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What is the difference between positional arguments and keyword arguments when calling a function? How do default parameter values work, and when are they evaluated? 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?
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What does it mean that functions are 'first-class objects' in Python?

Beginner

Answer

Functions can be assigned to variables, passed as arguments to other functions, returned from functions, and stored in data structures like lists or dicts -- treated just like any other value (an int or a string) rather than being a special, restricted kind of construct.
def greet():
    return "Hello!"

say_hello = greet          # assigned to a variable
functions = [greet, print]  # stored in a list

def call_it(func):           # passed as an argument
    return func()

print(call_it(greet))  # "Hello!"
Real-world example Passing a specific comparison or transformation function as an argument to a generic sorting or processing utility.

Common follow-ups: How does this first-class function support directly enable Python's decorator syntax?

Decorators
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