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

As of pydantic 2.0, we can use the @field_serializer decorator for serialization, and @field_validator for validation.

Taken from pydantic docs:

from datetime import datetime, timezone

from pydantic import BaseModel, field_serializer


class WithCustomEncoders(BaseModel):

    dt: datetime

    @field_serializer('dt')
    def serialize_dt(self, dt: datetime, _info):
        return dt.timestamp()


m = WithCustomEncoders(
    dt=datetime(2032, 6, 1, tzinfo=timezone.utc)
)
print(m.model_dump_json())
#> {"dt":1969660800.0}

And for validation:

from pydantic_core.core_schema import FieldValidationInfo

from pydantic import BaseModel, ValidationError, field_validator


class UserModel(BaseModel):
    name: str
    username: str
    password1: str
    password2: str

    @field_validator('name')
    def name_must_contain_space(cls, v):
        if ' ' not in v:
            raise ValueError('must contain a space')
        return v.title()

    @field_validator('password2')
    def passwords_match(cls, v, info: FieldValidationInfo):
        if 'password1' in info.data and v != info.data['password1']:
            raise ValueError('passwords do not match')
        return v

    @field_validator('username')
    def username_alphanumeric(cls, v):
        assert v.isalnum(), 'must be alphanumeric'
        return v


user = UserModel(
    name='samuel colvin',
    username='scolvin',
    password1='zxcvbn',
    password2='zxcvbn',
)
print(user)
"""
name='Samuel Colvin' username='scolvin' password1='zxcvbn' password2='zxcvbn'
"""