---
name: clean-functions
description: Use when writing, fixing, editing, or refactoring Python functions. Enforces Clean Code principles—maximum 3 arguments, single responsibility, no flag parameters.
when_to_use: |
  Also trigger on: functions with 4+ parameters, boolean flag parameters like `enabled=True`, functions that mutate their arguments in place, one-off `util`/`helper` functions that are never called, or asks like "too many arguments", "split this function", "is this still used".
---

# Clean Functions

## F1: Too Many Arguments (Maximum 3)

```python
# Bad - too many parameters
def create_user(name, email, age, country, timezone, language, newsletter):
    ...

# Good - use a dataclass or dict
@dataclass
class UserData:
    name: str
    email: str
    age: int
    country: str
    timezone: str
    language: str
    newsletter: bool

def create_user(data: UserData):
    ...
```

More than 3 arguments means your function is doing too much or needs 
a data structure.

## F2: No Output Arguments

Don't modify arguments as side effects. Return values instead.

```python
# Bad - modifies argument
def append_footer(report: Report) -> None:
    report.append("\n---\nGenerated by System")

# Good - returns new value
def with_footer(report: Report) -> Report:
    return report + "\n---\nGenerated by System"
```

## F3: No Flag Arguments

Boolean flags mean your function does at least two things.

```python
# Bad - function does two different things
def render(is_test: bool):
    if is_test:
        render_test_page()
    else:
        render_production_page()

# Good - split into two functions
def render_test_page(): ...
def render_production_page(): ...
```

## F4: Delete Dead Functions

If it's not called, delete it. No "just in case" code. Git preserves history.