Python Default & Keyword Arguments

A default argument is a parameter with a value ready when a call passes nothing. A keyword argument passes a value by parameter name.

The usual value written every time

Without a default, every usual call writes the same value again.

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

greet("friend")
greet("friend")
greet("Ada")

A default value

The parameter name has a default of friend. You may still pass your own value.

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

greet()
greet("Ada")

Keyword arguments

Name each argument in the call. The order can then change safely.

def show(name, city):
    print(f"{name} lives in {city}.")

show(city="Paris", name="Ana")

Mix the two styles

Give the first value by position. Give the rest by name.

def show(name, city):
    print(f"{name} lives in {city}.")

show("Ana", city="Paris")

Defaults come last

The required parameter item comes first here. The optional parameter size comes after it.

def order(item, size="medium"):
    print(item, size)

order("tea")

Example

def make_drink(size="medium"):
    print(f"One {size} drink, please.")

make_drink()
make_drink("small")
make_drink("large")
make_drink(size="large")

Key points

Default value: a value a parameter uses when the call passes nothing for it
Use a default value to make an argument optional
Keyword argument: an argument passed by parameter name, like size="large"
Put parameters with defaults after parameters without defaults

In this example, size has a default value of medium. The first call passes nothing, so size is medium. The second call passes small by position. The third call passes large by position. The last call passes large by keyword. Each call prints one drink line.

Note

In a call, positional arguments must come first.

Keyword arguments come after them.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Python Playground

def make_drink(size="medium"):
    print(f"One {size} drink, please.")

make_drink()
make_drink("small")
make_drink("large")
make_drink(size="large")

Exercise

Which call uses a keyword argument?

make_drink("small") make_drink() make_drink(size="small") def make_drink(size)