pydantic-basesettings-class-attributes

Pydantic BaseSettings Class Attributes Pitfall

Overview

When defining class attributes in Pydantic BaseSettings classes, Pydantic treats them as model fields and raises an error. Move constants outside the class as module-level private constants or use ClassVar annotation.

When to Use

• Defining constants in Pydantic BaseSettings or BaseModel subclasses
• Refactoring hardcoded mappings/configs into reusable data structures
• Encountering "non-annotated attribute" errors

The Problem

Pydantic treats class attributes as model fields:

PydanticUserError: A non-annotated attribute was detected: `MAPPING = {...}`.
All model fields require a type annotation; if `MAPPING` is not meant to be a field,
you may be able to resolve this error by annotating it as a `ClassVar`

Solution

Recommended: Move constants outside the class as module-level private constants

# Correct approach
_APOLLO_MAPPING = {
    "grok.api.key": "grok_api_key",
    "remote.timeout": "timeout_seconds",
}

_BOOLEAN_FIELDS = ["debug", "enabled"]

class Settings(BaseSettings):
    grok_api_key: str = ""
    timeout_seconds: int = 30

    @model_validator(mode="before")
    @classmethod
    def load_from_apollo(cls, data):
        for apollo_key, field in _APOLLO_MAPPING.items():
            ...

Alternative: Use ClassVar type annotation (works but less clean)

from typing import ClassVar

class Settings(BaseSettings):
    MAPPING: ClassVar[dict[str, str]] = {...}

Common Mistakes

• Defining mappings/constants directly in Pydantic classes without annotation
• Using ClassVar when module-level constants would be cleaner
• Forgetting that Pydantic validates all class attributes by default