Default arguments

Sometimes a parameter has a sensible default, and you'd rather not force the caller to supply it every time. Python lets you give a parameter a default value.

Giving a parameter a default

Assign a value in the def line. That value is used whenever the caller leaves the argument out:

Python

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

greet("Alice")          # Hello, Alice   (uses the default)
greet("Sam", "Hi")      # Hi, Sam        (overrides the default)

The first call doesn't pass greeting, so Python uses "Hello". The second call passes "Hi", which overrides the default.

The rule: defaults come last

Parameters with defaults must come after parameters without them:

Python

def greet(name, greeting="Hello"):   # OK
    ...

# def greet(greeting="Hello", name):  # ERROR

This keeps Python from getting confused about which argument is which.

Key takeaways

A default value is given in the def: def f(x, y=10):.
Callers can omit a defaulted argument or pass one to override it.
Parameters with defaults must come after those without.