Pydantic root validator unable to access class attributes

Here's a minimal example:

class BaseClass(BaseModel):
    description: str

    @root_validator(pre=True)
    def initialize(cls, values: Dict[str, Any]) -> Dict[str, Any]:
        print(cls.description) 
        # also tried print(values.get("description"), which is None
        return values


class SubClass(BaseClass):
    description = "foo"

Throws error

type object 'SubClass' has no attribute 'description'", "errorType": "AttributeError


I need to do some validation based on the fields provided by the child class. This validation will be common across all the child classes.

The solution is to use a ClassVar annotation for description.

With Pydantic models, simply adding a name: type or name: type = value in the class namespace will create a field on that model, not a class attribute. If you are interested, I explained in a bit more detail how Pydantic fields are different from regular attributes in this post. (The topic there is private attributes, but the explanation is still relevant.)

You want description to be class attribute, i.e. not a field of the model that can be assigned different values on different instances. The section on automatically excluded attributes of the Pydantic docs say: (emphasis mine)

> Class variables which begin with an underscore and attributes annotated with typing.ClassVar will be automatically excluded from the model.

So to achieve what you want, you need to explicitly tell the model that description is supposed to be a ClassVar, not a normal field:

from typing import Any, ClassVar

from pydantic import BaseModel, root_validator


class BaseClass(BaseModel):
    description: ClassVar[str]  # not a field
    spam: str
    eggs: int

    @root_validator(pre=True)
    def initialize(cls, values: dict[str, Any]) -> dict[str, Any]:
        if cls.description == "foo":
            values["spam"] = values["spam"].upper()
            values["eggs"] = values["eggs"] ** 2
        return values


class SubClass(BaseClass):
    description = "foo"


instance = SubClass(spam="abc", eggs=2)
print(instance.json(indent=4))

Output:

{
    "spam": "ABC",
    "eggs": 4
}

There is an error because cls inside the root_validator is referring to the class object itself, you cannot access it directly from cls because description is an attribute of instances of SubClass.

You can access the class attributes in the initialize root validator via the values dictionary

Also you gonna need to remove pre=True because it processes the validator before the model initialization.

from pydantic import BaseModel, root_validator
from typing import Dict, Any

class BaseClass(BaseModel):
    description: str

    @root_validator
    def initialize(cls, values: Dict[str, Any]) -> Dict[str, Any]:
        print(values.get("description")) 
        return values

class SubClass(BaseClass):
    description = "foo"

SubClass()

To access the description attribute from the child classes, you can use the class attribute, which refers to the class object itself. The "initialize" method is a class-level method meaning that is cannot access attributes of the instance or the class. You can try using:

print(cls.__dict__.get("description"))