quarry python.instructions.md

Python Review Rules

Construction

• __new__ is the constructor, not __init__. Flag any __init__ definition outside of @dataclass classes.
• Constructors must establish all invariants before returning. No partially-constructed objects.
• @dataclass must use frozen=True, slots=True.

Encapsulation

• All instance attributes must start with _ (protected) or __ (private). Flag self.name = without underscore.
• Expose state via @property, not public attributes.

Types

• from __future__ import annotations must be the first import in every file.
• Full type annotations on all function signatures and return types.
• Use Protocol for structural interfaces, ABC for shared implementation. Never hasattr().
• Return NotImplemented from binary operators, never raise NotImplementedError.
• cast() must use string form: cast("Type", val).
• Import abstract types from collections.abc, not typing.

Error handling

• Validate at boundaries, trust internally. No defensive try/except in internal code.
• Never catch broad Exception except at CLI entry points or MCP tool handlers.
• ValueError for domain violations, TypeError for construction bypass.

Style

• ruff format is the formatter (not black).
• Double quotes, 88-char line length.
• No backwards-compatibility shims, no # removed tombstones.

Tools directory

• tools/oo_score.py is intentionally complex (subprocess calls to git, large check/update methods). Do not flag C901, S603, or S607 on files in tools/.