Default Parameters

We learned that functions take parameters. But sometimes, most of the time a parameter has the same value, and only occasionally do we want to change it. It would be annoying to pass it every single time

For example, imagine a function that creates a player profile

def create_player(name, health, city):
    print(f"{name} | HP: {health} | City: {city}")

create_player("Tommy", 100, "Vice City")
create_player("Lance", 100, "Vice City")
create_player("Cortez", 100, "Vice City")
create_player("Phil", 50, "Vice City")

We wrote 100 and "Vice City" three out of four times. That's a lot of repetition. What if we could say "health is 100 by default and city is Vice City by default, unless I say otherwise"?

def create_player(name, health=100, city="Vice City"):
    print(f"{name} | HP: {health} | City: {city}")

create_player("Tommy")
create_player("Lance")
create_player("Phil", 50)
create_player("Cortez", 100, "San Andreas")

Output

Tommy | HP: 100 | City: Vice City
Lance | HP: 100 | City: Vice City
Phil | HP: 50 | City: Vice City
Cortez | HP: 100 | City: San Andreas

health=100 and city="Vice City" are default values. If we don't pass anything for those parameters, Python uses the defaults. If we do pass something, it overrides the default

Important rule: parameters with default values must come after parameters without defaults

def f(a, b=10, c=20):
    print(a, b, c)

This is fine. a has no default, b and c do

def f(a=10, b, c):
    print(a, b, c)

Run this. Python gives us a SyntaxError. You can't put a parameter without a default after one that has a default. Think about it: Python wouldn't know which value belongs to which parameter

We can also use keyword arguments to skip parameters

def create_player(name, health=100, city="Vice City"):
    print(f"{name} | HP: {health} | City: {city}")

create_player("Tommy", city="Liberty City")

Output Tommy | HP: 100 | City: Liberty City

We skipped health (kept the default 100) and only changed city by using its name. Without keyword arguments, we'd have to write create_player("Tommy", 100, "Liberty City"), passing 100 even though it's the default

Mission: Docking Log

Write a function dock(ship, bay="A1", priority="normal") that prints ship docked at bay (priority). The bay and priority parameters have default values, so they can be left out.

The three ship names are already in the starter as variables ship1, ship2, and ship3. Call dock exactly like this (the calls are already in the starter too):

dock(ship1)
dock(ship2, "B7")
dock(ship3, priority="urgent")

Notice the last call uses a keyword argument (priority="urgent") to skip bay and keep its default while still setting the priority.

Output

Voyager docked at A1 (normal)
Odyssey docked at B7 (normal)
Pioneer docked at A1 (urgent)