json_encoders inheritance

[art049]

Feature Request

Currently, defining json_encoders in child classes completely overrides the one defined from the parent classes:

from datetime import datetime, timedelta
from pydantic import BaseModel

class A(BaseModel):
    dt: datetime = datetime.now()
    class Config:
        json_encoders = {datetime: lambda _: "encoded_from_A"}

print(A().json())
#> {"dt": "encoded_from_A"}
class B(A):
    timedt: timedelta = timedelta(hours=100)
    class Config:
        json_encoders = {timedelta: lambda _: "encoded_from_B"}

print(B().json())
#> {"dt": "2020-10-23T01:11:20.613624", "timedt": "encoded_from_B"}

I think it would be handy to merge the JSON encoders with the one defined in the parent ones, thus the result would be:

print(B().json())
#> {"dt": "encoded_from_A", "timedt": "encoded_from_B"}

As an extension, it should still be possible to override the parent ones:

class Bbis(A):
    timedt: timedelta = timedelta(hours=100)

    class Config:
        json_encoders = {
            datetime: lambda _: "encoded_from_Bbis",
            timedelta: lambda _: "encoded_from_Bbis",
        }

print(Bbis().json()) 
#> {"dt": "encoded_from_Bbis", "timedt": "encoded_from_Bbis"}

I'm eager to implement this feature if you think it's worth it 😄