How do add an assembled field to a Pydantic model

Question

Say I have model

class UserDB(BaseModel):
    first_name: Optional[str] = None
    last_name: Optional[str] = None

How do I make another model that is constructed from this one and has a field that changes based on the fields in this model?

For instance, something like this

class User(BaseModel):
    full_name: str = first_name + ' ' + last_name

Constructed like this maybe

User.parse_obj(UserDB)

Thanks!

Asked By: Florian van Kampen-Wright

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Source

Answer 1

If you do not want to keep first_name and last_name in User then you can

customize __init__.
use validator for setting full_name.

Both methods do what you want:

from typing import Optional
from pydantic import BaseModel, validator


class UserDB(BaseModel):
    first_name: Optional[str] = None
    last_name: Optional[str] = None


class User_1(BaseModel):
    location: str  # for a change
    full_name: Optional[str] = None

    def __init__(self, user_db: UserDB, **data):
        super().__init__(full_name=f"{user_db.first_name} {user_db.last_name}", **data)


user_db = UserDB(first_name="John", last_name="Stark")
user = User_1(user_db, location="Mars")
print(user)


class User_2(BaseModel):
    first_name: Optional[str] = None
    last_name: Optional[str] = None
    full_name: Optional[str] = None

    @validator('full_name', always=True)
    def ab(cls, v, values) -> str:
        return f"{values['first_name']} {values['last_name']}"


user = User_2(**user_db.dict())
print(user)

output

location='Mars' full_name='John Stark'
first_name='John' last_name='Stark' full_name='John Stark'

UPDATE:
For working with response_model you can customize __init__ in such way:

class User_1(BaseModel):
    location: str  # for a change
    full_name: Optional[str] = None

    # def __init__(self, user_db: UserDB, **data):
    def __init__(self, first_name, last_name, **data):
        super().__init__(full_name=f"{first_name} {last_name}", **data)


user_db = UserDB(first_name="John", last_name="Stark")
user = User_1(**user_db.dict(), location="Mars")
print(user)

Answered By: alex_noname

Answer 2

I created a pip package that seems to do exactly what you need. Here is the link: https://pypi.org/project/pydantic-computed/

Your example would then look like this:

from pydantic import BaseModel
from pydantic_computed import Computed, computed

class UserDB(BaseModel):
    first_name: Optional[str] = None
    last_name: Optional[str] = None

class User(UserDB):
    full_name: Computed[str]

    @computed('full_name')
    def compute_full_name(first_name: str, last_name: str):
        return first_name + ' ' + last_name


# parsing also works as normal:
user_db = UserDB(first_name='John', last_name='Doe')
user = User.parse_obj(user_db)
print(user.full_name) # Outputs "John Doe"

This will also work for response_model (e.g. in FastAPI) since the computed value is actually set on the full_name property.

Answered By: Jakob Leibetseder

How do add an assembled field to a Pydantic model

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