Is there any post_load in pydantic?

Question

Previously I used the marshmallow library with the Flask. Some time ago I have tried FastAPI with Pydantic. At first glance pydantic seems similar to masrhmallow but on closer inspection they differ. And for me the main difference between them is post_load methods which are from marshmallow. I can't find any analogs for it in pydantic.

post_load is decorator for post-processing methods. Using it I can handle return object on my own, can do whatever I want:

class ProductSchema(Schema):
    alias = fields.Str()
    category = fields.Str()
    brand = fields.Str()

    @post_load
    def check_alias(self, params, **kwargs):
        """One of the fields must be filled"""
        if not any([params.get('alias'), params.get('category'), params.get('brand')]):
            raise ValidationError('No alias provided', field='alias')
        return params

Besides it used not only for validation. Code example is just for visual understanding, do not analyze it, I have just invented it.

So my question is: is there any analog for post_load in pydantic?

Answer 1

It is not obvious but pydantic's validator returns value of the field.

Pydantic v1

There are two ways to handle post_load conversions: validator and root_validator.

validator gets the field value as argument and returns its value. root_validator is the same but manipulates with the whole object.

from pydantic import validator, root_validator


class PaymentStatusSchema(BaseModel):
    order_id: str = Param(..., title="Order id in the shop")
    order_number: str = Param(..., title="Order number in chronological order")
    status: int = Param(..., title="Payment status")

    @validator("status")
    def convert_status(cls, status):
        return "active" if status == 1 else "inactive"

    @root_validator
    def validate_order_id(cls, values):
        """Check order id"""
        if not values.get('orderNumber') and not values.get('mdOrder'):
            raise HTTPException(status_code=400, detail='No order data provided')
        return values

By default pydantic runs validators as post-processing methods. For pre-processing you should use validators with pre argument:

    @root_validator(pre=True)
    def validate_order_id(cls, values):
        """Check order id"""
        # some code here
        return values

Pydantic v2

In Pydantic v2 validators migrated: validator to field_validator and root_validator to model_validator.

@field_validator('status')
@classmethod
def convert_status(cls, status: int):
    return "active" if status == 1 else "inactive"

@model_validator(mode='after')
@classmethod
def validate_order_id(cls, values):
    """Check order id"""
    if not values.get('orderNumber') and not values.get('mdOrder'):
        raise HTTPException(status_code=400, detail='No order data provided')
    return values

For post or pre-processing use mode parameter.