python type_error ai_generated true

TypeError: FieldInfo.__init__() got an unexpected keyword argument 'default_factory' when used with a positional default in Field(...)

ID: python/pydantic-field-info-missing-default

Also available as: JSON · Markdown · 中文
80%Fix Rate
84%Confidence
0Evidence
2024-05-08First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
2.x active — — —

Root Cause

Field() is called with both a positional default value and default_factory, which is ambiguous and rejected by Pydantic v2's stricter FieldInfo signature.

generic

中文

Field() 同时传入了位置默认值和 default_factory，存在歧义，被 Pydantic v2 更严格的 FieldInfo 签名拒绝。

Workarounds

  1. 98% success 
    Use only default_factory:

class M(BaseModel):
    items: list[int] = Field(default_factory=list)
  2. 85% success 
    Or use only a positional default:

class M(BaseModel):
    items: list[int] = []  # careful: mutable default is copied by pydantic

Dead Ends

Common approaches that don't work:

  1. 90% fail

    Raises TypeError at import time before any model is instantiated.

  2. 50% fail

    Breaks immutability guarantees and validation order; often still yields None when callers inspect the field.