redirect log to python console (#3090)

* redir log to python console

* fix pylint

* Apply suggestions from code review

* Update basic.py

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* Update c_api.h

* Apply suggestions from code review

* Apply suggestions from code review

* super-minor: better wording
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: StrikerRUS <nekit94-12@hotmail.com>

include/LightGBM/c_api.h

@@ -39,6 +39,13 @@ typedef void* BoosterHandle;  /*!< \brief Handle of booster. */
  */
 LIGHTGBM_C_EXPORT const char* LGBM_GetLastError();
 
+/*!
+ * \brief Register a callback function for log redirecting. 
+ * \param callback The callback function to register
+ * \return 0 when succeed, -1 when failure happens
+ */
+LIGHTGBM_C_EXPORT int LGBM_RegisterLogCallback(void (*callback)(const char*));
+
 // --- start Dataset interface
 
 /*!

include/LightGBM/utils/log.h

 /*!
  * Copyright (c) 2016 Microsoft Corporation. All rights reserved.
- * Licensed under the MIT License. See LICENSE file in the project root for license information.
+ * Licensed under the MIT License. See LICENSE file in the project root for
+ * license information.
  */
 #ifndef LIGHTGBM_UTILS_LOG_H_
 #define LIGHTGBM_UTILS_LOG_H_
 
-#include <string>
 #include <cstdarg>
 #include <cstdio>
 #include <cstdlib>
@@ -13,12 +13,13 @@
 #include <exception>
 #include <iostream>
 #include <stdexcept>
+#include <string>
 
 #ifdef LGB_R_BUILD
-  #define R_NO_REMAP
-  #define R_USE_C99_IN_CXX
-  #include <R_ext/Error.h>
-  #include <R_ext/Print.h>
+#define R_NO_REMAP
+#define R_USE_C99_IN_CXX
+#include <R_ext/Error.h>
+#include <R_ext/Print.h>
 #endif
 
 namespace LightGBM {
@@ -31,8 +32,9 @@ namespace LightGBM {
 
 #ifndef CHECK
 #define CHECK(condition)                                                    \
-  if (!(condition)) Log::Fatal("Check failed: " #condition \
-     " at %s, line %d .\n", __FILE__,  __LINE__);
+  if (!(condition))                                                         \
+    Log::Fatal("Check failed: " #condition " at %s, line %d .\n", __FILE__, \
+               __LINE__);
 #endif
 
 #ifndef CHECK_EQ
@@ -61,10 +63,12 @@ namespace LightGBM {
 
 #ifndef CHECK_NOTNULL
 #define CHECK_NOTNULL(pointer)                                         \
-  if ((pointer) == nullptr) LightGBM::Log::Fatal(#pointer " Can't be NULL at %s, line %d .\n", __FILE__,  __LINE__);
+  if ((pointer) == nullptr)                                            \
+    LightGBM::Log::Fatal(#pointer " Can't be NULL at %s, line %d .\n", \
+                         __FILE__, __LINE__);
 #endif
 
-enum class LogLevel: int {
+enum class LogLevel : int {
   Fatal = -1,
   Warning = 0,
   Info = 1,
@@ -72,17 +76,18 @@ enum class LogLevel: int {
 };
 
 /*!
-* \brief A static Log class
-*/
+ * \brief A static Log class
+ */
 class Log {
  public:
+  using Callback = void (*)(const char *);
   /*!
    * \brief Resets the minimal log level. It is INFO by default.
    * \param level The new minimal log level.
    */
-  static void ResetLogLevel(LogLevel level) {
-    GetLevel() = level;
-  }
+  static void ResetLogLevel(LogLevel level) { GetLevel() = level; }
+
+  static void ResetCallBack(Callback callback) { GetLogCallBack() = callback; }
 
   static void Debug(const char *format, ...) {
     va_list val;
@@ -113,38 +118,57 @@ class Log {
 #endif
     va_end(val);
 
-    // R code should write back to R's error stream,
-    // otherwise to stderr
-    #ifndef LGB_R_BUILD
+// R code should write back to R's error stream,
+// otherwise to stderr
+#ifndef LGB_R_BUILD
     fprintf(stderr, "[LightGBM] [Fatal] %s\n", str_buf);
     fflush(stderr);
     throw std::runtime_error(std::string(str_buf));
-    #else
+#else
     Rf_error("[LightGBM] [Fatal] %s\n", str_buf);
-    #endif
+#endif
   }
 
  private:
-  static void Write(LogLevel level, const char* level_str, const char *format, va_list val) {
+  static void Write(LogLevel level, const char *level_str, const char *format,
+                    va_list val) {
     if (level <= GetLevel()) {  // omit the message with low level
-      // R code should write back to R's output stream,
-      // otherwise to stdout
-      #ifndef LGB_R_BUILD
+// R code should write back to R's output stream,
+// otherwise to stdout
+#ifndef LGB_R_BUILD
+      if (GetLogCallBack() == nullptr) {
         printf("[LightGBM] [%s] ", level_str);
         vprintf(format, val);
         printf("\n");
         fflush(stdout);
-      #else
+      } else {
+        const size_t kBufSize = 512;
+        char buf[kBufSize];
+        snprintf(buf, kBufSize, "[LightGBM] [%s] ", level_str);
+        GetLogCallBack()(buf);
+        vsnprintf(buf, kBufSize, format, val);
+        GetLogCallBack()(buf);
+        GetLogCallBack()("\n");
+      }
+#else
       Rprintf("[LightGBM] [%s] ", level_str);
       Rvprintf(format, val);
       Rprintf("\n");
-      #endif
+#endif
     }
   }
 
   // a trick to use static variable in header file.
   // May be not good, but avoid to use an additional cpp file
-  static LogLevel& GetLevel() { static THREAD_LOCAL LogLevel level = LogLevel::Info; return level; }
+  static LogLevel &GetLevel() {
+    static THREAD_LOCAL LogLevel level = LogLevel::Info;
+    return level;
+  }
+
+  static Callback &GetLogCallBack() {
+    static THREAD_LOCAL Callback callback = nullptr;
+    return callback;
+  }
 };
 
 }  // namespace LightGBM

python-package/lightgbm/basic.py

 # coding: utf-8
 """Wrapper for C API of LightGBM."""
-from __future__ import absolute_import
+from __future__ import absolute_import, print_function
 
 import copy
 import ctypes
@@ -21,6 +21,11 @@ from .compat import (PANDAS_INSTALLED, DataFrame, Series, is_dtype_sparse,
 from .libpath import find_lib_path
 
 
+def _log_callback(msg):
+    """Redirect logs from native library into Python console."""
+    print("{0:s}".format(decode_string(msg)), end='')
+
+
 def _load_lib():
     """Load LightGBM library."""
     lib_path = find_lib_path()
@@ -28,6 +33,10 @@ def _load_lib():
         return None
     lib = ctypes.cdll.LoadLibrary(lib_path[0])
     lib.LGBM_GetLastError.restype = ctypes.c_char_p
+    callback = ctypes.CFUNCTYPE(None, ctypes.c_char_p)
+    lib.callback = callback(_log_callback)
+    if lib.LGBM_RegisterLogCallback(lib.callback) != 0:
+        raise LightGBMError(decode_string(lib.LGBM_GetLastError()))
     return lib
 
 

src/c_api.cpp

@@ -610,6 +610,12 @@ const char* LGBM_GetLastError() {
   return LastErrorMsg();
 }
 
+int LGBM_RegisterLogCallback(void (*callback)(const char*)) {
+  API_BEGIN();
+  Log::ResetCallBack(callback);
+  API_END();
+}
+
 int LGBM_DatasetCreateFromFile(const char* filename,
                                const char* parameters,
                                const DatasetHandle reference,