Allow dictionaries to overwrite entries with #fairseq:overwrite comment (#1073)

Summary:
[This commit](dd1298e) made it so that duplicate entries in a dictionary are ignored. Unfortunately the Camembert model depends on overwriting `<unk>`, `<s>` and `</s>`.

The proposed solution here is to allow the dictionary to have entries like:
```
<unk> 999 #fairseq:overwrite
<s> 999 #fairseq:overwrite
</s> 999 #fairseq:overwrite
, 999
▁de 999
. 999
(...)
```

These will preserve the old overwriting behavior. Thus we can release a new `camembert.v0.tar.gz` with a dictionary like above and it works.
Pull Request resolved: fairinternal/fairseq-py#1073

Reviewed By: kahne

Differential Revision: D20284569

Pulled By: myleott

fbshipit-source-id: bf78fbff13c94bf8a6485cbdda62305ddc30c056

Author: Myle Ott

fairseq/data/dictionary.py

@@ -91,9 +91,9 @@ def unk_string(self, escape=False):
         else:
             return self.unk_word
 
-    def add_symbol(self, word, n=1):
+    def add_symbol(self, word, n=1, overwrite=False):
         """Adds a word to the dictionary"""
-        if word in self.indices:
+        if word in self.indices and not overwrite:
             idx = self.indices[word]
             self.count[idx] = self.count[idx] + n
             return idx
@@ -215,15 +215,31 @@ def add_from_file(self, f):
 
         lines = f.readlines()
         indices_start_line = self._load_meta(lines)
+
         for line in lines[indices_start_line:]:
-            idx = line.rfind(" ")
-            if idx == -1:
+            try:
+                line, field = line.rstrip().rsplit(" ", 1)
+                if field == "#fairseq:overwrite":
+                    overwrite = True
+                    line, field = line.rsplit(" ", 1)
+                else:
+                    overwrite = False
+                count = int(field)
+                word = line
+                if word in self and not overwrite:
+                    raise RuntimeError(
+                        "Duplicate word found when loading Dictionary: '{}'. "
+                        "Duplicate words can overwrite earlier ones by adding the "
+                        "#fairseq:overwrite flag at the end of the corresponding row "
+                        "in the dictionary file. If using the Camembert model, please "
+                        "download an updated copy of the model file."
+                        .format(word)
+                    )
+                self.add_symbol(word, n=count, overwrite=overwrite)
+            except ValueError:
                 raise ValueError(
-                    "Incorrect dictionary format, expected '<token> <cnt>'"
+                    "Incorrect dictionary format, expected '<token> <cnt> [flags]'"
                 )
-            word = line[:idx]
-            count = int(line[idx + 1 :])
-            self.add_symbol(word, n=count)
 
     def _save(self, f, kv_iterator):
         if isinstance(f, str):

tests/test_dictionary.py

@@ -3,6 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
+import io
 import tempfile
 import unittest
 
@@ -65,6 +66,51 @@ def assertMatch(ids, ref_ids):
             assertMatch(reload_ids, ref_ids2)
             assertMatch(finalized_ids, reload_ids)
 
+    def test_overwrite(self):
+        # for example, Camembert overwrites <unk>, <s> and </s>
+        dict_file = io.StringIO(
+            "<unk> 999 #fairseq:overwrite\n"
+            "<s> 999 #fairseq:overwrite\n"
+            "</s> 999 #fairseq:overwrite\n"
+            ", 999\n"
+            "▁de 999\n"
+        )
+        d = Dictionary()
+        d.add_from_file(dict_file)
+        self.assertEqual(d.index('<pad>'), 1)
+        self.assertEqual(d.index('foo'), 3)
+        self.assertEqual(d.index('<unk>'), 4)
+        self.assertEqual(d.index('<s>'), 5)
+        self.assertEqual(d.index('</s>'), 6)
+        self.assertEqual(d.index(','), 7)
+        self.assertEqual(d.index('▁de'), 8)
+
+    def test_no_overwrite(self):
+        # for example, Camembert overwrites <unk>, <s> and </s>
+        dict_file = io.StringIO(
+            "<unk> 999\n"
+            "<s> 999\n"
+            "</s> 999\n"
+            ", 999\n"
+            "▁de 999\n"
+        )
+        d = Dictionary()
+        with self.assertRaisesRegex(RuntimeError, 'Duplicate'):
+            d.add_from_file(dict_file)
+
+    def test_space(self):
+        # for example, character models treat space as a symbol
+        dict_file = io.StringIO(
+            "  999\n"
+            "a 999\n"
+            "b 999\n"
+        )
+        d = Dictionary()
+        d.add_from_file(dict_file)
+        self.assertEqual(d.index(' '), 4)
+        self.assertEqual(d.index('a'), 5)
+        self.assertEqual(d.index('b'), 6)
+
 
 if __name__ == '__main__':
     unittest.main()