How to change date format in pydantic

Question

How to change date format in pydantic for validation and serialization? For validation I am using @validator. Is there an solution for both cases?

Answer 1

You can implement a custom json serializer by using pydantic's custom json encoders. Then, together with pydantic's custom validator, you can have both functionalities.

from datetime import datetime, timezone
from pydantic import BaseModel, validator


def convert_datetime_to_iso_8601_with_z_suffix(dt: datetime) -> str:
    return dt.strftime('%Y-%m-%dT%H:%M:%SZ')


def transform_to_utc_datetime(dt: datetime) -> datetime:
    return dt.astimezone(tz=timezone.utc)


class DateTimeSpecial(BaseModel):
    datetime_in_utc_with_z_suffix: datetime

    # custom input conversion for that field
    _normalize_datetimes = validator(
        "datetime_in_utc_with_z_suffix",
        allow_reuse=True)(transform_to_utc_datetime)

    class Config:
        json_encoders = {
            # custom output conversion for datetime
            datetime: convert_datetime_to_iso_8601_with_z_suffix
        }


if __name__ == "__main__":
    special_datetime = DateTimeSpecial(datetime_in_utc_with_z_suffix="2042-3-15T12:45+01:00")  # note the different timezone

    # input conversion
    print(special_datetime.datetime_in_utc_with_z_suffix)  # 2042-03-15 11:45:00+00:00

    # output conversion
    print(special_datetime.json())  # {"datetime_in_utc_with_z_suffix": "2042-03-15T11:45:00Z"}

This variant also works in fastapi's serializer where I am actually using it in that way.

Answer 2

I think that pre validator can help here.

from datetime import datetime, date

from pydantic import BaseModel, validator


class OddDate(BaseModel):
    birthdate: date

    @validator("birthdate", pre=True)
    def parse_birthdate(cls, value):
        return datetime.strptime(
            value,
            "%d/%m/%Y"
        ).date()


if __name__ == "__main__":
    odd_date = OddDate(birthdate="12/04/1992")
    print(odd_date.json()) #{"birthdate": "1992-04-12"}