Pydantic: Dynamic defaults set using default_factory show up in exported JSON schema

[jnik-aarhus]

Question

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.5.1
            pydantic compiled: False
                 install path: [home]/repos/fastapi-test/.venv/lib/python3.8/site-packages/pydantic
               python version: 3.8.2 (default, Apr  8 2020, 14:31:25)  [GCC 9.3.0]

Hello!
I'm currently developing an API using FastAPI, and, by extension, Pydantic.
There are a few fields which require dynamic default values, e.g. created_date, and per https://github.com/samuelcolvin/pydantic/issues/866 I've been using default_factory to do so. However, when a field has a default value defined using a factory, the dynamic value shows up in the generated JSON schema, which might not be desirable?

Example:

from datetime import datetime, timezone
from uuid import uuid4
from pydantic import BaseModel, Field


class SchemaTest(BaseModel):
    uuid = Field(default_factory=uuid4)
    created_date: datetime = Field(
        default_factory=lambda: datetime.now(timezone.utc)
    )
print(SchemaTest.schema_json(indent=2))

The above outputs

{
  "title": "SchemaTest",
  "type": "object",
  "properties": {
    "created_date": {
      "title": "Created Date",
      "default": "2020-05-15T10:04:18.692173+00:00",
      "type": "string",
      "format": "date-time"
    },
    "uuid": {
      "title": "Uuid",
      "default": "79a35356-4619-49da-9bbe-4be75a696485",
      "type": "string",
      "format": "uuid"
    }
  }
}

These values are of course different each time the schema is generated.
I understand why it happens, but I'm not sure it's entirely correct to expose the value in the JSON schema like this. It's worth noting that this is not a problem when using the "old" method of setting defaults via a validator.

Please let me know if I'm missing something, or that this is indeed appropriate behavior. Thanks for developing such a great tool!

[StephenBrown2]

You can probably give it a note in the description= that this is dynamically generated to be the current datetime, and the default is only an example of the output.

[PrettyWood]

Hello @jnik-aarhus !

1504 should also solve your issue.

[samuelcolvin]

I don't see how #1504 would fix this?

[PrettyWood]

@samuelcolvin Because of this line where I set default as None instead of calling the default_factory. But I need to have a look at your answer