dignified-python-313

name

dignified-python-313

description

This skill should be used when editing Python code in the erk codebase. Use when writing, reviewing, or refactoring Python to ensure adherence to LBYL exception handling patterns, Python 3.13+ type syntax (list[str], str | None), pathlib operations, ABC-based interfaces, absolute imports, and explicit error boundaries at CLI level. Also provides production-tested code smell patterns from Dagster Labs for API design, parameter complexity, and code organization. Essential for maintaining erk's dignified Python standards.

Dignified Python - Python 3.13+ Coding Standards

Write explicit, predictable code that fails fast at proper boundaries.

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Quick Reference - Check Before Coding

If you're about to write...	Check this rule
try: or except:	→ Exception Handling - Default: let exceptions bubble
from __future__ import annotations	→ FORBIDDEN - Python 3.13+ doesn't need it
List[...], Dict[...], Union[...]	→ Use list[...], dict[...], X | Y
dict[key] without checking	→ Use if key in dict: or .get()
path.resolve() or path.is_relative_to()	→ Check path.exists() first
typing.Protocol	→ Use abc.ABC instead
from .module import	→ Use absolute imports only
__all__ = ["..."] in __init__.py	→ See references/core-standards.md#code-in-initpy-and-all-exports
print(...) in CLI code	→ Use click.echo()
subprocess.run(...)	→ Add check=True
@property with I/O or expensive computation	→ See references/core-standards.md#performance-expectations
Function with many optional parameters	→ See references/code-smells-dagster.md
repr() for sorting or hashing	→ See references/code-smells-dagster.md
Context object passed everywhere	→ See references/code-smells-dagster.md
Function with 10+ local variables	→ See references/code-smells-dagster.md
Class with 50+ methods	→ See references/code-smells-dagster.md

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CRITICAL RULES (Top 6)

1. Exception Handling - NEVER for Control Flow 🔴

ALWAYS use LBYL (Look Before You Leap), NEVER EAFP

# ✅ CORRECT: Check before acting
if key in mapping:
    value = mapping[key]
else:
    handle_missing_key()

# ❌ WRONG: Using exceptions for control flow
try:
    value = mapping[key]
except KeyError:
    handle_missing_key()

Details: See references/core-standards.md#exception-handling for complete patterns

2. Type Annotations - Python 3.13+ Syntax Only 🔴

FORBIDDEN: from __future__ import annotations

# ✅ CORRECT: Modern Python 3.13+ syntax
def process(items: list[str]) -> dict[str, int]: ...
def find_user(id: int) -> User | None: ...

# ❌ WRONG: Legacy syntax
from typing import List, Dict, Optional
def process(items: List[str]) -> Dict[str, int]: ...

Details: See references/core-standards.md#type-annotations for all patterns

3. Path Operations - Check Exists First 🔴

# ✅ CORRECT: Check exists first
if path.exists():
    resolved = path.resolve()

# ❌ WRONG: Using exceptions
try:
    resolved = path.resolve()
except OSError:
    pass

Details: See references/core-standards.md#path-operations

4. Dependency Injection - ABC Not Protocol 🔴

# ✅ CORRECT: Use ABC
from abc import ABC, abstractmethod

class MyOps(ABC):
    @abstractmethod
    def operation(self) -> None: ...

# ❌ WRONG: Using Protocol
from typing import Protocol

Details: See references/core-standards.md#dependency-injection

5. Imports - Module-Level and Absolute 🔴

ALL imports must be at module level unless preventing circular imports

# ✅ CORRECT: Module-level, absolute imports
from erk.config import load_config
from pathlib import Path
import click

# ❌ WRONG: Inline imports (unless for circular import prevention)
def my_function():
    from erk.config import load_config  # WRONG unless circular import
    return load_config()

# ❌ WRONG: Relative imports
from .config import load_config

Exception: Inline imports are ONLY acceptable when preventing circular imports. Always document why:

def create_context():
    # Inline import to avoid circular dependency with tests
    from tests.fakes.gitops import FakeGitOps
    return FakeGitOps()

Details: See references/core-standards.md#imports

6. No Silent Fallback Behavior 🔴

# ❌ WRONG: Silent fallback
try:
    result = primary_method()
except:
    result = fallback_method()  # Untested, brittle

# ✅ CORRECT: Let error bubble up
result = primary_method()

Details: See references/core-standards.md#anti-patterns

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When to Load References

Load references/core-standards.md when:

• Writing exception handling code (LBYL patterns)
• Working with type annotations (Python 3.13+ syntax)
• Implementing path operations (exists() checks)
• Creating ABC interfaces (dependency injection)
• Organizing imports (absolute imports, module-level)
• Working with CLI code (Click patterns)
• Using dataclasses and immutability
• Avoiding anti-patterns (silent fallback, exception swallowing)
• Implementing @property or __len__ (performance expectations)

Load references/code-smells-dagster.md when:

• Designing function APIs (default parameters, keyword arguments)
• Managing parameter complexity (parameter anxiety, invalid combinations)
• Refactoring large functions/classes (god classes, local variables)
• Working with context managers (assignment patterns)
• Using repr() programmatically (string representation abuse)
• Passing context objects (context coupling)
• Dealing with error boundaries (early validation)

Load references/patterns-reference.md when:

• Developing CLI commands with Click
• Working with file I/O and pathlib
• Implementing dataclasses and frozen structures
• Managing subprocess operations
• Reducing code nesting (early returns, helper functions)

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Progressive Disclosure Guide

This skill uses a three-level loading system:

1. This file (SKILL.md): Core rules and navigation (~350 lines)
2. Reference files: Detailed patterns and examples (loaded as needed)
3. Quick lookup: Use the tables above to find what you need

Claude loads reference files only when needed based on the current task. The reference files contain:

• core-standards.md: Foundational Python patterns from this skill
• code-smells-dagster.md: Production-tested anti-patterns from Dagster Labs
• patterns-reference.md: Common implementation patterns and examples

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Philosophy

Write dignified Python code that:

• Fails fast at proper boundaries (not deep in the stack)
• Makes invalid states unrepresentable (use the type system)
• Expresses intent clearly (LBYL over EAFP)
• Minimizes cognitive load (explicit over implicit)
• Enables confident refactoring (test what you build)

Default stances:

• Let exceptions bubble up (handle at boundaries only)
• Break APIs and migrate immediately (no unnecessary backwards compatibility)
• Check conditions proactively (LBYL)
• Use modern Python 3.13+ syntax

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Quick Decision Tree

About to write Python code?

1. Using try/except?

   • Can you use LBYL instead? → Do that
   • Is this an error boundary? → OK to handle
   • Otherwise → Let it bubble

2. Using type hints?

   • Use list[str], str | None, not List, Optional
   • NO from __future__ import annotations

3. Working with paths?

   • Check .exists() before .resolve()
   • Use pathlib.Path, not os.path

4. Writing CLI code?

   • Use click.echo(), not print()
   • Exit with raise SystemExit(1)

5. Too many parameters?

   • See references/code-smells-dagster.md#parameter-anxiety

6. Class getting large?

   • See references/code-smells-dagster.md#god-classes

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Checklist Before Writing Code

Before writing try/except:

• Can I check the condition proactively? (LBYL)
• Is this at an error boundary? (CLI/API level)
• Am I adding meaningful context or just hiding the error?

Before using type hints:

• Am I using Python 3.13+ syntax? (list, dict, |)
• Have I removed all typing imports except essentials?

Before path operations:

• Did I check .exists() before .resolve()?
• Am I using pathlib.Path?
• Did I specify encoding="utf-8"?

Before adding backwards compatibility:

• Did the user explicitly request it?
• Is this a public API?
• Default: Break and migrate immediately

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Common Patterns Summary

Scenario	Preferred Approach	Avoid
Dictionary access	if key in dict: or .get(key, default)	try: dict[key] except KeyError:
File existence	if path.exists():	try: open(path) except FileNotFoundError:
Type checking	if isinstance(obj, Type):	try: obj.method() except AttributeError:
Value validation	if is_valid(value):	try: process(value) except ValueError:
Path resolution	if path.exists(): path.resolve()	try: path.resolve() except OSError:

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References

• Core Standards: references/core-standards.md - Detailed LBYL patterns, type annotations, imports
• Code Smells: references/code-smells-dagster.md - Production-tested anti-patterns
• Pattern Reference: references/patterns-reference.md - CLI, file I/O, dataclasses
• Python 3.13 docs: https://docs.python.org/3.13/