Lean Code

Lean Code Skill

When to Use

Apply this skill whenever you generate, modify, or refactor code.

Pre-Generation Checks (Mandatory Before Writing Code)

1. Does this code have external users? If not, no fallback is needed.
2. Is there a concrete reason to support older versions? If not, no compatibility code is needed.
3. Does this logic already exist somewhere in the codebase? If yes, reuse it.

Disallowed Patterns

If you are about to write any of the following patterns, stop and remove them first.

# NG: Fallback to a legacy format that does not exist
if hasattr(obj, 'new_method'):
    obj.new_method()
else:
    obj.old_method()  # old_method does not exist

# NG: Defensive default value "just in case"
value = config.get('key', some_complex_fallback_logic())

# NG: Same validation duplicated in two places
def create_user(name):
    if not name or len(name) > 100:  # Use validate_name()
        raise ValueError()

# NG: Optional arguments nobody uses
def process(data, legacy_mode=False, compat_version=None):
    ...

Recommended Patterns

# OK: Write directly
obj.new_method()

# OK: Raise an error when config is missing (do not hide issues)
value = config['key']

# OK: Keep validation in one place
def validate_name(name):
    if not name or len(name) > 100:
        raise ValueError()

# OK: Keep only arguments needed now
def process(data):
    ...

Post-Generation Self-Review

Before outputting code, ask yourself:

• Is there defensive logic that starts with "if ..."? Is that scenario actually possible now?
• Did I write the same logic twice?
• Is there code that can be removed without affecting current behavior? If yes, remove it.

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