Default and Keyword Arguments

By the end of this lesson you can:

• Give a parameter a default value, and call the function with or without it.
• Pass arguments by name, in any order, at the call.
• State the two ordering rules — defaults last in the def, positional before keyword in the call.
• Decide which arguments deserve names and which are obvious without them.

You find this line in a classmate's program:

make_drink("Hot chocolate", 1, True, 0, False)

• What is the 1? What is the 0?
• There are two True/False values. Which is which?
• How would you find out — and how long would it take?

1 · A default value goes in the definition, with an =. If the caller says nothing, the default is used.

def order(item, qty=1):
    """Order qty of an item. One, if you don't say."""
    print(f"{qty} x {item}")


order("Pancake")        # 1 x Pancake
order("Pancake", 3)     # 3 x Pancake

The parameter with a default has become optional. Nothing else changed: it is still an ordinary parameter, and you can still pass it by position.

2 · Naming an argument at the call uses the same =, but on the other side:

def make_drink(name, size="regular", ice=True, sugar=1):
    """Print one drink order."""
    ice_text = "with ice" if ice else "no ice"
    print(f"{size} {name}, {ice_text}, {sugar} sugar")


make_drink("Hot chocolate")
make_drink("Cocoa", size="large")
make_drink("Orange juice", sugar=0, ice=False)
make_drink("Strawberry milk", ice=False, size="large")

Output

regular Hot chocolate, with ice, 1 sugar
large Cocoa, with ice, 1 sugar
regular Orange juice, with ice, 0 sugar
large Strawberry milk, no ice, 1 sugar

Look at the last two calls. sugar before ice in one, ice before size in the other — named arguments have no order, because the name says where the value goes. You also skip anything you do not care about, and its default fills in.

When to name. Not every argument needs it. A good habit:

• greet("Mia") — obvious. Leave it.
• set_alarm(7, 30) — probably hours and minutes. Borderline.
• report(marks, True, False) — name them. Two bare booleans are unreadable a week later.

Defaults let one function serve the common case in one word and the unusual case in two. Type this and run it.

def receipt(item, price, qty=1, discount=0, currency="$"):
    """Return one receipt line, with an optional per-item discount."""
    gross = price * qty
    net = gross - gross * discount
    line = f"{qty} x {item:<14} {currency} {net:>6.2f}"
    if discount:
        line += f"   ({int(discount * 100)}% off)"
    return line


print(receipt("Pizza", 4.50))
print(receipt("Hot chocolate", 2.00, 3))
print(receipt("Pancake", 1.50, qty=10, discount=0.15))
print(receipt("Iced latte", 12.00, currency="SGD"))

Output

1 x Pizza     $   4.50
3 x Hot chocolate      $   6.00
10 x Pancake    $  12.75   (15% off)
1 x Iced latte     SGD  12.00

Four calls, four different shapes, one function. Read them again and notice what you did not have to write: no 0 for "no discount", no "$" on every line, no 1 for a single item.

One trap to know about now, before it bites you. A default is created once, when the def is read — not each time you call. That is invisible for numbers and strings, but it matters for lists:

def add_item(item, basket=[]):     # DON'T do this
    basket.append(item)
    return basket

print(add_item("pancake"))            # ['pancake']
print(add_item("tea"))             # ['pancake', 'tea']  <- the same list!

The safe pattern is basket=None, then build a fresh list inside:

def add_item(item, basket=None):
    """Add an item to a basket, starting a new one if none was given."""
    if basket is None:
        basket = []
    basket.append(item)
    return basket

A default value in the def makes a parameter optional, and those parameters must come after the required ones. Naming arguments at the call frees you from the order and makes the call readable — but once you start naming, everything after it must be named too. Good defaults describe the normal case, so the common call is short. And never use a list as a default: use None and build a fresh one inside.