Pydantic should parse simple yyyy-MM-dd for datetime

[aersam]

Initial Checks

• I have searched Google & GitHub for similar requests and couldn't find anything
• I have read and followed the docs and still think this feature is missing

Description

from pydantic import BaseModel
from datetime import datetime
import json

datetime.fromisoformat("2021-01-01")  # this works fine


class Model(BaseModel):
    dt: datetime


Model.model_validate_json(json.dumps({"dt": "2021-01-01T00:00"})) # works, of course
Model.model_validate_json(json.dumps({"dt": "2021-01-01"})) # fails

I think pydantic is too strict here, especially since datetime.fromisoformat works without trouble.

Affected Components

• Compatibility between releases
• Data validation/parsing
• Data serialization - .model_dump() and .model_dump_json()
• JSON Schema
• Dataclasses
• Model Config
• Field Types - adding or changing a particular data type
• Function validation decorator
• Generic Models
• Other Model behaviour - model_construct(), pickling, private attributes, ORM mode
• Plugins and integration with other tools - mypy, FastAPI, python-devtools, Hypothesis, VS Code, PyCharm, etc.

[sydney-runkle]

@aersam,

Agreed, we will implement support for this in pydantic-core :).

[sydney-runkle]

@aersam,

Just added support for this, will be released in 2.6.0 :)

[aersam]

Thanks a lot! Will make some of my fast api url's a bit cleaner 🙂

[miikka]

Hey, what's the best way to disable this new feature? We do not want to allow yyyy-MM-dd for datetimes.

[sydney-runkle]

@miikka,

You can use strict mode on that datetime type (or at the model level):

from datetime import datetime, timezone
from typing import Annotated

from pydantic import BaseModel, Strict, ValidationError

StrictDatetime = Annotated[datetime, Strict()]

class Model(BaseModel):
    dt_strict: StrictDatetime
    dt_lax: datetime

try:
   m = Model(dt_strict='2021-01-01', dt_lax='2021-01-01')
   print(repr(m))
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    dt_strict
    Input should be a valid datetime [type=datetime_type, input_value='2021-01-01', input_type=str]
        For further information visit https://errors.pydantic.dev/2.6/v/datetime_type
    """

[miikka]

@sydney-runkle Thanks, that's neat! Unfortunately it turns out that doesn't help with FastAPI's path parameters coercion (which uses Pydantic under the hood with TypeAdapter.validate_python which does not accept strings at all for strict mode datetimes) but maybe that's better discussed somewhere else than in this issue.

[sydney-runkle]

@miikka,

Feel free to open another issue and we can continue discussion there!