python type_error ai_generated true

pydantic.errors.UnionError: Unable to validate input for union

ID: python/pydantic-union-type-validation

Also available as: JSON · Markdown · 中文
80%Fix Rate
83%Confidence
0Evidence
2024-05-05First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
2.x active — — —

Root Cause

When a field is typed as Union[A, B], Pydantic tries each type in order; if none match, it raises UnionError, often due to ambiguous types or strict validation.

generic

中文

当字段类型为 Union[A, B] 时，Pydantic 会按顺序尝试每种类型；如果都不匹配，就会抛出 UnionError，通常是由于类型歧义或严格验证。

Workarounds

  1. 90% success 
    Use discriminated unions with Literal types: Field(discriminator='type')
  2. 85% success 
    Define a custom validator to handle complex union logic

Dead Ends

Common approaches that don't work:

  1. 70% fail

    Order matters; if first type accepts too broadly, it may consume valid inputs for second type.

  2. 60% fail

    Optional[Union] can still fail if input is None but not allowed.