[python] make possibility to create Booster from string official (#2098)

python-package/lightgbm/basic.py

@@ -397,7 +397,7 @@ class _InnerPredictor(object):
             self.num_total_iteration = out_num_iterations.value
             self.pandas_categorical = None
         else:
-            raise TypeError('Need Model file or Booster handle to create a predictor')
+            raise TypeError('Need model_file or booster_handle to create a predictor')
 
         pred_parameter = {} if pred_parameter is None else pred_parameter
         self.pred_parameter = param_dict_to_str(pred_parameter)
@@ -1578,7 +1578,7 @@ class Dataset(object):
 class Booster(object):
     """Booster in LightGBM."""
 
-    def __init__(self, params=None, train_set=None, model_file=None, silent=False):
+    def __init__(self, params=None, train_set=None, model_file=None, model_str=None, silent=False):
         """Initialize the Booster.
 
         Parameters
@@ -1589,6 +1589,8 @@ class Booster(object):
             Training dataset.
         model_file : string or None, optional (default=None)
             Path to the model file.
+        model_str : string or None, optional (default=None)
+            Model will be loaded from this string.
         silent : bool, optional (default=False)
             Whether to print messages during construction.
         """
@@ -1666,10 +1668,11 @@ class Booster(object):
                 ctypes.byref(out_num_class)))
             self.__num_class = out_num_class.value
             self.pandas_categorical = _load_pandas_categorical(file_name=model_file)
-        elif 'model_str' in params:
-            self.model_from_string(params['model_str'], False)
+        elif model_str is not None:
+            self.model_from_string(model_str, not silent)
         else:
-            raise TypeError('Need at least one training dataset or model file to create booster instance')
+            raise TypeError('Need at least one training dataset or model file or model string '
+                            'to create Booster instance')
         self.params = params
 
     def __del__(self):
@@ -1689,7 +1692,7 @@ class Booster(object):
 
     def __deepcopy__(self, _):
         model_str = self.model_to_string(num_iteration=-1)
-        booster = Booster({'model_str': model_str})
+        booster = Booster(model_str=model_str)
         return booster
 
     def __getstate__(self):

tests/python_package_test/test_engine.py

@@ -583,7 +583,7 @@ class TestEngine(unittest.TestCase):
         model_str = gbm4.model_to_string()
         gbm4.model_from_string(model_str, False)
         pred5 = gbm4.predict(X_test)
-        gbm5 = lgb.Booster({'model_str': model_str})
+        gbm5 = lgb.Booster(model_str=model_str)
         pred6 = gbm5.predict(X_test)
         np.testing.assert_almost_equal(pred0, pred1)
         np.testing.assert_almost_equal(pred0, pred2)