redirect log to python console

include/LightGBM/c_api.h

@@ -39,6 +39,8 @@ typedef void* BoosterHandle;  /*!< \brief Handle of booster. */
  */
 LIGHTGBM_C_EXPORT const char* LGBM_GetLastError();
 
+LIGHTGBM_C_EXPORT int LGBM_RegisterLogCallback(void (*callback)(const char*));
+
 // --- start Dataset interface
 
 /*!

include/LightGBM/utils/log.h

@@ -1,24 +1,25 @@
 /*!
  * 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>
 #include <cstring>
 #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 {
@@ -30,9 +31,10 @@ namespace LightGBM {
 #endif
 
 #ifndef CHECK
-#define CHECK(condition)                                   \
-  if (!(condition)) Log::Fatal("Check failed: " #condition \
-     " at %s, line %d .\n", __FILE__,  __LINE__);
+#define CHECK(condition)                                                    \
+  if (!(condition))                                                         \
+    Log::Fatal("Check failed: " #condition " at %s, line %d .\n", __FILE__, \
+               __LINE__);
 #endif
 
 #ifndef CHECK_EQ
@@ -60,29 +62,32 @@ namespace LightGBM {
 #endif
 
 #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__);
+#define CHECK_NOTNULL(pointer)                                         \
+  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,
   Debug = 2,
 };
 
 /*!
-* \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;
-  }
+   * \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 ResetCallBack(Callback callback) { GetLogCallBack() = callback; }
 
   static void Debug(const char *format, ...) {
     va_list val;
@@ -113,39 +118,58 @@ class Log {
 #endif
     va_end(val);
 
-    // 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
-      Rf_error("[LightGBM] [Fatal] %s\n", str_buf);
-    #endif
+// 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
+    Rf_error("[LightGBM] [Fatal] %s\n", str_buf);
+#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
-        Rprintf("[LightGBM] [%s] ", level_str);
-        Rvprintf(format, val);
-        Rprintf("\n");
-      #endif
+      } else {
+        const size_t buf_size = 512;
+        char buf[buf_size];
+        snprintf(buf, buf_size, "[LightGBM] [%s] ", level_str);
+        GetLogCallBack()(buf);
+        vsnprintf(buf, buf_size, format, val);
+        GetLogCallBack()(buf);
+        GetLogCallBack()("\n");
+      }
+#else
+      Rprintf("[LightGBM] [%s] ", level_str);
+      Rvprintf(format, val);
+      Rprintf("\n");
+#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
-#endif   // LightGBM_UTILS_LOG_H_
+#endif  // LightGBM_UTILS_LOG_H_

python-package/lightgbm/basic.py

@@ -21,13 +21,21 @@
 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()
     if len(lib_path) == 0:
         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,