How to set a Pydantic field value depending on other fields

Question

from pydantic import BaseModel


class Grafana(BaseModel):
    user: str
    password: str
    host: str
    port: str
    api_key: str | None = None
    GRAFANA_URL = f"http://{user}:{password}@{host}:{port}"
    API_DATASOURCES = "/api/datasources"
    API_KEYS = "/api/auth/keys"

With Pydantic I get two unbound variables error messages for user, password, etc. in GRAFANA_URL.

Is there a way to solve this? In a regular class, I would just create GRAFANA_URL in the __init__ method. With Pydantic, I'm not sure how to proceed.

Answer 1

Option A: Use a @validator

See the validators documentation for details.

from typing import Any

from pydantic import BaseModel, validator


class Model(BaseModel):
    foo: str
    bar: str
    foobar: str = ""

    @validator("foobar", always=True)
    def set_if_empty(cls, v: str, values: dict[str, Any]) -> str:
        if v == "":
            return values["foo"] + values["bar"]
        return v


obj = Model(foo="a", bar="b")
print(obj)  # foo='a' bar='b' foobar='ab'

That way foobar remains a regular model field.

Note that for this to work, foobar must be defined after foo and bar. Otherwise you will have to use a root validator.

Option B: Make it a @property

from pydantic import BaseModel


class Model(BaseModel):
    foo: str
    bar: str

    @property
    def foobar(self) -> str:
        return self.foo + self.bar


obj = Model(foo="a", bar="b")
print(obj)         # foo='a' bar='b'
print(obj.foobar)  # ab

Then foobar will not be a model field anymore and therefore not part of the schema. That may or may not be relevant to you.

Option C: Make it a @computed_field (Pydantic v2 only!)

Defining computed fields will be available for Pydantic 2.

from pydantic import BaseModel, computed_field


class Model(BaseModel):
    foo: str
    bar: str

    @computed_field
    @property
    def foobar(self) -> str:
        return self.foo + self.bar


obj = Model(foo="a", bar="b")
print(obj)         # foo='a' bar='b' foobar='ab'