[WIP] expose 'load/save model from/to string' to c_api (#241)

* expose save model to string to c_api

* add 'save-model-to-string' to python; use it to copy model

* remove boosting_type

include/LightGBM/c_api.h

@@ -259,6 +259,17 @@ LIGHTGBM_C_EXPORT int LGBM_BoosterCreateFromModelfile(
   int* out_num_iterations,
   BoosterHandle* out);
 
+/*!
+* \brief load an existing boosting from string
+* \param model_str model string
+* \param out_num_iterations number of iterations of this booster
+* \param out handle of created Booster
+* \return 0 when succeed, -1 when failure happens
+*/
+LIGHTGBM_C_EXPORT int LGBM_BoosterLoadModelFromString(
+  const char* model_str,
+  int* out_num_iterations,
+  BoosterHandle* out);
 
 /*!
 * \brief free obj in handle
@@ -558,6 +569,21 @@ LIGHTGBM_C_EXPORT int LGBM_BoosterSaveModel(BoosterHandle handle,
   int num_iteration,
   const char* filename);
 
+/*!
+* \brief save model to string
+* \param handle handle
+* \param num_iteration, <= 0 means save all
+* \param buffer_len string buffer length, if buffer_len < out_len, re-allocate buffer
+* \param out_len actual output length
+* \param out_str string of model, need to pre-allocate memory before call this
+* \return 0 when succeed, -1 when failure happens
+*/
+LIGHTGBM_C_EXPORT int LGBM_BoosterSaveModelToString(BoosterHandle handle,
+  int num_iteration,
+  int buffer_len,
+  int* out_len,
+  char* out_str);
+
 /*!
 * \brief dump model to json
 * \param handle handle

python-package/lightgbm/basic.py

@@ -1213,6 +1213,8 @@ class Booster(object):
                     self.pandas_categorical = eval(last_line[len('pandas_categorical:'):])
                 else:
                     self.pandas_categorical = None
+        elif 'model_str' in params:
+            self.__load_model_from_string(params['model_str'])
         else:
             raise TypeError('Need at least one training dataset or model file to create booster instance')
 
@@ -1224,9 +1226,10 @@ class Booster(object):
         return self.__deepcopy__(None)
 
     def __deepcopy__(self, _):
-        with _temp_file() as f:
-            self.save_model(f.name)
-            return Booster(model_file=f.name)
+        model_str = self.__save_model_to_string()
+        booster = Booster({'model_str': model_str})
+        booster.pandas_categorical = self.pandas_categorical
+        return booster
 
     def __getstate__(self):
         this = self.__dict__.copy()
@@ -1234,22 +1237,18 @@ class Booster(object):
         this.pop('train_set', None)
         this.pop('valid_sets', None)
         if handle is not None:
-            with _temp_file() as f:
-                self.save_model(f.name)
-                this["handle"] = f.readlines()
+            this["handle"] = self.__save_model_to_string()
         return this
 
     def __setstate__(self, state):
-        model = state['handle']
-        if model is not None:
+        model_str = state.get('handle', None)
+        if model_str is not None:
             handle = ctypes.c_void_p()
             out_num_iterations = ctypes.c_int(0)
-            with _temp_file() as f:
-                f.writelines(model)
-                _safe_call(_LIB.LGBM_BoosterCreateFromModelfile(
-                    c_str(f.name),
-                    ctypes.byref(out_num_iterations),
-                    ctypes.byref(handle)))
+            _safe_call(_LIB.LGBM_BoosterLoadModelFromString(
+                c_str(model_str),
+                ctypes.byref(out_num_iterations),
+                ctypes.byref(handle)))
             state['handle'] = handle
         self.__dict__.update(state)
 
@@ -1472,6 +1471,46 @@ class Booster(object):
         with open(filename, 'a') as f:
             f.write('\npandas_categorical:' + repr(self.pandas_categorical))
 
+    def __load_model_from_string(self, model_str):
+        """[Private] Load model from string"""
+        out_num_iterations = ctypes.c_int(0)
+        _safe_call(_LIB.LGBM_BoosterLoadModelFromString(
+            c_str(model_str),
+            ctypes.byref(out_num_iterations),
+            ctypes.byref(self.handle)))
+        out_num_class = ctypes.c_int(0)
+        _safe_call(_LIB.LGBM_BoosterGetNumClasses(
+            self.handle,
+            ctypes.byref(out_num_class)))
+        self.__num_class = out_num_class.value
+
+    def __save_model_to_string(self, num_iteration=-1):
+        """[Private] Save model to string"""
+        if num_iteration <= 0:
+            num_iteration = self.best_iteration
+        buffer_len = 1 << 20
+        tmp_out_len = ctypes.c_int(0)
+        string_buffer = ctypes.create_string_buffer(buffer_len)
+        ptr_string_buffer = ctypes.c_char_p(*[ctypes.addressof(string_buffer)])
+        _safe_call(_LIB.LGBM_BoosterSaveModelToString(
+            self.handle,
+            ctypes.c_int(num_iteration),
+            ctypes.c_int(buffer_len),
+            ctypes.byref(tmp_out_len),
+            ptr_string_buffer))
+        actual_len = tmp_out_len.value
+        '''if buffer length is not long enough, re-allocate a buffer'''
+        if actual_len > buffer_len:
+            string_buffer = ctypes.create_string_buffer(actual_len)
+            ptr_string_buffer = ctypes.c_char_p(*[ctypes.addressof(string_buffer)])
+            _safe_call(_LIB.LGBM_BoosterSaveModelToString(
+                self.handle,
+                ctypes.c_int(num_iteration),
+                ctypes.c_int(actual_len),
+                ctypes.byref(tmp_out_len),
+                ptr_string_buffer))
+        return string_buffer.value.decode()
+
     def dump_model(self, num_iteration=-1):
         """
         Dump model to json format

src/c_api.cpp

@@ -31,6 +31,10 @@ public:
     boosting_.reset(Boosting::CreateBoosting(filename));
   }
 
+  Booster() {
+    boosting_.reset(Boosting::CreateBoosting("gbdt", nullptr));
+  }
+
   Booster(const Dataset* train_data,
     const char* parameters) {
     auto param = ConfigBase::Str2Map(parameters);
@@ -181,6 +185,14 @@ public:
     boosting_->SaveModelToFile(num_iteration, filename);
   }
 
+  void LoadModelFromString(const char* model_str) {
+    boosting_->LoadModelFromString(model_str);
+  }
+
+  std::string SaveModelToString(int num_iteration) {
+    return boosting_->SaveModelToString(num_iteration);
+  }
+
   std::string DumpModel(int num_iteration) {
     return boosting_->DumpModel(num_iteration);
   }
@@ -605,6 +617,18 @@ LIGHTGBM_C_EXPORT int LGBM_BoosterCreateFromModelfile(
   API_END();
 }
 
+LIGHTGBM_C_EXPORT int LGBM_BoosterLoadModelFromString(
+  const char* model_str,
+  int* out_num_iterations,
+  BoosterHandle* out) {
+  API_BEGIN();
+  auto ret = std::unique_ptr<Booster>(new Booster());
+  ret->LoadModelFromString(model_str);
+  *out_num_iterations = ret->GetBoosting()->GetCurrentIteration();
+  *out = ret.release();
+  API_END();
+}
+
 LIGHTGBM_C_EXPORT int LGBM_BoosterFree(BoosterHandle handle) {
   API_BEGIN();
   delete reinterpret_cast<Booster*>(handle);
@@ -889,6 +913,21 @@ LIGHTGBM_C_EXPORT int LGBM_BoosterSaveModel(BoosterHandle handle,
   API_END();
 }
 
+LIGHTGBM_C_EXPORT int LGBM_BoosterSaveModelToString(BoosterHandle handle,
+  int num_iteration,
+  int buffer_len,
+  int* out_len,
+  char* out_str) {
+  API_BEGIN();
+  Booster* ref_booster = reinterpret_cast<Booster*>(handle);
+  std::string model = ref_booster->SaveModelToString(num_iteration);
+  *out_len = static_cast<int>(model.size()) + 1;
+  if (*out_len <= buffer_len) {
+    std::strcpy(out_str, model.c_str());
+  }
+  API_END();
+}
+
 LIGHTGBM_C_EXPORT int LGBM_BoosterDumpModel(BoosterHandle handle,
   int num_iteration,
   int buffer_len,