All 15 Functions & Scope questions
More in Functions & Scope
What is the difference between positional arguments and keyword arguments when calling a function? What does it mean that functions are 'first-class objects' in Python? What is Python's LEGB rule for variable scope resolution, and what does each letter stand for? Why do you need the 'global' keyword to MODIFY a global variable from inside a function, but not to simply READ it? How does the 'nonlocal' keyword let a nested function modify a variable in its ENCLOSING (not global) scope? How do *args and **kwargs let a function accept an arbitrary number of positional and keyword arguments? Why is using a mutable default argument (like an empty list) a classic Python bug, and how do you avoid it? How do keyword-only arguments (using a bare '*' in the parameter list) force certain parameters to be passed by name only? How do positional-only parameters (using a bare '/' in the parameter list, Python 3.8+) restrict how certain parameters can be passed? How does a closure capture variables from its enclosing scope, and what pitfall occurs when creating multiple closures inside a loop?
Advertisement
Python › Functions & Scope

How do default parameter values work, and when are they evaluated?

Beginner

Answer

A default value (`def func(param=default):`) is used when the caller omits that argument — critically, the default expression is evaluated ONCE, at FUNCTION DEFINITION time, not on every call, which matters for mutable defaults.
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Sam"))              # 'Hello, Sam!' -- uses the default
print(greet("Sam", "Hi"))        # 'Hi, Sam!' -- default overridden
Real-world example Providing a sensible default value for an optional configuration parameter, like a default timeout or retry count.

Common follow-ups: Why is using a mutable object (like a list) as a default value considered a classic Python pitfall?

Data Types & Structures
All 15 Functions & Scope questions