langchain-local-dev-loop

# LangChain Local Dev Loop ## Overview Configure a rapid local development workflow for LangChain applications with testing, debugging, and hot reload capabilities. ## Prerequisites - Completed `langchain-install-auth` setup - Python 3.9+ with virtual environment - pytest and related testing tools - IDE with Python support (VS Code recommended) ## Instructions ### Step 1: Set Up Project Structure ``` my-langchain-app/ ├── src/ │ ├── __init__.py │ ├── chains/ │ │ └── __init__.py │ ├── agents/ │ │ └── __init__.py │ └── prompts/ │ └── __init__.py ├── tests/ │ ├── __init__.py │ ├── conftest.py │ └── test_chains.py ├── .env ├── .env.example ├── pyproject.toml └── README.md ``` ### Step 2: Configure Testing ```python # tests/conftest.py import pytest from unittest.mock import MagicMock from langchain_core.messages import AIMessage @pytest.fixture def mock_llm(): """Mock LLM for unit tests without API calls.""" mock = MagicMock() mock.invoke.return_value = AIMessage(content="Mocked response") return mock @pytest.fixture def sample_prompt(): """Sample prompt for testing.""" from langchain_core.prompts import ChatPromptTemplate return ChatPromptTemplate.from_template("Test: {input}") ``` ### Step 3: Create Test File ```python # tests/test_chains.py def test_chain_construction(mock_llm, sample_prompt): """Test that chain can be constructed.""" from langchain_core.output_parsers import StrOutputParser chain = sample_prompt | mock_llm | StrOutputParser() assert chain is not None def test_chain_invoke(mock_llm, sample_prompt): """Test chain invocation with mock.""" from langchain_core.output_parsers import StrOutputParser chain = sample_prompt | mock_llm | StrOutputParser() result = chain.invoke({"input": "test"}) assert result == "Mocked response" ``` ### Step 4: Set Up Development Tools ```toml # pyproject.toml [project] name = "my-langchain-app" version = "0.1.0" requires-python = ">=3.9" dependencies = [ "langchain>=0.3.0", "langchain-openai>=0.2.0", "python-dotenv>=1.0.0", ] [project.optional-dependencies] dev = [ "pytest>=8.0.0", "pytest-asyncio>=0.23.0", "pytest-cov>=4.0.0", "ruff>=0.1.0", "mypy>=1.0.0", ] [tool.pytest.ini_options] asyncio_mode = "auto" testpaths = ["tests"] [tool.ruff] line-length = 100 ``` ## Output - Organized project structure with separation of concerns - pytest configuration with fixtures for mocking LLMs - Development dependencies configured - Ready for rapid iteration ## Error Handling | Error | Cause | Solution | |-------|-------|----------| | Import Error | Missing package | Install with `pip install -e ".[dev]"` | | Fixture Not Found | conftest.py issue | Ensure conftest.py is in tests/ directory | | Async Test Error | Missing marker | Add `@pytest.mark.asyncio` decorator | | Env Var Missing | .env not loaded | Use `python-dotenv` and load_dotenv() | ## Examples ### Running Tests ```bash # Run all tests pytest # Run with coverage pytest --cov=src --cov-report=html # Run specific test pytest tests/test_chains.py::test_chain_invoke -v # Watch mode (requires pytest-watch) ptw ``` ### Integration Test Example ```python # tests/test_integration.py import pytest from dotenv import load_dotenv load_dotenv() @pytest.mark.integration def test_real_llm_call(): """Integration test with real LLM (requires API key).""" from langchain_openai import ChatOpenAI llm = ChatOpenAI(model="gpt-4o-mini", temperature=0) response = llm.invoke("Say 'test passed'") assert "test" in response.content.lower() ``` ## Resources - [pytest Documentation](https://docs.pytest.org/) - [LangChain Testing Guide](https://python.langchain.com/docs/contributing/testing) - [python-dotenv](https://pypi.org/project/python-dotenv/) ## Next Steps Proceed to `langchain-sdk-patterns` for production-ready code patterns.