fix: eliminate some mutable default arguments in SPARQL code (#2301)

This change eliminates some situations where a mutable object (i.e., a dictionary) was used as the default value for functions in the `rdflib.plugins.sparql.processor` module and related code. It replaces these situations with `typing.Optinal` that defaults to None, and is then handled within the function. Luckily, some of the code that the SPARQL Processor relied on already had this style, meaning not a lot of changes had to be made.

This change also makes a small update to the logic in the SPARQL Processor's query function to simplify the if/else statement. This better mirrors the implementation in the `UpdateProcessor`.

3 files changed, 26 insertions, 18 deletions

diff --git a/rdflib/plugins/sparql/evaluate.py b/rdflib/plugins/sparql/evaluate.py
index 4f8d687b..764250c8 100644
--- a/rdflib/plugins/sparql/evaluate.py
+++ b/rdflib/plugins/sparql/evaluate.py
@@ -642,7 +642,7 @@ def evalDescribeQuery(ctx: QueryContext, query) -> Dict[str, Union[str, Graph]]:
 def evalQuery(
     graph: Graph,
     query: Query,
-    initBindings: Mapping[str, Identifier],
+    initBindings: Optional[Mapping[str, Identifier]] = None,
     base: Optional[str] = None,
 ) -> Mapping[Any, Any]:
     """
@@ -661,7 +661,7 @@ def evalQuery(
         documentation.
     """
 
-    initBindings = dict((Variable(k), v) for k, v in initBindings.items())
+    initBindings = dict((Variable(k), v) for k, v in (initBindings or {}).items())
 
     ctx = QueryContext(graph, initBindings=initBindings)
 
diff --git a/rdflib/plugins/sparql/processor.py b/rdflib/plugins/sparql/processor.py
index c2fb7e54..f10f372b 100644
--- a/rdflib/plugins/sparql/processor.py
+++ b/rdflib/plugins/sparql/processor.py
@@ -19,22 +19,30 @@ from rdflib.term import Identifier
 
 
 def prepareQuery(
-    queryString: str, initNs: Mapping[str, Any] = {}, base: Optional[str] = None
+    queryString: str,
+    initNs: Optional[Mapping[str, Any]] = None,
+    base: Optional[str] = None,
 ) -> Query:
     """
     Parse and translate a SPARQL Query
     """
+    if initNs is None:
+        initNs = {}
     ret = translateQuery(parseQuery(queryString), base, initNs)
     ret._original_args = (queryString, initNs, base)
     return ret
 
 
 def prepareUpdate(
-    updateString: str, initNs: Mapping[str, Any] = {}, base: Optional[str] = None
+    updateString: str,
+    initNs: Optional[Mapping[str, Any]] = None,
+    base: Optional[str] = None,
 ) -> Update:
     """
     Parse and translate a SPARQL Update
     """
+    if initNs is None:
+        initNs = {}
     ret = translateUpdate(parseUpdate(updateString), base, initNs)
     ret._original_args = (updateString, initNs, base)
     return ret
@@ -43,8 +51,8 @@ def prepareUpdate(
 def processUpdate(
     graph: Graph,
     updateString: str,
-    initBindings: Mapping[str, Identifier] = {},
-    initNs: Mapping[str, Any] = {},
+    initBindings: Optional[Mapping[str, Identifier]] = None,
+    initNs: Optional[Mapping[str, Any]] = None,
     base: Optional[str] = None,
 ) -> None:
     """
@@ -73,8 +81,8 @@ class SPARQLUpdateProcessor(UpdateProcessor):
     def update(
         self,
         strOrQuery: Union[str, Update],
-        initBindings: Mapping[str, Identifier] = {},
-        initNs: Mapping[str, Any] = {},
+        initBindings: Optional[Mapping[str, Identifier]] = None,
+        initNs: Optional[Mapping[str, Any]] = None,
     ) -> None:
         """
         .. caution::
@@ -108,8 +116,8 @@ class SPARQLProcessor(Processor):
     def query(  # type: ignore[override]
         self,
         strOrQuery: Union[str, Query],
-        initBindings: Mapping[str, Identifier] = {},
-        initNs: Mapping[str, Any] = {},
+        initBindings: Optional[Mapping[str, Identifier]] = None,
+        initNs: Optional[Mapping[str, Any]] = None,
         base: Optional[str] = None,
         DEBUG: bool = False,
     ) -> Mapping[str, Any]:
@@ -132,9 +140,7 @@ class SPARQLProcessor(Processor):
            documentation.
         """
 
-        if not isinstance(strOrQuery, Query):
-            parsetree = parseQuery(strOrQuery)
-            query = translateQuery(parsetree, base, initNs)
-        else:
-            query = strOrQuery
-        return evalQuery(self.graph, query, initBindings, base)
+        if isinstance(strOrQuery, str):
+            strOrQuery = translateQuery(parseQuery(strOrQuery), base, initNs)
+
+        return evalQuery(self.graph, strOrQuery, initBindings, base)
diff --git a/rdflib/plugins/sparql/update.py b/rdflib/plugins/sparql/update.py
index f27ee9b3..5ce86f39 100644
--- a/rdflib/plugins/sparql/update.py
+++ b/rdflib/plugins/sparql/update.py
@@ -280,7 +280,9 @@ def evalCopy(ctx: QueryContext, u: CompValue) -> None:
 
 
 def evalUpdate(
-    graph: Graph, update: Update, initBindings: Mapping[str, Identifier] = {}
+    graph: Graph,
+    update: Update,
+    initBindings: Optional[Mapping[str, Identifier]] = None,
 ) -> None:
     """
 
@@ -315,7 +317,7 @@ def evalUpdate(
     """
 
     for u in update.algebra:
-        initBindings = dict((Variable(k), v) for k, v in initBindings.items())
+        initBindings = dict((Variable(k), v) for k, v in (initBindings or {}).items())
 
         ctx = QueryContext(graph, initBindings=initBindings)
         ctx.prologue = u.prologue