Custom JSON encoder not working with nested model

Question

I am trying to create a custom JSON encoding for a nested Pydantic model. I have simplified the problem to the following example:

from pydantic import BaseModel


class SubModel(BaseModel):
    name: str
    short_name: str


class TestModel(BaseModel):
    sub_model: SubModel

    class Config:
        json_encoders = {SubModel: lambda s: s.short_name}


model = TestModel(sub_model=SubModel(name="Sub Model", short_name="SM"))

print(model)
print(model.json())

I am expecting the final line to output:

{"sub_model": "SM"}

But instead I am getting the output as if I never even defined my own json_encoders:

{"sub_model": {"name": "Sub Model", "short_name": "SM"}}

How can I correctly define a JSON encoder for another Pydantic model?

github.com/samuelcolvin/pydantic/issues/2277

duyue
– duyue

2022-06-28 09:25:47 +00:00
Commented Jun 28, 2022 at 9:25 — duyue
– duyue, Commented Jun 28, 2022 at 9:25

Mornie · Accepted Answer · 2022-08-14 10:42:37Z

2

There's nothing wrong with the json_encoder you specified. You just need to add models_as_dict=False to the json() method to override the default dict-like serialization:

print(model.json(models_as_dict=False))

See https://pydantic-docs.helpmanual.io/usage/exporting_models/#serialising-self-reference-or-other-models for the reference.

answered Aug 14, 2022 at 10:42

Mornie

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1 Comment

shadowtalker Over a year ago

Pydantic not having a proper API Reference doc is extremely painful and makes it really difficult to discover features like this. Especially when the user hides information like this behind complicated headings like "Serialising self-reference or other models" that most people aren't likely to read, thinking that it's not relevant in this situation because it's not a "self-reference".

Christian Kasim Loan · Accepted Answer · 2022-08-21 17:58:12Z

1

I ran into the same problem. What worked for me is setting the JSON encoders on the global BaseConfig instead of the class


from pydantic import BaseConfig
BaseConfig.json_encoders = {
    MyClass: lambda v: v.as_str(),
}

Reference: https://github.com/pydantic/pydantic/issues/2277#issuecomment-764010272

answered Aug 21, 2022 at 17:58

Christian Kasim Loan

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