Python Conventions

Python Conventions

This file is self-contained for day-to-day use. Deeper reference (needs the full harness checkout, not just this skill — these aren't bundled here since they're documentation, not something this skill runs): languages/python/CONVENTIONS.md (complete examples) and languages/python/COPILOT_INSTRUCTIONS.md (general agent operating principles for Python repos — inspect before changing, scope discipline, never claim a command passed without running it).

Naming

• snake_case — functions, variables. PascalCase — classes. UPPER_SNAKE_CASE — module-level constants.
• Single underscore prefix for private (_internal); avoid double underscore except for genuine name-mangling needs.
• Exceptions: PascalCase + Error/Exception suffix, specific not generic (ValidationError, not BadThing).

Imports & structure

Standard library → third-party → local, each group separated by a blank line. Absolute imports over relative. No from module import *.

Type hints

Use them on function parameters and returns. Match syntax to the project's minimum Python version — check requires-python in pyproject.toml before using list[str] (3.9+), X | None (3.10+), or match/case (3.10+). Don't silently raise the minimum version.

Pitfalls to catch in review

# Mutable default argument — persists across calls, shared state bug
def f(item, container=[]):  # WRONG
def f(item, container=None):  # RIGHT — default to None, create inside

# Bare/broad except — swallows KeyboardInterrupt, hides real errors
except:  # WRONG
except Exception:  # still too broad, usually
except ValueError as e:  # RIGHT — catch what you expect

# `is` for value comparison — checks identity, not equality
if user_id is 5:  # WRONG
if user_id == 5:  # RIGHT (== for values, `is` only for None/True/False)

Testing

tests/ directory, test_*.py files, test_* functions, pytest fixtures for setup. See patterns/testing/TDD.md for the broader methodology — this skill covers Python-specific structure only.

Formatting & tooling

Defer to the project's pyproject.toml — don't impose Black/Ruff defaults over an existing configured formatter/linter. If unconfigured: Black or ruff format (88-char lines), ruff check, mypy for typing, pytest for tests.