Assign an empty mapping array to empty fields of NamespaceSwappingField (#4403)

Author: Pengcheng YIN

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed a bug with the sharded dataset reader where it would only read a fraction of the instances
   in distributed training.
 - Fixed checking equality of `ArrayField`s.
+- Fixed a bug where `NamespaceSwappingField` did not work correctly with `.empty_field()`.
 
 ### Added
 

allennlp/data/fields/namespace_swapping_field.py

@@ -49,7 +49,10 @@ def as_tensor(self, padding_lengths: Dict[str, int]) -> torch.Tensor:
 
     @overrides
     def empty_field(self) -> "NamespaceSwappingField":
-        return NamespaceSwappingField([], self._target_namespace)
+        empty_field = NamespaceSwappingField([], self._target_namespace)
+        empty_field._mapping_array = []
+
+        return empty_field
 
     def __len__(self):
         return len(self._source_tokens)