Setting a default value based on another value

Question

I have the following Pydantic model:

from pydantic import BaseModel
import key

class Wallet(BaseModel):
    private_key: str = Field(default_factory=key.generate_private_key)
    address: str

I want address to have a default_factory as a function that takes a private_key as input and returns an address. My intentions would be something along the lines of the following:

address: str = Field(default_factory=key.generate_address(self.private_key)

How can I achieve this?

Hernán Alarcón · Accepted Answer · 2023-02-04T04:15:31.980

Another option is to just use @validator because in it you can access previously validated fields. From the documentation:

validators are "class methods", so the first argument value they receive is the UserModel class, not an instance of UserModel.

the second argument is always the field value to validate; it can be named as you please

you can also add any subset of the following arguments to the signature (the names must match):

values: a dict containing the name-to-value mapping of any previously-validated fields

Example:

class Wallet(BaseModel):
    private_key: str = Field(default_factory=key.generate_private_key)
    address: str = "" # "" seems better than None to use the correct type

    @validator("address", always=True)
    def get_address(cls, address: str, values: Dict[str, Any]) -> str:
        if address == "" and "private_key" in values:
            return key.generate_address(values["private_key"])
        return address

It can be argued that @validator should be preferred over @root_validator if you just want to generate a value for a single field.

There are two important aspects of this approach that must be considered:

The "previously-validated fields" from the documentation means that in your case private_key must be defined before address. The values of fields defined after the field that is validated are not available to the validator.
If the field that is validated has a default value and you still want the validator to be executed in that case, you have to use always=True.

It is worth adding that if the private key is not set, get_address is still called, and will result in an undesirable exception (instead of a validation error). Changing this line: `if address == "":` To: `if address == "" and 'private_key' in values:` Will help with that — Adam, Feb 02 '23 at 11:34
@Adam, thanks for the suggestion, I added it. I agree with you that it is always better to check if a given key is in the `values` dict because any previously validated field could be invalid. In this particular case `private_key` has a `default_factory`, so the `KeyError` will be caused not by not setting it, but by setting it with an invalid value (something that pydantic cannot parse as a `str`). — Hernán Alarcón, Feb 04 '23 at 04:22

score 0 · Answer 2 · answered Dec 01 '21 at 13:16

I was able to acheive it using a root_validator:

class Wallet(BaseModel):
    address: str = None
    private_key: str = Field(default_factory=key.generate_private_key)

    @root_validator
    def get_address(cls, values) -> dict:
        if values.get("address") is None:
            values["address"] = key.generate_address(values.get("private_key"))
        return values

Not sure if this is the best way to acheieve this though

Setting a default value based on another value

2 Answers2