improve docs on custom result formatters

Adds section on formatting errors and using CLI. Improves wording
in others.

diff --git a/docs/customize/Result-Formatting.md b/docs/customize/Result-Formatting.md
index c9e9245b4a655dfe9873de3ee168de9788864349..cd817afb82f471690a20482a339133fc7dcd1e1d 100644 (file)
--- a/docs/customize/Result-Formatting.md
+++ b/docs/customize/Result-Formatting.md
@@ -95,14 +95,32 @@ def _format_status_text(result, _):
     if result.status:
         return f"The server is currently not running. {result.message}"
 
     if result.status:
         return f"The server is currently not running. {result.message}"
 

-    return f"Good news! The server is running just fine."
+    return "Good news! The server is running just fine."

 ```
 
 That's all. Nominatim will automatically pick up the new format name and
 ```
 
 That's all. Nominatim will automatically pick up the new format name and

-will allow the user to use it. Make sure to really define formatters for
-**all** result types. If they are for endpoints that you do not intend to
-use, you can simply return some static string but the function needs to be
-there.
+will allow the user to use it. There is no need to implement formatter
+functions for all the result types, when you invent a new one. The
+available formats will be determined for each API endpoint separately.
+To find out which formats are available, you can use the `--list-formats`
+option of the CLI tool:
+
+```
+me@machine:planet-project$ nominatim status --list-formats
+2024-08-16 19:54:00: Using project directory: /home/nominatim/planet-project
+text
+json
+chatty
+debug
+me@machine:planet-project$
+```
+
+The `debug` format listed in the last line will always appear. It is a
+special format that enables debug output via the command line (the same
+as the `debug=1` parameter enables for the server API). To not clash
+with this built-in function, you shouldn't name your own format 'debug'.
+
+### Content type of new formats

 
 All responses will be returned with the content type application/json by
 default. If your format produces a different content type, you need
 
 All responses will be returned with the content type application/json by
 default. If your format produces a different content type, you need


@@ -121,6 +139,47 @@ The `content_types` module used above provides constants for the most
 frequent content types. You set the content type to an arbitrary string,
 if the content type you need is not available.
 
 frequent content types. You set the content type to an arbitrary string,
 if the content type you need is not available.
 

+## Formatting error messages
+
+Any exception thrown during processing of a request is given to
+a special error formatting function. It takes the requested content type,
+the status code and the error message. It should return the error message
+in a form appropriate for the given content type.
+
+You can overwrite the default formatting function with the decorator
+`error_format_func`:
+
+``` python
+import nominatim_api.server.content_types as ct
+
+@dispatch.error_format_func
+def _format_error(content_type: str, msg: str, status: int) -> str:
+    if content_type == ct.CONTENT_XML:
+        return f"""<?xml version="1.0" encoding="UTF-8" ?>
+                     <message>{msg}</message>
+                """
+    if content_type == ct.CONTENT_JSON:
+        return f'"{msg}"'
+
+    return f"ERROR: {msg}"
+```
+
+
+## Debugging custom formatters
+
+The easiest way to try out your custom formatter is by using the Nominatim
+CLI commands. Custom formats can be chosen with the `--format` parameter:
+
+```
+me@machine:planet-project$ nominatim status --format chatty
+2024-08-16 19:54:00: Using project directory: /home/nominatim/planet-project
+Good news! The server is running just fine.
+me@machine:planet-project$
+```
+
+They will also emit full error messages when there is a problem with the
+code you need to debug.
+

 ## Reference
 
 ### FormatDispatcher
 ## Reference
 
 ### FormatDispatcher