CODEWITHSUNDEEP
pythongenericspydantic
Syed Jafri
Syed Jafri

Reputation: 476

Pydantic inherit generic class

New to python and pydantic, I come from a typescript background. I was wondering if you can inherit a generic class?

In typescript the code would be as follows

interface GenericInterface<T> {
  value: T
}

interface ExtendsGeneric<T> extends GenericInterface<T> {
  // inherit value from GenericInterface
  otherValue: string
}

const thing: ExtendsGeneric<Number> = {
  value: 1,
  otherValue: 'string'
}

What I have been trying is something along the lines of

#python3.9
from pydantic.generics import GenericModel
from typing import TypeVar
from typing import Generic

T = TypeVar("T", int, str)

class GenericField(GenericModel, Generic[T]):
    value: T

class ExtendsGenericField(GenericField[T]):
    otherValue: str

ExtendsGenericField[int](value=1, otherValue="other value")

And I get the error of TypeError: Too many parameters for ExtendsGenericField; actual 1, expected 0. This sort of checks out because in the Pydantic docs it explicitly states "In order to declare a generic model...Use the TypeVar instances as annotations where you will want to replace them..." The easy workaround is to make ExtendsGeneric inherit from GenericModel and have value in its own class definition, but I was trying to reuse classes.

Is inheriting a value from a generic class possible?

Upvotes: 9

Views: 16133

Answers (3)

juanmirocks
juanmirocks

Reputation: 6171

Since Pydantic v2 (at least with v2.6.x), the following works (reference):

# at least with Python 3.9
from typing import Generic, TypeVar
from pydantic import BaseModel

T = TypeVar("T")

class BaseClass(BaseModel, Generic[T]):
    value: T

class ChildClass(BaseClass[T], Generic[T]):
    other_value: str


x = ChildClass[int](value=1, other_value="other value")

type(x)
# __main__.ChildClass[int]

Upvotes: 2

Abhishek
Abhishek

Reputation: 111

Not sure but referring yours my code worked :)

from datetime import datetime
from pydantic import BaseModel, parse_obj_as
from enum import Enum
import json
from typing import TypeVar, Generic
from pydantic.generics import GenericModel

class TradeOverlapping(BaseModel):
    COBDate: str
    DBServer: str

class EventType(Enum):
    STH_READY= 'STH_READY'

T = TypeVar("T")
class KafkaEvent(GenericModel,Generic[T]):
    Source: str
    Version: str
    TimeStamp: datetime
    Event: EventType
    Data: T

p = KafkaEvent[TradeOverlapping].parse_obj({"Source":"TOPIC","Version":"1.0","TimeStamp":"2022-08-03 06:49:17","Event":EventType('STH_READY'),"Data":TradeOverlapping(COBDate="2023-04-05", DBServer="DB_SERVER")})
print("Producer:")
print(p.json())

print("----------------------------")

value = '{"Source":"TOPIC","Version":"1.0","TimeStamp":"2022-08-03 06:49:17","Event":"STH_READY","Data":{"COBDate":"2023-04-05","DBServer":"DB_SERVER"}}'
obj = parse_obj_as(KafkaEvent[TradeOverlapping], json.loads(value))
print("Consumer:")
print(obj)

Upvotes: 1

jkinkead
jkinkead

Reputation: 4411

Generics are a little weird in Python, and the problem is that ExtendsGenericField itself isn't declared as generic. To solve, just add Generic[T] as a super class of ExtendsGenericField:

from pydantic.generics import GenericModel
from typing import TypeVar
from typing import Generic

T = TypeVar("T", int, str)

class GenericField(GenericModel, Generic[T]):
    value: T

class ExtendsGenericField(GenericField[T], Generic[T]):
    otherValue: str

ExtendsGenericField[int](value=1, otherValue="other value")

Upvotes: 14

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