python data_error ai_generated true

pydantic_core._pydantic_core.ValidationError: 3 validation errors for TaggedUnion cat.kind Input should be 'cat' [type=literal_error, input_value='dog', input_type=str]

ID: python/pydantic-union-tag-discriminator-error

Also available as: JSON · Markdown · 中文
80%Fix Rate
84%Confidence
0Evidence
2024-09-03First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
2.x active — — —

Root Cause

A discriminated union with a Literal discriminator received a tag value that does not match any union member, so every branch fails validation.

generic

中文

使用 Literal 判别器的可辨识联合收到一个不匹配任何联合成员的标签值，导致每个分支都校验失败。

Workarounds

  1. 90% success 
    kind = data.get('kind')
mapping = {'cat': Cat, 'dog': Dog}
if kind not in mapping:
    raise ValueError(f'unknown kind: {kind}')
obj = mapping[kind].model_validate(data)
  2. 85% success 
    from typing import Annotated
from pydantic import BeforeValidator

Normalize = Annotated[str, BeforeValidator(lambda v: v.lower())]

class Cat(BaseModel):
    kind: Literal['cat']
    name: Normalize

Dead Ends

Common approaches that don't work:

  1. 65% fail

    Falls back to slow smart-union matching and produces confusing errors when multiple members overlap.

  2. 80% fail

    Discriminator requires all Literal values to be unique; a str member makes the union ambiguous and Pydantic raises at build time.