Add merged json_encoders inheritance (#2064)

* Implement merged json_encoders inheritance

* json_encoders inheritance documentation

changes/2064-art049.md

@@ -0,0 +1 @@
+Add merged `json_encoders` inheritance

docs/examples/exporting_models_json_encoders_merge.py

@@ -0,0 +1,24 @@
+from datetime import datetime, timedelta
+from pydantic import BaseModel
+from pydantic.json import timedelta_isoformat
+
+
+class BaseClassWithEncoders(BaseModel):
+    dt: datetime
+    diff: timedelta
+
+    class Config:
+        json_encoders = {
+            datetime: lambda v: v.timestamp()
+        }
+
+
+class ChildClassWithEncoders(BaseClassWithEncoders):
+    class Config:
+        json_encoders = {
+            timedelta: timedelta_isoformat
+        }
+
+
+m = ChildClassWithEncoders(dt=datetime(2032, 6, 1), diff=timedelta(hours=100))
+print(m.json())

docs/usage/exporting_models.md

@@ -99,11 +99,19 @@ _(This script is complete, it should run "as is")_
 By default, `timedelta` is encoded as a simple float of total seconds. The `timedelta_isoformat` is provided
 as an optional alternative which implements ISO 8601 time diff encoding.
 
+The `json_encoders` are also merged during the models inheritance with the child
+encoders taking precedence over the parent one.
+
+```py
+{!.tmp_examples/exporting_models_json_encoders_merge.py!}
+```
+_(This script is complete, it should run "as is")_
+
 ### Serialising subclasses
 
 !!! note
     New in version **v1.5**.
-    
+
     Subclasses of common types were not automatically serialised to JSON before **v1.5**.
 
 Subclasses of common types are automatically encoded like their super-classes:

pydantic/main.py

@@ -169,13 +169,19 @@ def prepare_field(cls, field: 'ModelField') -> None:
 
 
 def inherit_config(self_config: 'ConfigType', parent_config: 'ConfigType') -> 'ConfigType':
+    namespace = {}
     if not self_config:
         base_classes = (parent_config,)
     elif self_config == parent_config:
         base_classes = (self_config,)
     else:
         base_classes = self_config, parent_config  # type: ignore
-    return type('Config', base_classes, {})
+        namespace['json_encoders'] = {
+            **getattr(parent_config, 'json_encoders', {}),
+            **getattr(self_config, 'json_encoders', {}),
+        }
+
+    return type('Config', base_classes, namespace)
 
 
 EXTRA_LINK = 'https://pydantic-docs.helpmanual.io/usage/model_config/'

tests/test_json.py

@@ -170,6 +170,35 @@ class Config:
     assert m.json() == '{"x": "P0DT0H2M3.000000S"}'
 
 
+def test_json_encoder_simple_inheritance():
+    class Parent(BaseModel):
+        dt: datetime.datetime = datetime.datetime.now()
+        timedt: datetime.timedelta = datetime.timedelta(hours=100)
+
+        class Config:
+            json_encoders = {datetime.datetime: lambda _: 'parent_encoder'}
+
+    class Child(Parent):
+        class Config:
+            json_encoders = {datetime.timedelta: lambda _: 'child_encoder'}
+
+    assert Child().json() == '{"dt": "parent_encoder", "timedt": "child_encoder"}'
+
+
+def test_json_encoder_inheritance_override():
+    class Parent(BaseModel):
+        dt: datetime.datetime = datetime.datetime.now()
+
+        class Config:
+            json_encoders = {datetime.datetime: lambda _: 'parent_encoder'}
+
+    class Child(Parent):
+        class Config:
+            json_encoders = {datetime.datetime: lambda _: 'child_encoder'}
+
+    assert Child().json() == '{"dt": "child_encoder"}'
+
+
 def test_custom_encoder_arg():
     class Model(BaseModel):
         x: datetime.timedelta