Conventions

Conventions

Project coding conventions for all Python code. Covers structure, style, quality, and logging. For detailed tool configurations see tooling.md. For file templates see templates.md.

General Principles

• Prioritize readability over cleverness
• Follow principle of least surprise
• Keep functions small and focused (single responsibility)
• Profile before optimizing — never guess at bottlenecks
• Write tests before or alongside implementation

Project Structure

Use src-layout with layered architecture:

${PROJECT_NAME}/
├── pyproject.toml
├── mkdocs.yml
├── docs/
│   ├── index.md
│   └── scripts/
│       └── gen_ref_pages.py
├── src/
│   └── ${PROJECT_NAME}/
│       ├── __init__.py
│       ├── _version.py        # Auto-generated by hatch-vcs
│       ├── contracts/         # API schemas — single source of truth
│       ├── core/              # Infrastructure only (logger, config, exceptions)
│       ├── models/            # Shared domain models (Pydantic)
│       ├── services/          # Business logic (framework-agnostic)
│       └── interfaces/        # Entry points
│           ├── cli/           # CLI (Typer)
│           ├── rest/          # REST API (FastAPI)
│           └── rpc/           # JSON-RPC over WebSocket
└── tests/
    ├── conftest.py
    └── ...                    # Mirror src/ structure

Layer Rules

• contracts/: API schemas (OpenAPI, OpenRPC) — single source of truth, written first
• core/: Infrastructure only — NO business logic (logger, config, exceptions, utils)
• models/: Pydantic models matching contract schemas — written before implementation
• services/: Business logic — framework-agnostic, no Typer/FastAPI imports, fully testable
• interfaces/: Thin wrappers — call services, handle I/O, minimal logic

Python Version

Target Python 3.13+:

[project]
requires-python = ">=3.13"

Imports

1. Standard library
2. Third-party packages
3. Local imports

import os
from pathlib import Path

import httpx
from pydantic import BaseModel

from ${PROJECT_NAME}.core import logger
from ${PROJECT_NAME}.services.processor import process_data

• Prefer from x import y for specific items
• Use absolute imports, not relative

Type Hints

Always use type hints on all function signatures:

def process_file(
    filepath: Path,
    options: dict[str, Any] | None = None,
) -> ProcessResult:
    ...

Modern Typing (Python 3.13+)

list[str]           # not List[str]
dict[str, int]      # not Dict[str, int]
str | None          # not Optional[str]
int | str           # not Union[int, str]

Docstrings

Google-style for all public functions:

def fetch_data(url: str, timeout: int = 30) -> dict[str, Any]:
    """Fetch data from a URL and return parsed JSON.

    Args:
        url: The URL to fetch data from.
        timeout: Request timeout in seconds. Defaults to 30.

    Returns:
        Parsed JSON response as a dictionary.

    Raises:
        httpx.HTTPError: If the request fails.
    """
    ...

Naming

• Variables: explicit names (file_count not fc, user_data not ud)
• Constants: ALL_CAPS at module level — extract magic numbers
• Functions/methods: snake_case, imperative verbs

MAX_RETRIES = 3
DEFAULT_TIMEOUT = 30
SUPPORTED_FORMATS = {"json", "yaml", "toml"}

Logging

Never use print() — always use logger.

from ${PROJECT_NAME}.core import logger

logger.debug(f"Processing item {i} of {total}")
logger.info(f"Processing {count} files")
logger.success(f"Completed: {result}")
logger.warning(f"Rate limit approaching")
logger.error(f"Failed to process: {error}")
logger.critical("System failure")

Key Patterns

# Log exceptions with context
try:
    process()
except Exception as e:
    logger.error(f"Processing failed: {e}", exc_info=True)
    raise

# Progress logging for long operations
for i, item in enumerate(items, 1):
    process_item(item)
    if i % 100 == 0:
        logger.info(f"Progress: {i}/{total} items processed")
logger.success(f"Completed processing {total} items")

Testing with Logging

def test_function_logs_correctly(caplog):
    with caplog.at_level("INFO"):
        my_function()
        assert "Expected message" in caplog.text

Error Handling

# Specific exceptions, not bare except
try:
    result = process(data)
except FileNotFoundError:
    logger.error(f"File not found: {filepath}")
    raise
except ValueError as e:
    logger.error(f"Invalid data: {e}")
    raise

# Custom exceptions in core/exceptions.py
class ProcessingError(Exception):
    """Raised when processing fails."""
    pass

# Validate early, fail fast
def process(data: str) -> Result:
    if not data:
        raise ValueError("data cannot be empty")
    ...

Configuration

Use environment variables via pydantic-settings:

from pydantic_settings import BaseSettings

class Settings(BaseSettings):
    debug: bool = False
    api_key: str
    database_url: str

    model_config = {"env_prefix": "${PROJECT_NAME}_"}

settings = Settings()

Refactoring

• Look for duplication, long functions, unclear names
• Extract common functionality to reusable functions
• Keep refactoring commits separate from feature commits

Code Review Checklist

Before committing:

• Type hints complete and accurate
• Google-style docstrings on public APIs
• Logger used, not print()
• Magic numbers extracted to constants
• Specific exception handling (no bare except)
• Tests comprehensive and passing
• No sensitive info in logs or code
• uv run ruff check . passes
• uv run pyright passes

Quick Reference

See Also

• spec-driven — contracts-first development and models placement
• project-name — resolves ${PROJECT_NAME}
• documentation — MkDocs setup and API doc generation
• tdd — test-driven development with pytest