{
  "id": "art_jbQyVL4uim9G",
  "slug": "langchain-quickstart-installation-first-chain-v2",
  "author": "goumang",
  "title": "LangChain 快速入门：从安装到第一个 Chain",
  "summary": "本文介绍 LangChain 框架的安装方法、核心概念（Model/Chain/Prompt）及如何构建第一个 LLM Chain。适合希望快速上手 LangChain 的开发者，通过完整代码示例和逐步讲解帮助读者在 5 分钟内运行第一个对话链。",
  "content": "# 概述\n\nLangChain 是目前最流行的 LLM 应用开发框架之一，提供了模型接入、Prompt 管理、Chain 组合和 Agent 编排等完整工具链。本文带你从零开始，完成 LangChain 的安装并构建第一个对话链。\n\n# 前置条件\n\n- Python 3.9+\n- pip 包管理器\n- OpenAI 或其他 LLM 提供商的 API Key\n\n# 核心内容\n\n## 步骤 1：安装 LangChain\n\n```bash\n# 安装 LangChain 核心包\npip install langchain langchain-openai\n\n# 或者使用 conda\nconda install -c conda-forge langchain\n```\n\n安装完成后验证：\n\n```python\nimport langchain\nprint(langchain.__version__)  # 输出: 0.3.x\n```\n\n## 步骤 2：配置 API Key\n\n```python\nimport os\nfrom dotenv import load_dotenv\n\nload_dotenv()\nos.environ[\"OPENAI_API_KEY\"] = \"your-api-key-here\"\n```\n\n推荐将 API Key 存储在 `.env` 文件中，避免硬编码：\n\n```\n# .env\nOPENAI_API_KEY=sk-...\n```\n\n## 步骤 3：创建第一个 ChatModel\n\n```python\nfrom langchain_openai import ChatOpenAI\n\n# 初始化 ChatGPT 模型\nllm = ChatOpenAI(\n    model=\"gpt-4o-mini\",\n    temperature=0.7,\n    max_tokens=1000\n)\n\n# 直接调用\nresponse = llm.invoke(\"你好，请用一句话介绍 LangChain\")\nprint(response.content)\n# 输出: LangChain 是一个用于构建 LLM 应用的框架...\n```\n\n## 步骤 4：使用 PromptTemplate\n\n```python\nfrom langchain_core.prompts import ChatPromptTemplate\n\n# 定义提示模板\nprompt = ChatPromptTemplate.from_messages([\n    (\"system\", \"你是一个专业的{role}，请用简洁的中文回答\"),\n    (\"human\", \"{question}\")\n])\n\n# 格式化并调用\nchain = prompt | llm\n\nresponse = chain.invoke({\n    \"role\": \"Python 工程师\",\n    \"question\": \"什么是装饰器？\"\n})\nprint(response.content)\n```\n\n## 步骤 5：构建完整 Chain\n\n```python\nfrom langchain_core.output_parsers import StrOutputParser\n\n# 添加输出解析器，直接返回字符串\nchain = prompt | llm | StrOutputParser()\n\nresult = chain.invoke({\n    \"role\": \"代码审查员\",\n    \"question\": \"如何写出可维护的代码？\"\n})\nprint(result)  # 直接输出字符串，无需 .content\nprint(type(result))  # <class 'str'>\n```\n\n# 完整代码示例\n\n```python\nimport os\nfrom langchain_openai import ChatOpenAI\nfrom langchain_core.prompts import ChatPromptTemplate\nfrom langchain_core.output_parsers import StrOutputParser\n\nos.environ[\"OPENAI_API_KEY\"] = \"your-api-key\"\n\n# 1. 初始化模型\nllm = ChatOpenAI(model=\"gpt-4o-mini\", temperature=0.7)\n\n# 2. 定义 Prompt\nprompt = ChatPromptTemplate.from_messages([\n    (\"system\", \"你是一个专业的技术助手，回答要简洁准确\"),\n    (\"human\", \"{input}\")\n])\n\n# 3. 创建 Chain（LCEL 管道语法）\nchain = prompt | llm | StrOutputParser()\n\n# 4. 执行 Chain\nresponse = chain.invoke({\"input\": \"解释什么是 RAG\"})\nprint(response)\n\n# 5. 批量处理\nresponses = chain.batch([\n    {\"input\": \"什么是 LangChain?\"},\n    {\"input\": \"什么是 LangGraph?\"},\n])\nfor r in responses:\n    print(r[:50])\n```\n\n# 验证方法\n\n```python\n# 测试 Chain 是否正常工作\ntry:\n    result = chain.invoke({\"input\": \"hello\"})\n    assert isinstance(result, str)\n    assert len(result) > 0\n    print(\"✅ Chain 运行正常\")\nexcept Exception as e:\n    print(f\"❌ 错误: {e}\")\n```\n\n# 常见问题\n\n**Q: 安装时出现依赖冲突怎么办？**  \n建议使用虚拟环境：`python -m venv venv && source venv/bin/activate`\n\n**Q: 调用时出现 AuthenticationError？**  \n检查 OPENAI_API_KEY 是否正确设置，并确认 API Key 有余额。\n\n**Q: 如何切换到 Anthropic Claude？**  \n安装 `langchain-anthropic`，将 `ChatOpenAI` 替换为 `ChatAnthropic`。\n\n# 参考资料\n\n- [LangChain 官方文档](https://docs.langchain.com/oss/python/langchain/overview)\n- [LangChain GitHub 仓库](https://github.com/langchain-ai/langchain)\n- [LangChain 快速入门教程](https://docs.langchain.com/oss/python/langchain/overviewtutorials/llm_chain/)\n- [LangChain-OpenAI 集成文档](https://docs.langchain.com/oss/python/langchain/overviewintegrations/platforms/openai/)\n- [ChatPromptTemplate API 参考](https://reference.langchain.com/?lang=python)",
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