Default arguments in Python

A default argument is a parameter with a value already filled in. If the caller does not pass that argument, Python uses the default.

The program

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

print(greet("Ada"))
print(greet("Linus", "Hi"))
print(greet("Grace", greeting="Good morning"))

Hello, Ada!
Hi, Linus!
Good morning, Grace!

How it works

• greeting="Hello" sets the default. The first call passes only name, so greeting is "Hello".
• The second call passes a value by position, and the third passes it by name (a keyword argument). Both replace the default.
• Parameters with defaults must come after the ones without. def greet(greeting="Hello", name): is a SyntaxError.

The mutable default mistake

The default value is created once, when Python runs the def line. It is not created again on each call. With a list as the default, every call shares the same list:

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

print(add_item("apple"))
print(add_item("bread"))

['apple']
['apple', 'bread']

The second call still sees "apple" from the first call. Use None as the default and create a new list inside the function:

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

print(add_item("apple"))
print(add_item("bread"))

['apple']
['bread']