Default, Keyword and Variable Arguments

Why this matters

Real functions are flexible. print("a", "b", sep="-") takes any number of values and an optional separator. You can write functions like that too.

The idea

Default values make a parameter optional:

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

greet("Ada")             # 'Hello, Ada!'
greet("Ada", "Welcome")  # 'Welcome, Ada!'

Keyword arguments say which parameter you mean, so the order no longer matters and calls read well:

greet(greeting="Hi", name="Bo")

*args collects any number of extra positional arguments into a tuple:

def total(*numbers):
    return sum(numbers)

total(1, 2, 3)           # 6

**kwargs collects extra keyword arguments into a dictionary:

def show(**info):
    return info

show(a=1, b=2)           # {'a': 1, 'b': 2}

A parameter placed after *args can only be given by keyword (sep= below). Parameters with defaults come after those without.

Try it

Write box(text, char="*") that returns the text surrounded by char on each side. Call it with and without the second argument.

Your turn

Write three functions that return their answers:

• power(base, exponent=2): power(3) is 9 and power(2, 10) is 1024.
• join_words(*words, sep=" "): join_words("a", "b", "c") is "a b c", and with sep="-" it is "a-b-c".
• profile(name, **details): the name, then the details sorted by key: profile("Ada", city="London", age=36) is "Ada (age=36, city=London)". With no details it is just "Ada".

Recap

• def f(x, y=10) gives y a default; callers may leave it out.
• Keyword arguments (f(y=1, x=2)) name the parameter; *args and **kwargs gather extras into a tuple and a dict.
• Defaults are evaluated once: use immutable defaults like numbers, text or None.

Common mistakes

• A required parameter after a default one: SyntaxError: non-default argument follows default argument.
• A list as a default (def f(items=[])): the same list is shared by every call. Use None and create the list inside.
• Giving a parameter twice, positionally and by keyword: TypeError: got multiple values.