implement target_name(field_name) and target_names(field_name) for Do…

…cument

src/pytorch_ie/core/document.py

@@ -518,6 +518,26 @@ def annotation_fields(cls) -> Set[dataclasses.Field]:
             if typing.get_origin(ann_field_types[f.name]) is AnnotationLayer
         }
 
+    @classmethod
+    def target_names(cls, field_name: str) -> Set[str]:
+        a_field = next((f for f in cls.annotation_fields() if f.name == field_name), None)
+        if a_field is None:
+            raise ValueError(f"'{field_name}' is not an annotation field of {cls.__name__}.")
+        result = a_field.metadata.get("targets")
+        if result is None:
+            raise ValueError(f"Annotation field '{field_name}' has no targets.")
+        return set(result)
+
+    @classmethod
+    def target_name(cls, field_name: str) -> str:
+        target_names = cls.target_names(field_name)
+        if len(target_names) != 1:
+            raise ValueError(
+                f"The annotation field '{field_name}' has more or less than one target, "
+                f"can not return a single target name: {sorted(target_names)}"
+            )
+        return list(target_names)[0]
+
     def __getitem__(self, key: str) -> AnnotationLayer:
         if key not in self._annotation_fields:
             raise KeyError(f"Document has no attribute '{key}'.")

tests/core/test_document.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+from pytorch_ie.annotations import BinaryRelation
 from pytorch_ie.core import Annotation
 from pytorch_ie.core.document import (
     AnnotationLayer,
@@ -347,6 +348,35 @@ class MyDocument(Document):
     assert annotation_field_names == {"words"}
 
 
+def test_document_target_names():
+    @dataclasses.dataclass
+    class MyDocument(Document):
+        text: str
+        words: AnnotationLayer[Span] = annotation_field(target="text")
+        sentences: AnnotationLayer[Span] = annotation_field(target="text")
+        belongs_to: AnnotationLayer[BinaryRelation] = annotation_field(
+            targets=["words", "sentences"]
+        )
+
+    # request target names for annotation field
+    assert MyDocument.target_names("words") == {"text"}
+    assert MyDocument.target_name("words") == "text"
+
+    # requested field is not an annotation field
+    with pytest.raises(ValueError) as excinfo:
+        MyDocument.target_names("text")
+    assert str(excinfo.value) == f"'text' is not an annotation field of {MyDocument.__name__}."
+
+    # requested field has two targets
+    assert MyDocument.target_names("belongs_to") == {"words", "sentences"}
+    with pytest.raises(ValueError) as excinfo:
+        MyDocument.target_name("belongs_to")
+    assert (
+        str(excinfo.value)
+        == f"The annotation field 'belongs_to' has more or less than one target, can not return a single target name: ['sentences', 'words']"
+    )
+
+
 def test_document_copy():
     @dataclasses.dataclass
     class MyDocument(Document):