Specifying a different input type for a Pydantic model field (comma-separated string input as a list of strings)

Using Pydantic, how can I specify an attribute that has an input type different from its actual type?

For example I have a systems field that contains a list of systems (so a list of strings) and the user can provide this systems list as a comma separated string (e.g. "system1,system2"); then I use a validator to split this string into a list of strings.

The code below is doing that and it's working but the type hinting is wrong as the systems field is actually a list of strings, not a string; the validator is splitting the original string into a list of strings.

How can I fix this?

import typing

from pydantic import BaseSettings, Field, validator


class Config(BaseSettings):
    systems: str = Field([], description="list of systems as a comma separated list (e.g. 'sys1,sys2')")

    @validator("systems")
    def set_systems(cls, v) -> typing.List[str]:
        if v == "":
            return []
        systems = list(filter(None, v.split(",")))
        return systems


if __name__ == "__main__":
    c = Config(**{"systems": "foo,bar"})
    print(c)

Always annotate model fields with the types you actually want in your schema!

If you want the field systems to be a list of strings, then annotate it accordingly. A comma-separated string is the exception after all. To allow it, use a pre=True validator to intercept that string before the default field validators get to it (and raise an error). Then you can split it and return the list as you wish:

from pydantic import BaseSettings, Field, validator


class Config(BaseSettings):
    systems: list[str] = Field(default_factory=list)

    @validator("systems", pre=True)
    def split_comma_separated(cls, v: object) -> object:
        if isinstance(v, str):
            v = v.strip()
            return [] if v == "" else v.split(",")
        return v


if __name__ == "__main__":
    print(Config.parse_obj({"systems": "foo,bar"}))
    print(Config.parse_obj({"systems": ""}))
    print(Config())

Output:

systems=['foo', 'bar']
systems=[]
systems=[]