Default and keyword arguments

Default values

A parameter can have a default value, used when the caller doesn't pass that argument:

def greet(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

greet("Asha")
greet("Leo", "Good morning")

Parameters with defaults must come after those without them. def greet(greeting="Hello", name): is a syntax error.

def power(base, exponent=2):
    return base ** exponent

print(power(5))        # 25
print(power(5, 3))     # 125

You have already used defaults in built-in functions: print()'s sep=" " and end="\n", and round()'s ndigits.

Keyword arguments

When calling, you can name the parameter you're giving a value to. Keyword arguments can be in any order:

def describe_pet(animal, name, age=1):
    print(f"{name} the {animal} is {age} year(s) old.")

describe_pet(animal="cat", name="Mittens")
describe_pet(name="Rex", age=4, animal="dog")
describe_pet("parrot", age=12, name="Polly")

Keyword arguments make calls self-explanatory: send_email(to="mei@example.com", urgent=True) is far clearer than send_email("mei@example.com", True).

Positional arguments must come before keyword arguments in a call:

def f(a, b):
    print(a, b)

f(b=2, 1)

The mutable default trap

Default values are created once, when the function is defined, not each time it's called. With a list as a default, every call shares the same list:

def add_item(item, basket=[]):
    basket.append(item)
    return basket

print(add_item("apple"))
print(add_item("pear"))     # the apple is still there!

The fix is to default to None and create the list inside:

def add_item(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

print(add_item("apple"))
print(add_item("pear"))

Use None as the default for lists, dictionaries and sets, every time.

Keyword-only parameters

Parameters after a * must be passed by keyword. This forces callers to be explicit about settings:

def connect(host, *, timeout=10, secure=True):
    print(f"Connecting to {host} (timeout={timeout}, secure={secure})")

connect("example.com", timeout=5)