add escape hatch for custom JSON serialization

kombu/utils/json.py

@@ -32,7 +32,9 @@ def default(self, o):
 
         for t, (marker, encoder) in _encoders.items():
             if isinstance(o, t):
-                return _as(marker, encoder(o))
+                return (
+                    encoder(o) if marker is None else _as(marker, encoder(o))
+                )
 
         # Bytes is slightly trickier, so we cannot put them directly
         # into _encoders, because we use two formats: bytes, and base64.
@@ -50,7 +52,11 @@ def _as(t: str, v: Any):
 
 
 def dumps(
-    s, _dumps=json.dumps, cls=JSONEncoder, default_kwargs=None, **kwargs
+    s,
+    _dumps=json.dumps,
+    cls=JSONEncoder,
+    default_kwargs=None,
+    **kwargs
 ):
     """Serialize object to json string."""
     default_kwargs = default_kwargs or {}
@@ -94,35 +100,47 @@ def loads(s, _loads=json.loads, decode_bytes=True, object_hook=object_hook):
 
 def register_type(
     t: type[T],
-    marker: str,
+    marker: str | None,
     encoder: Callable[[T], EncodedT],
-    decoder: Callable[[EncodedT], T],
+    decoder: Callable[[EncodedT], T] = lambda d: d,
 ):
-    """Add support for serializing/deserializing native python type."""
+    """Add support for serializing/deserializing native python type.
+
+    If marker is `None`, the encoding is a pure transformation and the result
+    is not placed in an envelope, so `decoder` is unnecessary. Decoding must
+    instead be handled outside this library.
+    """
     _encoders[t] = (marker, encoder)
-    _decoders[marker] = decoder
+    if marker is not None:
+        _decoders[marker] = decoder
 
 
-_encoders: dict[type, tuple[str, EncoderT]] = {}
+_encoders: dict[type, tuple[str | None, EncoderT]] = {}
 _decoders: dict[str, DecoderT] = {
     "bytes": lambda o: o.encode("utf-8"),
     "base64": lambda o: base64.b64decode(o.encode("utf-8")),
 }
 
-# NOTE: datetime should be registered before date,
-# because datetime is also instance of date.
-register_type(datetime, "datetime", datetime.isoformat, datetime.fromisoformat)
-register_type(
-    date,
-    "date",
-    lambda o: o.isoformat(),
-    lambda o: datetime.fromisoformat(o).date(),
-)
-register_type(time, "time", lambda o: o.isoformat(), time.fromisoformat)
-register_type(Decimal, "decimal", str, Decimal)
-register_type(
-    uuid.UUID,
-    "uuid",
-    lambda o: {"hex": o.hex},
-    lambda o: uuid.UUID(**o),
-)
+
+def _register_default_types():
+    # NOTE: datetime should be registered before date,
+    # because datetime is also instance of date.
+    register_type(datetime, "datetime", datetime.isoformat,
+                  datetime.fromisoformat)
+    register_type(
+        date,
+        "date",
+        lambda o: o.isoformat(),
+        lambda o: datetime.fromisoformat(o).date(),
+    )
+    register_type(time, "time", lambda o: o.isoformat(), time.fromisoformat)
+    register_type(Decimal, "decimal", str, Decimal)
+    register_type(
+        uuid.UUID,
+        "uuid",
+        lambda o: {"hex": o.hex},
+        lambda o: uuid.UUID(**o),
+    )
+
+
+_register_default_types()

t/unit/utils/test_json.py

@@ -3,6 +3,7 @@
 import sys
 import uuid
 from collections import namedtuple
+from dataclasses import dataclass
 from datetime import datetime
 from decimal import Decimal
 
@@ -11,7 +12,8 @@
 from hypothesis import strategies as st
 
 from kombu.utils.encoding import str_to_bytes
-from kombu.utils.json import dumps, loads
+from kombu.utils.json import (_register_default_types, dumps, loads,
+                              register_type)
 
 if sys.version_info >= (3, 9):
     from zoneinfo import ZoneInfo
@@ -28,6 +30,10 @@ def __json__(self):
 
 
 class test_JSONEncoder:
+    @pytest.fixture(autouse=True)
+    def reset_registered_types(self):
+        _register_default_types()
+
     @pytest.mark.freeze_time("2015-10-21")
     def test_datetime(self):
         now = datetime.utcnow()
@@ -82,6 +88,41 @@ def test_UUID(self):
             assert loaded_value == {'u': id}
             assert loaded_value["u"].version == id.version
 
+    def test_register_type_overrides_defaults(self):
+        # This type is already registered by default, let's override it
+        register_type(uuid.UUID, "uuid", lambda o: "custom", lambda o: o)
+        value = uuid.uuid4()
+        loaded_value = loads(dumps({'u': value}))
+        assert loaded_value == {'u': "custom"}
+
+    def test_register_type_with_new_type(self):
+        # Guaranteed never before seen type
+        @dataclass()
+        class SomeType:
+            a: int
+
+        register_type(SomeType, "some_type", lambda o: "custom", lambda o: o)
+        value = SomeType(42)
+        loaded_value = loads(dumps({'u': value}))
+        assert loaded_value == {'u': "custom"}
+
+    def test_register_type_with_empty_marker(self):
+        register_type(
+            datetime,
+            None,
+            lambda o: o.isoformat(),
+            lambda o: "should never be used"
+        )
+        now = datetime.utcnow()
+        serialized_str = dumps({'now': now})
+        deserialized_value = loads(serialized_str)
+
+        assert "__type__" not in serialized_str
+        assert "__value__" not in serialized_str
+
+        # Check that there is no extra deserialization happening
+        assert deserialized_value == {'now': now.isoformat()}
+
     def test_default(self):
         with pytest.raises(TypeError):
             dumps({'o': object()})