SetAttributeValueWhenValidating

In Pydantic, you can use the @validator decorator to set the value of one attribute based on the value of another during validation. To achieve this, you can use the pre=True parameter if you want to validate the attributes before any other validation rules are applied.

Here's an example of how to set the value of attB when validating the value of attA:

from pydantic import BaseModel, validator

class MyModel(BaseModel):
    attA: int
    attB: int = None  # Default value can be None or some other value

    @validator('attA', pre=True)
    def set_attB_from_attA(cls, attA_value, values):
        # Here, we can set attB based on attA
        values['attB'] = attA_value * 2  # Example logic: attB is twice the value of attA
        return attA_value

# Example usage
model = MyModel(attA=5)
print(model.attA)  # Output: 5
print(model.attB)  # Output: 10

Explanation:

1. BaseModel: MyModel inherits from BaseModel, which is the base class for creating Pydantic models.
2. Attributes: attA is defined as an integer, and attB is also an integer with a default value of None.
3. Validator:
   • The @validator('attA', pre=True) decorator is applied to set_attB_from_attA.
   • This method takes attA_value (the value for attA) and values (a dictionary of attribute values, which may include other validated fields).
   • Inside the method, attB is set based on the value of attA. In this case, attB is set to be double the value of attA.
4. Return Value: The validated value of attA is returned, which allows the validation process to continue.

When you create an instance of MyModel, attB will automatically be set based on the value of attA during validation.