configurable error formatting and content type in result formatter

src/nominatim_api/result_formatting.py

@@ -12,8 +12,11 @@ from collections import defaultdict
 from pathlib import Path
 import importlib
 
+from .server.content_types import CONTENT_JSON
+
 T = TypeVar('T') # pylint: disable=invalid-name
 FormatFunc = Callable[[T, Mapping[str, Any]], str]
+ErrorFormatFunc = Callable[[str, str, int], str]
 
 
 class FormatDispatcher:
@@ -21,7 +24,11 @@ class FormatDispatcher:
         a module using decorators.
     """
 
-    def __init__(self) -> None:
+    def __init__(self, content_types: Optional[Mapping[str, str]] = None) -> None:
+        self.error_handler: ErrorFormatFunc = lambda ct, msg, status: f"ERROR {status}: {msg}"
+        self.content_types: Dict[str, str] = {}
+        if content_types:
+            self.content_types.update(content_types)
         self.format_functions: Dict[Type[Any], Dict[str, FormatFunc[Any]]] = defaultdict(dict)
 
 
@@ -37,6 +44,15 @@ class FormatDispatcher:
         return decorator
 
 
+    def error_format_func(self, func: ErrorFormatFunc) -> ErrorFormatFunc:
+        """ Decorator for a function that formats error messges.
+            There is only one error formatter per dispatcher. Using
+            the decorator repeatedly will overwrite previous functions.
+        """
+        self.error_handler = func
+        return func
+
+
     def list_formats(self, result_type: Type[Any]) -> List[str]:
         """ Return a list of formats supported by this formatter.
         """
@@ -58,6 +74,32 @@ class FormatDispatcher:
         return self.format_functions[type(result)][fmt](result, options)
 
 
+    def format_error(self, content_type: str, msg: str, status: int) -> str:
+        """ Convert the given error message into a response string
+            taking the requested content_type into account.
+
+            Change the format using the error_format_func decorator.
+        """
+        return self.error_handler(content_type, msg, status)
+
+
+    def set_content_type(self, fmt: str, content_type: str) -> None:
+        """ Set the content type for the given format. This is the string
+            that will be returned in the Content-Type header of the HTML
+            response, when the given format is choosen.
+        """
+        self.content_types[fmt] = content_type
+
+
+    def get_content_type(self, fmt: str) -> str:
+        """ Return the content type for the given format.
+
+            If no explicit content type has been defined, then
+            JSON format is assumed.
+        """
+        return self.content_types.get(fmt, CONTENT_JSON)
+
+
 def load_format_dispatcher(api_name: str, project_dir: Optional[Path]) -> FormatDispatcher:
     """ Load the dispatcher for the given API.