Add error format from config replacement objects

sanic/app.py

@@ -1472,7 +1472,9 @@ class Sanic(BaseSanic, metaclass=TouchUpMeta):
     async def _startup(self):
         self.signalize()
         self.finalize()
-        ErrorHandler.finalize(self.error_handler)
+        ErrorHandler.finalize(
+            self.error_handler, fallback=self.config.FALLBACK_ERROR_FORMAT
+        )
         TouchUp.run(self)
 
     async def _server_event(

sanic/handlers.py

@@ -38,7 +38,14 @@ class ErrorHandler:
         self.base = base
 
     @classmethod
-    def finalize(cls, error_handler):
+    def finalize(cls, error_handler, fallback: Optional[str] = None):
+        if (
+            fallback
+            and fallback != "auto"
+            and error_handler.fallback == "auto"
+        ):
+            error_handler.fallback = fallback
+
         if not isinstance(error_handler, cls):
             error_logger.warning(
                 f"Error handler is non-conforming: {type(error_handler)}"

tests/test_errorpages.py

@@ -1,6 +1,7 @@
 import pytest
 
 from sanic import Sanic
+from sanic.config import Config
 from sanic.errorpages import HTMLRenderer, exception_response
 from sanic.exceptions import NotFound, SanicException
 from sanic.handlers import ErrorHandler
@@ -313,3 +314,31 @@ def test_setting_fallback_to_non_default_raise_warning(app):
         app.config.FALLBACK_ERROR_FORMAT = "json"
 
     assert app.error_handler.fallback == "json"
+
+
+def test_allow_fallback_error_format_in_config_injection():
+    class MyConfig(Config):
+        FALLBACK_ERROR_FORMAT = "text"
+
+    app = Sanic("test", config=MyConfig())
+
+    @app.route("/error", methods=["GET", "POST"])
+    def err(request):
+        raise Exception("something went wrong")
+
+    request, response = app.test_client.get("/error")
+    assert request.app.error_handler.fallback == "text"
+    assert response.status == 500
+    assert response.content_type == "text/plain; charset=utf-8"
+
+
+def test_allow_fallback_error_format_in_config_replacement(app):
+    class MyConfig(Config):
+        FALLBACK_ERROR_FORMAT = "text"
+
+    app.config = MyConfig()
+
+    request, response = app.test_client.get("/error")
+    assert request.app.error_handler.fallback == "text"
+    assert response.status == 500
+    assert response.content_type == "text/plain; charset=utf-8"