{
  "id": "python/pydantic-nested-dict-default-mutable",
  "signature": "pydantic.errors.PydanticUserError: Field 'tags' requires a default value or default_factory; mutable defaults are not allowed in Pydantic v2",
  "signature_zh": "pydantic.errors.PydanticUserError：字段 'tags' 需要默认值或 default_factory；Pydantic v2 不允许可变默认值。",
  "regex": "pydantic\\.errors\\.PydanticUserError:\\ Field\\ 'tags'\\ requires\\ a\\ default\\ value\\ or\\ default_factory;\\ mutable\\ defaults\\ are\\ not\\ allowed\\ in\\ Pydantic\\ v2",
  "domain": "python",
  "category": "config_error",
  "subcategory": null,
  "root_cause": "A mutable default (list, dict, set) was provided directly to a field, which Pydantic v2 rejects to avoid shared-state bugs.",
  "root_cause_type": "generic",
  "root_cause_zh": "字段直接使用了可变默认值（list、dict、set），Pydantic v2 为避免共享状态 bug 而拒绝此写法。",
  "versions": [
    {
      "version": "2.x",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    }
  ],
  "os_specific": {},
  "dead_ends": [
    {
      "action": "",
      "why_fails": "Same mutable default problem; Pydantic raises at class definition.",
      "fail_rate": 0.7,
      "condition": "",
      "sources": []
    },
    {
      "action": "",
      "why_fails": "Callers must handle None everywhere; downstream iteration crashes.",
      "fail_rate": 0.5,
      "condition": "",
      "sources": []
    },
    {
      "action": "",
      "why_fails": "Bypasses Pydantic field metadata; model_dump and JSON schema lose the default.",
      "fail_rate": 0.55,
      "condition": "",
      "sources": []
    }
  ],
  "workarounds": [
    {
      "action": "",
      "success_rate": 0.98,
      "how": "from pydantic import BaseModel, Field\nclass M(BaseModel):\n    tags: list[str] = Field(default_factory=list)",
      "condition": "",
      "sources": []
    },
    {
      "action": "",
      "success_rate": 0.9,
      "how": "class M(BaseModel):\n    tags: list[str] = []  # Pydantic copies per-instance",
      "condition": "",
      "sources": []
    }
  ],
  "workarounds_zh": [],
  "transition_graph": {
    "leads_to": [],
    "preceded_by": [],
    "frequently_confused_with": []
  },
  "official_doc_url": null,
  "official_doc_section": null,
  "error_code": null,
  "verification_tier": "ai_generated",
  "confidence": 0.89,
  "fix_success_rate": 0.8,
  "resolvable": "true",
  "first_seen": "2024-02-08",
  "last_confirmed": "2025-01-01",
  "last_updated": "2025-01-01",
  "evidence_count": 0,
  "tags": [],
  "locale": "en",
  "aliases": []
}