Deprecate not configuring a default content type.

docs/history.rst

@@ -6,6 +6,7 @@ Version History
 - Add type annotations (see :ref:`type-info`)
 - Return a "406 Not Acceptable" if the :http:header:`Accept` header values cannot be matched
   and there is no default content type configured
+- Deprecate not having a default content type configured
 
 :compare:`3.0.4 <3.0.3...3.0.4>` (2 Nov 2020)
 ---------------------------------------------

sprockets/mixins/mediatype/content.py

@@ -31,6 +31,8 @@ from __future__ import annotations
 
 import logging
 import typing
+import warnings
+
 try:
     from typing import Literal
 except ImportError:  # pragma: no cover
@@ -90,15 +92,15 @@ class ContentSettings:
 
     """
 
-    default_content_type: typing.Union[str, None]
     default_encoding: typing.Union[str, None]
-    _handlers: typing.Dict[str, type_info.Transcoder]
     _available_types: typing.List[datastructures.ContentType]
+    _default_content_type: typing.Union[str, None]
+    _handlers: typing.Dict[str, type_info.Transcoder]
 
     def __init__(self) -> None:
         self._handlers = {}
         self._available_types = []
-        self.default_content_type = None
+        self._default_content_type = None
         self.default_encoding = None
 
     def __getitem__(self, content_type: str) -> type_info.Transcoder:
@@ -137,6 +139,20 @@ class ContentSettings:
         """
         return self._available_types
 
+    @property
+    def default_content_type(self) -> typing.Union[str, None]:
+        return self._default_content_type
+
+    @default_content_type.setter
+    def default_content_type(self, new_value: typing.Union[str, None]) -> None:
+        if new_value is None:
+            warnings.warn(
+                DeprecationWarning(
+                    'Using sprockets.mixins.mediatype without a default'
+                    ' content type is deprecated and will become an error'
+                    ' in a future version'))
+        self._default_content_type = new_value
+
 
 def install(application: type_info.HasSettings,
             default_content_type: typing.Optional[str],

tests.py

@@ -285,6 +285,11 @@ class ContentSettingsTests(unittest.TestCase):
                          'json')
         self.assertEqual(settings['application/json; charset=utf-8'], handler)
 
+    def test_that_setting_no_default_content_type_warns(self):
+        settings = content.ContentSettings()
+        with self.assertWarns(DeprecationWarning):
+            settings.default_content_type = None
+
 
 class ContentFunctionTests(unittest.TestCase):
     def setUp(self):