Python中类型注解Literal的实现

class Document(BaseMedia):
    """Class for storing a piece of text and associated metadata.

    !!! note

        `Document` is for **retrieval workflows**, not chat I/O. For sending text
        to an LLM in a conversation, use message types from `langchain.messages`.

    Example:
        ```python
        from langchain_core.documents import Document

        document = Document(
            page_content="Hello, world!", metadata={"source": "https://example.com"}
        )
        ```
    """

    page_content: str
    """String text."""

    type: Literal["Document"] = "Document"

print(f"转换格式后：\n{json.dumps(document1_json, ensure_ascii=False, indent=2)}")

# 转换格式后，{
#   "lc": 1,
#   "type": "constructor",
#   "id": [
#     "langchain",
#     "schema",
#     "document",
#     "Document"
#   ],
#   "kwargs": {
#     "metadata": {
#       "author": "张三",
#       "page": 10
#     },
#     "page_content": "LangChain 是一个用于开发大语言模型应用的框架。",
#     "type": "Document" 
#   }
# }

from pydantic import BaseModel
from typing import Literal

class MyClass(BaseModel):
    type: Literal["red"] = "red"

obj = MyClass()
obj.type = "Image"  # ❌ Pydantic 会在运行时抛出 ValidationError