CODEWITHSUNDEEP
pythonpython-typingpydanticpylance
MrJ
MrJ

Reputation: 1514

Is Pylance working properly for Annotated Pydantic Fields?

Why does Pylance warn for Alice?

I expect that field seed should be identical for both classes User1 and User2.

import random
from typing import Annotated

from pydantic import BaseModel, Field

RandomInt = Annotated[int, Field(default_factory=lambda:random.randint(0, 1000))]


class User1(BaseModel):
    name : str
    seed : RandomInt


class User2(BaseModel):
    name : str
    seed : int = Field(default_factory=lambda: random.randint(0, 1000))


if __name__ == '__name__':
    alice = User1(name='Alice')

    bob = User2(name='Bob')

But instead Pylance warns

(variable) name: str
Argument missing for parameter "seed"PylancereportGeneralTypeIssues

Even though the code runs perfectly and the default_factory defined in RandomInt does its job.

Upvotes: 1

Views: 1365

Answers (2)

Anatoly Makarevich
Anatoly Makarevich

Reputation: 147

This is specific behavior from Pydantic which supports default_factory, but Python's type system doesn't know that Pydantic will generate a default value. To Python's type system (and thus Pylance) RandomInt is an annotated int, and Pylance (as well as MyPy and others) will ignore any annotations, since they're application-specific.

Note that pydantic.Field() is type-hinted itself as returning a value (I believe it's hinted as Any?), which is why Pylance won't give a warning about missing default value (because a default value exists) for User2, but warns about no default value in User1.

Upvotes: 0

DanielB
DanielB

Reputation: 2958

The usage in User1.seed and User2.seed is not equivalent. Pydantic's Field is not a type annotation, it must be used as a value (as is for User2.seed). Such, pydantic just interprets User1.seed as an int field, with no default value, and so requires you to provide a value on creation.

Annotated is used for providing non-type annotations alongside type annotations (type annotations are the norm, but annotations can be used for another applications).

Upvotes: 1

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