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

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"}

Answer 2

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 3

In case you don't necessarily want to apply this behavior to all datetimes, you can create a custom type extending datetime . For example, to make a custom type that always ensures we have a datetime with tzinfo set to UTC:

from datetime import datetime, timezone

from pydantic.datetime_parse import parse_datetime


class utc_datetime(datetime):
    @classmethod
    def __get_validators__(cls):
        yield parse_datetime  # default pydantic behavior
        yield cls.ensure_tzinfo

    @classmethod
    def ensure_tzinfo(cls, v):
        # if TZ isn't provided, we assume UTC, but you can do w/e you need
        if v.tzinfo is None:
            return v.replace(tzinfo=timezone.utc)
        # else we convert to utc
        return v.astimezone(timezone.utc)
    
    @staticmethod
    def to_str(dt:datetime) -> str:
        return dt.isoformat() # replace with w/e format you want

Then your pydantic models would look like:

from pydantic import BaseModel

class SomeObject(BaseModel):
    some_datetime_in_utc: utc_datetime

    class Config:
        json_encoders = {
            utc_datetime: utc_datetime.to_str
        }

Going this route helps with reusability and separation of concerns:)