fix: Add `to_dict` method to the JSON-LD `Context` class. (#2310)

`Context.to_dict` is used in JSON-LD serialization, but it was not implemented.
This change adds the method.

- Closes <https://github.com/RDFLib/rdflib/issues/2138>.

---------

Co-authored-by: Marc-Antoine Parent <maparent@acm.org>

3 files changed, 130 insertions, 2 deletions

diff --git a/rdflib/plugins/shared/jsonld/context.py b/rdflib/plugins/shared/jsonld/context.py
index d0224d4a..b19f6673 100644
--- a/rdflib/plugins/shared/jsonld/context.py
+++ b/rdflib/plugins/shared/jsonld/context.py
@@ -85,7 +85,7 @@ class Context(object):
         self.terms: Dict[str, Any] = {}
         # _alias maps NODE_KEY to list of aliases
         self._alias: Dict[str, List[str]] = {}
-        self._lookup: Dict[Tuple[str, Any, Union[Defined, str], bool], Any] = {}
+        self._lookup: Dict[Tuple[str, Any, Union[Defined, str], bool], Term] = {}
         self._prefixes: Dict[str, Any] = {}
         self.active = False
         self.parent: Optional[Context] = None
@@ -243,8 +243,10 @@ class Context(object):
 
         if isinstance(container, (list, set, tuple)):
             container = set(container)
-        else:
+        elif container is not UNDEF:
             container = set([container])
+        else:
+            container = set()
 
         term = Term(
             idref,
@@ -617,6 +619,37 @@ class Context(object):
             term = term.get(ID)
         return term
 
+    def _term_dict(self, term: Term) -> Union[Dict[str, Any], str]:
+        tdict: Dict[str, Any] = {}
+        if term.type != UNDEF:
+            tdict[TYPE] = self.shrink_iri(term.type)
+        if term.container:
+            tdict[CONTAINER] = list(term.container)
+        if term.language != UNDEF:
+            tdict[LANG] = term.language
+        if term.reverse:
+            tdict[REV] = term.id
+        else:
+            tdict[ID] = term.id
+        if tdict.keys() == {ID}:
+            return tdict[ID]
+        return tdict
+
+    def to_dict(self) -> Dict[str, Any]:
+        """
+        Returns a dictionary representation of the context that can be
+        serialized to JSON.
+
+        :return: a dictionary representation of the context.
+        """
+        r = {v: k for (k, v) in self._prefixes.items()}
+        r.update({term.name: self._term_dict(term) for term in self._lookup.values()})
+        if self.base:
+            r[BASE] = self.base
+        if self.language:
+            r[LANG] = self.language
+        return r
+
 
 Term = namedtuple(
     "Term",
diff --git a/test/jsonld/test_context.py b/test/jsonld/test_context.py
index 6578268a..034936d2 100644
--- a/test/jsonld/test_context.py
+++ b/test/jsonld/test_context.py
@@ -2,10 +2,12 @@
 JSON-LD Context Spec
 """
 
+import json
 from functools import wraps
 from pathlib import Path
 from typing import Any, Dict
 
+from rdflib.namespace import PROV, XSD, Namespace
 from rdflib.plugins.shared.jsonld import context, errors
 from rdflib.plugins.shared.jsonld.context import Context
 
@@ -234,3 +236,52 @@ def test_dict_source(tmp_path: Path) -> None:
     file.write_text(r"""{ "@context": { "ex": "http://example.com/" } }""")
     ctx = Context(source=[{"@context": file.as_uri()}])
     assert "http://example.com/" == ctx.terms["ex"].id
+
+
+EG = Namespace("https://example.com/")
+
+DIVERSE_CONTEXT = json.loads(
+    """
+        {
+            "@context": {
+                "ex": "https://example.com/",
+                "generatedAt": { "@id": "http://www.w3.org/ns/prov#generatedAtTime", "@type": "http://www.w3.org/2001/XMLSchema#dateTime" },
+                "graphMap": { "@id": "https://example.com/graphMap", "@container": ["@graph", "@id"] },
+                "occupation_en": { "@id": "https://example.com/occupation", "@language": "en" },
+                "children": { "@reverse": "https://example.com/parent" }
+            }
+        }
+        """
+)
+
+
+def test_parsing() -> None:
+    """
+    A `Context` can be parsed from a dict.
+    """
+    ctx = Context(DIVERSE_CONTEXT)
+    assert f"{EG}" == ctx.terms["ex"].id
+    assert f"{PROV.generatedAtTime}" == ctx.terms["generatedAt"].id
+    assert f"{XSD.dateTime}" == ctx.terms["generatedAt"].type
+    assert f"{EG.graphMap}" == ctx.terms["graphMap"].id
+    assert {"@graph", "@id"} == ctx.terms["graphMap"].container
+    assert f"{EG.occupation}" == ctx.terms["occupation_en"].id
+    assert "en" == ctx.terms["occupation_en"].language
+    assert False is ctx.terms["occupation_en"].reverse
+    assert True is ctx.terms["children"].reverse
+    assert f"{EG.parent}" == ctx.terms["children"].id
+
+
+def test_to_dict() -> None:
+    """
+    A `Context` can be converted to a dictionary.
+    """
+    ctx = Context()
+    ctx.add_term("ex", f"{EG}")
+    ctx.add_term("generatedAt", f"{PROV.generatedAtTime}", coercion=f"{XSD.dateTime}")
+    ctx.add_term("graphMap", f"{EG.graphMap}", container=["@graph", "@id"])
+    ctx.add_term("occupation_en", f"{EG.occupation}", language="en")
+    ctx.add_term("children", f"{EG.parent}", reverse=True)
+    result = ctx.to_dict()
+    result["graphMap"]["@container"] = sorted(result["graphMap"]["@container"])
+    assert DIVERSE_CONTEXT["@context"] == result
diff --git a/test/test_serializers/test_serializer_jsonld.py b/test/test_serializers/test_serializer_jsonld.py
new file mode 100644
index 00000000..aff0544e
--- /dev/null
+++ b/test/test_serializers/test_serializer_jsonld.py
@@ -0,0 +1,44 @@
+import json
+import logging
+import pprint
+from typing import Any, Dict, Union
+
+import pytest
+
+from rdflib import Graph
+from rdflib.namespace import Namespace
+from rdflib.plugins.shared.jsonld.context import Context
+
+EG = Namespace("http://example.org/")
+
+
+@pytest.mark.parametrize(
+    ["input"],
+    [
+        (
+            Context(
+                {
+                    "eg": f"{EG}",
+                }
+            ),
+        ),
+        ({"eg": f"{EG}"},),
+    ],
+)
+def test_serialize_context(input: Union[Dict[str, Any], Context]) -> None:
+    """
+    The JSON-LD serializer accepts and correctly serializes the context argument to the output.
+    """
+    graph = Graph()
+    graph.add((EG.subject, EG.predicate, EG.object0))
+    graph.add((EG.subject, EG.predicate, EG.object1))
+    context = Context(
+        {
+            "eg": f"{EG}",
+        }
+    )
+    logging.debug("context = %s", pprint.pformat(vars(context)))
+    data = graph.serialize(format="json-ld", context=context)
+    logging.debug("data = %s", data)
+    obj = json.loads(data)
+    assert obj["@context"] == {"eg": f"{EG}"}