Python Function Arguments

Positional arguments

The simplest way to pass values is by position. Python matches each argument to a parameter based on its order: the first argument fills the first parameter, the second fills the second, and so on. Because order determines meaning, swapping two positional arguments usually changes the result.

def introduce(name, role):
    print(name + " works as a " + role)

introduce("Asha", "designer")   # Asha works as a designer
introduce("designer", "Asha")   # designer works as a Asha

Keyword arguments

You can also pass values by naming the parameter directly in the call using the name=value form. These are called keyword arguments. Because each value is tied to a name rather than a position, the order no longer matters and the call reads more clearly.

def introduce(name, role):
    print(name + " works as a " + role)

introduce(role="engineer", name="Ravi")
# Ravi works as a engineer

Default parameter values

A parameter can be given a default value in the definition. If the caller omits that argument, the default is used; if they supply one, it overrides the default. Defaults are useful for options that rarely change, so callers only specify what differs from the norm.

def greet(name, greeting="Hello"):
    print(greeting + ", " + name + "!")

greet("Maya")               # Hello, Maya!
greet("Maya", "Good morning")  # Good morning, Maya!

Rules to keep in mind

• By default, you must pass one argument for every parameter that has no default value.
• Positional arguments must appear before any keyword arguments in a call.
• Avoid using mutable objects such as lists or dictionaries as default values, since the same object is shared across calls.
• Keyword arguments make calls with many parameters far easier to read.