Default parameter values

A parameter can have a default value, written with = after its name in the def line. When a call leaves that argument out, the parameter gets the default:

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


greet("Ada")
greet("Linus", "Good morning")
greet("Grace", "")

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

The last call shows that an empty string is still an argument: the default is used only when the argument is missing. You've relied on defaults all along: print puts a line break at the end unless you give end=..., and range(5) starts from 0 unless you give it a start.

Keyword arguments

With several defaults, a call can name the parameter it gives with a keyword argument, as you've done with sep= and end= in print, and keep the defaults of the others:

def line(length=20, character="-"):
    return character * length


print(line())
print(line(5))
print(line(8, "="))
print(line(character="*"))

--------------------
-----
========
********************

Parameters with a default come after those without one. The other way round, Python couldn't tell which argument belongs to which parameter, and it stops before running anything:

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

  File "main.py", line 1
    def greet(greeting="Hello", name):
                                ^^^^
SyntaxError: parameter without a default follows parameter with a default

A list as a default: a trap

A default value is made once, when the def line runs, not on every call. For a number or a string that doesn't matter. But a list can change, and then every call that uses the default shares the same list:

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


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

['apple']
['apple', 'pear']

The second call got a basket that already had an apple in it. The usual fix is None as the default, and a new list made inside the function: def add_item(item, basket=None):, with if basket is None: and basket = [] at the start of the body.