add default switch

fastNLP/core/vocabulary.py

@@ -28,15 +28,25 @@ class Vocabulary(object):
         vocab["word"]
         vocab.to_word(5)
     """
-    def __init__(self):
-        self.word2idx = deepcopy(DEFAULT_WORD_TO_INDEX)
-        self.padding_label = DEFAULT_PADDING_LABEL
-        self.unknown_label = DEFAULT_UNKNOWN_LABEL
+    def __init__(self, need_default=True):
+        """
+        :param bool need_default: set if the Vocabulary has default labels reserved.
+        """
+        if need_default:
+            self.word2idx = deepcopy(DEFAULT_WORD_TO_INDEX)
+            self.padding_label = DEFAULT_PADDING_LABEL
+            self.unknown_label = DEFAULT_UNKNOWN_LABEL
+        else:
+            self.word2idx = {}
+            self.padding_label = None
+            self.unknown_label = None
+
+        self.has_default = need_default
         self.idx2word = None
 
     def __len__(self):
         return len(self.word2idx)
-    
+
     def update(self, word):
         """add word or list of words into Vocabulary
         
@@ -73,9 +83,13 @@ class Vocabulary(object):
         return self[w]
     
     def unknown_idx(self):
+        if self.unknown_label is None: 
+            return None
         return self.word2idx[self.unknown_label]
     
     def padding_idx(self):
+        if self.padding_label is None:
+            return None
         return self.word2idx[self.padding_label]
 
     def build_reverse_vocab(self):