Piye cara paramèter fungsi lan argument default iku nggo?

Question

Accepted Answer

Fungsi Python ndhukung argument **positional** lan **keyword**, plus nilai **default**. Mangerteni carane padha kombinasi — lan siji pitakon default-argument sing misuwur — iku penting banget.

## Positional, keyword, lan argument default

```python
def greet(name, greeting="Hello", punctuation="!"):
    return f"{greeting}, {name}{punctuation}"

greet("Ann")                          # positional → "Hello, Ann!"
greet("Ann", "Hi")                    # "Hi, Ann!"
greet("Ann", punctuation=".")          # keyword arg → "Hello, Ann."
greet(name="Ann", greeting="Hey")      # all by keyword
```

Parameter sing duwe default iku optional; sampeyan bisa pass argument kanthi posisi utawa kanthi jeneng (keyword args nambah readability lan nggoyo sampeyan skip parameter optional sing luwih awal).

## Gotcha sing misuwur: mutable default argument

```python
# ❌ DANGER — the default list is created ONCE and SHARED across all calls
def add_item(item, items=[]):
    items.append(item)
    return items

add_item("a")   # ["a"]
add_item("b")   # ["a", "b"]  ← surprise! the SAME list persisted
```

Nilai default dievaluasi **sepisan** nalika fungsi didefinisikan, ora saben call. Mutable default (list/dict) mulane dishare antaraning call — bug klasik iku. Cara ndandani:

```python
# ✅ use None as the default and create a fresh object inside
def add_item(item, items=None):
    if items is None:
        items = []
    items.append(item)
    return items
```

## Keyword-only lan positional-only parameter

```python
def f(a, b, *, key):       # `*` → `key` MUST be passed by keyword
    ...
f(1, 2, key=3)             # ✅   f(1, 2, 3) ❌

def g(a, b, /, c):         # `/` → a, b MUST be positional
    ...
```

Tandha `*` lan `/` ngakoni sampeyan paksa carane argument dipass — guna kanggo API sing jelas lan stabil.

## Kenapa iku penting

Mangerteni parameter handling iku fundamental kanggo nulis lan manggil fungsi Python kanthi bener.

Default nggawe fungsi fleksibel, lan keyword argument nggawe call self-documenting.

Crucially, **mutable-default-argument trap** iku siji saka bug Python sing paling umum lan membingungkan — ngerti yen default dievaluasi sepisan (mulane mutable default tetep lantaran call) lan `None`-sentinel fix nyegah kelas sumunar error.

Kasuwan iki dasar-dasar nggabung fungsi API sing bersih lan predictable.