Merge pull request #1656 from RDFLib/roundtrip_hext

Allow hext to participate in RDF format roundtripping

2 files changed, 20 insertions, 7 deletions

diff --git a/rdflib/plugins/parsers/hext.py b/rdflib/plugins/parsers/hext.py
index 59e045cf..fab70c78 100644
--- a/rdflib/plugins/parsers/hext.py
+++ b/rdflib/plugins/parsers/hext.py
@@ -24,14 +24,23 @@ class HextuplesParser(Parser):
         pass
 
     def _load_json_line(self, line: str):
-        return [x if x != "" else None for x in json.loads(line)]
+        # this complex handing is because the 'value' component is
+        # allowed to be "" but not None
+        # all other "" values are treated as None
+        ret1 = json.loads(line)
+        ret2 = [x if x != "" else None for x in ret1]
+        if ret1[2] == "":
+            ret2[2] = ""
+        return ret2
 
     def _parse_hextuple(self, cg: ConjunctiveGraph, tup: List[Union[str, None]]):
         # all values check
         # subject, predicate, value, datatype cannot be None
         # language and graph may be None
         if tup[0] is None or tup[1] is None or tup[2] is None or tup[3] is None:
-            raise ValueError("subject, predicate, value, datatype cannot be None")
+            raise ValueError(
+                "subject, predicate, value, datatype cannot be None. Given: "
+                f"{tup}")
 
         # 1 - subject
         s: Union[URIRef, BNode]
diff --git a/rdflib/plugins/serializers/hext.py b/rdflib/plugins/serializers/hext.py
index c86882a2..3fdf3684 100644
--- a/rdflib/plugins/serializers/hext.py
+++ b/rdflib/plugins/serializers/hext.py
@@ -2,7 +2,8 @@
 HextuplesSerializer RDF graph serializer for RDFLib.
 See <https://github.com/ontola/hextuples> for details about the format.
 """
-from typing import IO, Optional, Union
+from typing import IO, Optional, Type, Union
+import json
 from rdflib.graph import Graph, ConjunctiveGraph
 from rdflib.term import Literal, URIRef, Node, BNode
 from rdflib.serializer import Serializer
@@ -19,7 +20,9 @@ class HextuplesSerializer(Serializer):
 
     def __init__(self, store: Union[Graph, ConjunctiveGraph]):
         self.default_context: Optional[Node]
+        self.graph_type: Type[Graph]
         if isinstance(store, ConjunctiveGraph):
+            self.graph_type = ConjunctiveGraph
             self.contexts = list(store.contexts())
             if store.default_context:
                 self.default_context = store.default_context
@@ -27,6 +30,7 @@ class HextuplesSerializer(Serializer):
             else:
                 self.default_context = None
         else:
+            self.graph_type = Graph
             self.contexts = [store]
             self.default_context = None
 
@@ -101,14 +105,14 @@ class HextuplesSerializer(Serializer):
             else:
                 language = ""
 
-            return '["%s", "%s", "%s", "%s", "%s", "%s"]\n' % (
+            return json.dumps([
                 self._iri_or_bn(triple[0]),
                 triple[1],
                 value,
                 datatype,
                 language,
-                self._context(context),
-            )
+                self._context(context)
+            ]) + "\n"
         else:  # do not return anything for non-IRIs or BNs, e.g. QuotedGraph, Subjects
             return None
 
@@ -121,7 +125,7 @@ class HextuplesSerializer(Serializer):
             return None
 
     def _context(self, context):
-        if self.default_context is None:
+        if self.graph_type == Graph:
             return ""
         if context.identifier == "urn:x-rdflib:default":
             return ""