show deprecation warning when using json_encoder/decoder

CHANGES.rst

@@ -3,6 +3,9 @@ Version 2.2.1
 
 Unreleased
 
+-   Setting or accessing ``json_encoder`` or ``json_decoder`` raises a
+    deprecation warning. :issue:`4732`
+
 
 Version 2.2.0
 -------------
@@ -50,18 +53,18 @@ Released 2022-08-01
         provider can be set to use a different JSON library.
         ``flask.jsonify`` will call ``app.json.response``, other
         functions in ``flask.json`` will call corresponding functions in
-        ``app.json``. :pr:`4688`
+        ``app.json``. :pr:`4692`
 
 -   JSON configuration is moved to attributes on the default
     ``app.json`` provider. ``JSON_AS_ASCII``, ``JSON_SORT_KEYS``,
     ``JSONIFY_MIMETYPE``, and ``JSONIFY_PRETTYPRINT_REGULAR`` are
-    deprecated. :pr:`4688`
+    deprecated. :pr:`4692`
 -   Setting custom ``json_encoder`` and ``json_decoder`` classes on the
     app or a blueprint, and the corresponding ``json.JSONEncoder`` and
     ``JSONDecoder`` classes, are deprecated. JSON behavior can now be
-    overridden using the ``app.json`` provider interface. :pr:`4688`
+    overridden using the ``app.json`` provider interface. :pr:`4692`
 -   ``json.htmlsafe_dumps`` and ``json.htmlsafe_dump`` are deprecated,
-    the function is built-in to Jinja now. :pr:`4688`
+    the function is built-in to Jinja now. :pr:`4692`
 -   Refactor ``register_error_handler`` to consolidate error checking.
     Rewrite some error messages to be more consistent. :issue:`4559`
 -   Use Blueprint decorators and functions intended for setup after

src/flask/app.py

@@ -1,5 +1,6 @@
 import functools
 import inspect
+import json
 import logging
 import os
 import sys
@@ -379,25 +380,86 @@ def use_x_sendfile(self, value: bool) -> None:
         )
         self.config["USE_X_SENDFILE"] = value
 
-    #: The JSON encoder class to use. Defaults to
-    #: :class:`~flask.json.JSONEncoder`.
-    #:
-    #: .. deprecated:: 2.2
-    #:      Will be removed in Flask 2.3. Customize
-    #:      :attr:`json_provider_class` instead.
-    #:
-    #: .. versionadded:: 0.10
-    json_encoder: None = None
+    _json_encoder: t.Union[t.Type[json.JSONEncoder], None] = None
+    _json_decoder: t.Union[t.Type[json.JSONDecoder], None] = None
 
-    #: The JSON decoder class to use. Defaults to
-    #: :class:`~flask.json.JSONDecoder`.
-    #:
-    #: .. deprecated:: 2.2
-    #:      Will be removed in Flask 2.3. Customize
-    #:      :attr:`json_provider_class` instead.
-    #:
-    #: .. versionadded:: 0.10
-    json_decoder: None = None
+    @property  # type: ignore[override]
+    def json_encoder(self) -> t.Type[json.JSONEncoder]:  # type: ignore[override]
+        """The JSON encoder class to use. Defaults to
+        :class:`~flask.json.JSONEncoder`.
+
+        .. deprecated:: 2.2
+             Will be removed in Flask 2.3. Customize
+             :attr:`json_provider_class` instead.
+
+        .. versionadded:: 0.10
+        """
+        import warnings
+
+        warnings.warn(
+            "'app.json_encoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+
+        if self._json_encoder is None:
+            from . import json
+
+            return json.JSONEncoder
+
+        return self._json_encoder
+
+    @json_encoder.setter
+    def json_encoder(self, value: t.Type[json.JSONEncoder]) -> None:
+        import warnings
+
+        warnings.warn(
+            "'app.json_encoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        self._json_encoder = value
+
+    @property  # type: ignore[override]
+    def json_decoder(self) -> t.Type[json.JSONDecoder]:  # type: ignore[override]
+        """The JSON decoder class to use. Defaults to
+        :class:`~flask.json.JSONDecoder`.
+
+        .. deprecated:: 2.2
+             Will be removed in Flask 2.3. Customize
+             :attr:`json_provider_class` instead.
+
+        .. versionadded:: 0.10
+        """
+        import warnings
+
+        warnings.warn(
+            "'app.json_decoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+
+        if self._json_decoder is None:
+            from . import json
+
+            return json.JSONDecoder
+
+        return self._json_decoder
+
+    @json_decoder.setter
+    def json_decoder(self, value: t.Type[json.JSONDecoder]) -> None:
+        import warnings
+
+        warnings.warn(
+            "'app.json_decoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        self._json_decoder = value
 
     json_provider_class: t.Type[JSONProvider] = DefaultJSONProvider
     """A subclass of :class:`~flask.json.provider.JSONProvider`. An

src/flask/blueprints.py

@@ -1,3 +1,4 @@
+import json
 import os
 import typing as t
 from collections import defaultdict
@@ -172,18 +173,76 @@ class Blueprint(Scaffold):
 
     _got_registered_once = False
 
-    #: Blueprint local JSON encoder class to use. Set to ``None`` to use
-    #: the app's :class:`~flask.Flask.json_encoder`.
-    #:
-    #: .. deprecated:: 2.2
-    #:      Will be removed in Flask 2.3.
-    json_encoder: None = None
-    #: Blueprint local JSON decoder class to use. Set to ``None`` to use
-    #: the app's :class:`~flask.Flask.json_decoder`.
-    #:
-    #: .. deprecated:: 2.2
-    #:      Will be removed in Flask 2.3.
-    json_decoder: None = None
+    _json_encoder: t.Union[t.Type[json.JSONEncoder], None] = None
+    _json_decoder: t.Union[t.Type[json.JSONDecoder], None] = None
+
+    @property  # type: ignore[override]
+    def json_encoder(  # type: ignore[override]
+        self,
+    ) -> t.Union[t.Type[json.JSONEncoder], None]:
+        """Blueprint-local JSON encoder class to use. Set to ``None`` to use the app's.
+
+        .. deprecated:: 2.2
+             Will be removed in Flask 2.3. Customize
+             :attr:`json_provider_class` instead.
+
+        .. versionadded:: 0.10
+        """
+        import warnings
+
+        warnings.warn(
+            "'bp.json_encoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return self._json_encoder
+
+    @json_encoder.setter
+    def json_encoder(self, value: t.Union[t.Type[json.JSONEncoder], None]) -> None:
+        import warnings
+
+        warnings.warn(
+            "'bp.json_encoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        self._json_encoder = value
+
+    @property  # type: ignore[override]
+    def json_decoder(  # type: ignore[override]
+        self,
+    ) -> t.Union[t.Type[json.JSONDecoder], None]:
+        """Blueprint-local JSON decoder class to use. Set to ``None`` to use the app's.
+
+        .. deprecated:: 2.2
+             Will be removed in Flask 2.3. Customize
+             :attr:`json_provider_class` instead.
+
+        .. versionadded:: 0.10
+        """
+        import warnings
+
+        warnings.warn(
+            "'bp.json_decoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return self._json_decoder
+
+    @json_decoder.setter
+    def json_decoder(self, value: t.Union[t.Type[json.JSONDecoder], None]) -> None:
+        import warnings
+
+        warnings.warn(
+            "'bp.json_decoder' is deprecated and will be removed in Flask 2.3."
+            " Customize 'app.json_provider_class' or 'app.json' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        self._json_decoder = value
 
     def __init__(
         self,

src/flask/json/provider.py

@@ -176,11 +176,11 @@ def dumps(self, obj: t.Any, **kwargs: t.Any) -> str:
         :param obj: The data to serialize.
         :param kwargs: Passed to :func:`json.dumps`.
         """
-        cls = self._app.json_encoder
+        cls = self._app._json_encoder
         bp = self._app.blueprints.get(request.blueprint) if request else None
 
-        if bp is not None and bp.json_encoder is not None:
-            cls = bp.json_encoder
+        if bp is not None and bp._json_encoder is not None:
+            cls = bp._json_encoder
 
         if cls is not None:
             import warnings
@@ -235,11 +235,11 @@ def loads(self, s: str | bytes, **kwargs: t.Any) -> t.Any:
         :param s: Text or UTF-8 bytes.
         :param kwargs: Passed to :func:`json.loads`.
         """
-        cls = self._app.json_decoder
+        cls = self._app._json_decoder
         bp = self._app.blueprints.get(request.blueprint) if request else None
 
-        if bp is not None and bp.json_decoder is not None:
-            cls = bp.json_decoder
+        if bp is not None and bp._json_decoder is not None:
+            cls = bp._json_decoder
 
         if cls is not None:
             import warnings

src/flask/scaffold.py

@@ -1,4 +1,5 @@
 import importlib.util
+import json
 import os
 import pathlib
 import pkgutil
@@ -78,14 +79,14 @@ class Scaffold:
     #:
     #: .. deprecated:: 2.2
     #:      Will be removed in Flask 2.3.
-    json_encoder: None = None
+    json_encoder: t.Union[t.Type[json.JSONEncoder], None] = None
 
     #: JSON decoder class used by :func:`flask.json.loads`. If a
     #: blueprint sets this, it will be used instead of the app's value.
     #:
     #: .. deprecated:: 2.2
     #:      Will be removed in Flask 2.3.
-    json_decoder: None = None
+    json_decoder: t.Union[t.Type[json.JSONDecoder], None] = None
 
     def __init__(
         self,