Functions and Parameter Passing

Packaging Logic

A function is a named, reusable block of code that performs a task. Defining a function once and calling it many times avoids repetition and gives a piece of logic a meaningful name — the foundation of organized programming.

def square(n):
    return n * n

y = square(5)   # y becomes 25

A function may take parameters — named inputs listed in its definition (n above). When you call it, you supply arguments — the actual values passed in (5 above). Inside, the parameter behaves like a local variable holding that argument. A function may return a value with return, which both ends the function and hands a result back to the caller; a function with no return yields nothing useful.

Scope governs where names are visible. Variables created inside a function are local — they exist only during the call and are invisible outside. This is a feature: it lets functions work without clobbering each other's variables. A variable declared outside all functions is global, visible everywhere (but relying on globals makes code harder to reason about).

Functions enable abstraction: a caller uses square(5) without caring how it computes the square, just as you use a phone without knowing its circuitry. Well-chosen functions turn a big problem into small, named, testable pieces.

Common pitfall: confusing parameters (the names in the definition) with arguments (the actual values in the call). def f(x) declares a parameter x; f(10) passes the argument 10. Also, expecting to use a function's local variables from outside the function — they simply do not exist there.