpu_stuff) == expected_log
+
+
+def test_register_invalid_logger():
+    class LoggerWithoutInfoMethod:
+        def warning(self, msg: str) -> None:
+            print(msg)
+
+    class LoggerWithoutWarningMethod:
+        def info(self, msg: str) -> None:
+            print(msg)
+
+    class LoggerWithAttributeNotCallable:
+        def __init__(self):
+            self.info = 1
+            self.warning = 2
+
+    expected_error_message = "Logger must provide 'info' and 'warning' method"
+
+    with pytest.raises(TypeError, match=expected_error_message):
+        lgb.register_logger(LoggerWithoutInfoMethod())
+
+    with pytest.raises(TypeError, match=expected_error_message):
+        lgb.register_logger(LoggerWithoutWarningMethod())
+
+    with pytest.raises(TypeError, match=expected_error_message):
+        lgb.register_logger(LoggerWithAttributeNotCallable())
+
+
+def test_register_custom_logger():
+    logged_messages = []
+
+    class CustomLogger:
+        def custom_info(self, msg: str) -> None:
+            logged_messages.append(msg)
+
+        def custom_warning(self, msg: str) -> None:
+            logged_messages.append(msg)
+
+    custom_logger = CustomLogger()
+    lgb.register_logger(
+        custom_logger,
+        info_method_name="custom_info",
+        warning_method_name="custom_warning"
+    )
+
+    lgb.basic._log_info("info message")
+    lgb.basic._log_warning("warning message")
+
+    expected_log = ["info message", "warning message"]
+    assert logged_messages == expected_log
+
+    logged_messages = []
+    X = np.array([[1, 2, 3],
+                  [1, 2, 4],
+                  [1, 2, 4],
+                  [1, 2, 3]],
+                 dtype=np.float32)
+    y = np.array([0, 1, 1, 0])
+    lgb_data = lgb.Dataset(X, y)
+    lgb.train(
+        {'objective': 'binary', 'metric': 'auc'},
+        lgb_data,
+        num_boost_round=10,
+        valid_sets=[lgb_data],
+        categorical_feature=[1]
+    )
+    assert logged_messages, "custom logger was not called"