Pydantic overriding default value with Field produces Pyright error, is it expected?

[rafrafek]

Hello,

I'm getting this error:

"expire_time" overrides a field of the same name but is missing a default value

for this code:

from datetime import timedelta

from pydantic import BaseModel, Field


class BaseSession(BaseModel):
    expire_time: timedelta = timedelta(hours=3)


class CustomSession(BaseSession):
    expire_time: timedelta = Field(timedelta(hours=3), alias="expireTime")
    # ^^^^^^^^^ missing a default value

Is it expected, or is it a bug in Pydantic or Pyright? Isn't Field a default value here?

[erictraut]

This is expected behavior. It's neither a bug in pydantic nor in pyright.

Pydantic provides a non-standard way to specify a default value via a positional argument, as you've done in your code sample. Field specifiers in stdlib dataclass do not support this; they require you to specify the default by keyword argument default=.

Standards-compliant type checkers like pyright do not know about pydantics idiosyncrasies. Their behavior is based on the dataclass_transform mechanism which assumes standard dataclass behaviors.

Pydantic also supports the standard dataclass behavior, so if you change your code to the following, it will satisfy both pydantic and standards-based type checkers like pyright.

class CustomSession(BaseSession):
    expire_time: timedelta = Field(default=timedelta(hours=3), alias="expireTime")

[rafrafek]

Thank you for clarifying it for me!