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I'm using the pydantic BaseModel with a validator like this:

from datetime import date

from typing import List, Optional

from pydantic import BaseModel, BaseConfig, validator



class Model(BaseModel):

    class Config(BaseConfig):

        allow_population_by_alias = True

        fields = {

            "some_date": {

                "alias": "some_list"

            }

        }

    some_date: Optional[date]

    some_list: List[date]



    @validator("some_date", pre=True, always=True)

    def validate_date(cls, value):

        if len(value) < 2: # here value is some_list

            return None

        return value[0] # return the first value - let's assume it's a date string



# This reproduces the problem

m = Model(some_list=['2019-01-03'])

I would like to compute the value of some_date based on the value of some_list and make it None if a certain condition met.

My JSON never contains the field some_date, it's always populated based on some_list hence pre=True, always=True. However the default validator for some_date will run after my custom one, which will fail if validate_date returns None.

Is there a way to create such a field which is only computed by another one and still can be Optional?

I'm inclined to say that this is not possible in the way you try.

You correctly pointed out that the default validator for date is called after the custom validate_date from your data model. The relevant source code can be found here: pydantic.fields.py (as of Jan 2019, git-hash: 19320bf). Apparently, the allow_none-mode is not supported for a cascade of validators, see pydantic.Field._apply_validators for details. More concretely, the output of a particular validator is never tested for None, that test is done further upstream in Model.validate.

From skim reading documentation and source of pydantic, I tend to to say that pydantic's validation mechanism currently has very limited support for type-transformations (list -> date, list -> NoneType) within the validation functions. If you have good arguments for your use-case, you may want to request that feature here.

Taking a step back, however, your approach using an alias and the flag allow_population_by_alias (which is not recommended, anyways, as stated in the documentation) seems a bit overloaded. some_date is needed only as a shortcut for some_list[0] if len(some_list) >= 2 else None, but it's never set independently from some_list. If that's really the case, why not opting for the following, much simpler option?

class Model(BaseModel):

    some_list: List[date] = ...



    @property 

    def some_date(self):

        return None if len(self.some_list) < 2 else self.some_list[0]