narrow: Use dict to map operator to by_* method in NarrowBuilder.

Updates the logic for identifying the method to use to extend the
query for the given term from a narrow to use a dictionary that
maps the operator string to the by_* method in the NarrowBuilder
class.

Previously, the by_* method was determined by building a string
based on the operator string and replacing dashes with underscores.

zerver/lib/narrow.py

@@ -256,6 +256,20 @@ class NarrowBuilder:
         self.msg_id_column = msg_id_column
         self.realm = realm
         self.is_web_public_query = is_web_public_query
+        self.by_method_map = {
+            "has": self.by_has,
+            "in": self.by_in,
+            "is": self.by_is,
+            "stream": self.by_stream,
+            "streams": self.by_streams,
+            "topic": self.by_topic,
+            "sender": self.by_sender,
+            "near": self.by_near,
+            "id": self.by_id,
+            "search": self.by_search,
+            "pm-with": self.by_pm_with,
+            "group-pm-with": self.by_group_pm_with,
+        }
 
     def add_term(self, query: Select, term: Dict[str, Any]) -> Select:
         """
@@ -278,9 +292,9 @@ class NarrowBuilder:
 
         negated = term.get("negated", False)
 
-        method_name = "by_" + operator.replace("-", "_")
-        method = getattr(self, method_name, None)
-        if method is None:
+        if operator in self.by_method_map:
+            method = self.by_method_map[operator]
+        else:
             raise BadNarrowOperatorError("unknown operator " + operator)
 
         if negated: