fix: supercharged ConjunctiveGraph.serialize to format as TriG by def…

…ault (#1674)

Created method ConjunctiveGraph.serialize(..., format='trig', ...).

Because of typing problems I replaced some annotations in the
overloaded methods of rdflib.Graph.serialize, so that in subclasses
serialize knows, that it returns type(self) and not Graph.

Because Graph.serialize annotations are still incompatible with
ConjunctiveGraph.serialize added type: ignore [overriden]
to let mypy skip that method.

rdflib/graph.py

@@ -1225,7 +1225,7 @@ def absolutize(self, uri: str, defrag: int = 1) -> URIRef:
     # no destination and non-None positional encoding
     @overload
     def serialize(
-        self,
+        self: _GraphT,
         destination: None,
         format: str,
         base: Optional[str],
@@ -1237,7 +1237,7 @@ def serialize(
     # no destination and non-None keyword encoding
     @overload
     def serialize(
-        self,
+        self: _GraphT,
         destination: None = ...,
         format: str = ...,
         base: Optional[str] = ...,
@@ -1250,7 +1250,7 @@ def serialize(
     # no destination and None encoding
     @overload
     def serialize(
-        self,
+        self: _GraphT,
         destination: None = ...,
         format: str = ...,
         base: Optional[str] = ...,
@@ -1262,25 +1262,25 @@ def serialize(
     # non-None destination
     @overload
     def serialize(
-        self,
+        self: _GraphT,
         destination: Union[str, pathlib.PurePath, IO[bytes]],
         format: str = ...,
         base: Optional[str] = ...,
         encoding: Optional[str] = ...,
         **args: Any,
-    ) -> "Graph":
+    ) -> _GraphT:
         ...
 
     # fallback
     @overload
     def serialize(
-        self,
+        self: _GraphT,
         destination: Optional[Union[str, pathlib.PurePath, IO[bytes]]] = ...,
         format: str = ...,
         base: Optional[str] = ...,
         encoding: Optional[str] = ...,
         **args: Any,
-    ) -> Union[bytes, str, "Graph"]:
+    ) -> Union[bytes, str, _GraphT]:
         ...
 
     def serialize(
@@ -2193,6 +2193,16 @@ def context_id(self, uri: str, context_id: Optional[str] = None) -> URIRef:
             context_id = "#context"
         return URIRef(context_id, base=uri)
 
+    def serialize(  # type: ignore [override]
+        self: _ConjunctiveGraphT,
+        destination: Optional[Union[str, pathlib.PurePath, IO[bytes]]] = None,
+        format: str = "trig",
+        base: Optional[str] = None,
+        encoding: Optional[str] = None,
+        **args: Any,
+    ) -> Union[bytes, str, _ConjunctiveGraphT]:
+        return super().serialize(destination, format, base, encoding, **args)
+
     def parse(
         self,
         source: Optional[