Functions & Scope

1. What Is It?

A function is a named block of code you can invoke with arguments to get a result. Scope is the rule that decides which names are visible from where. Together they form the basic unit of reuse: a function does one thing, takes inputs, returns an output, and its internals are hidden from the rest of the program.

Without functions, every program is one long script with no reuse and no way to isolate bugs. Without scope rules, every variable collides with every other — you'd need globally unique names for everything. Understanding both is what separates code that works once from code that works as a system.

2. How It Works

When a function is called:

1. A new stack frame is allocated for its local variables and parameters.
2. Arguments are bound to parameters (by value or by reference, depending on the language).
3. The function body executes.
4. On return, the result is passed back, the frame is torn down, and locals disappear.

# Python
def area(width, height):
    result = width * height       # 'result' is local to this frame
    return result

a = area(3, 4)                    # a = 12
# 'result' no longer exists here

Scope: where a name is visible

A scope is the region of code where a name can be resolved. Most languages use lexical (static) scope: you can determine a name's scope by reading the source. It doesn't depend on who called whom at runtime.

Python's scope lookup follows the LEGB rule:

• Local (inside the current function)
• Enclosing (inside any surrounding function, for closures)
• Global (module top level)
• Built-in (print, len, etc.)

x = "global"
def outer():
    x = "enclosing"
    def inner():
        # x = "local"  # uncomment to shadow
        print(x)        # "enclosing" — LEGB lookup finds it
    inner()
outer()

Java and C++ use block scope: a name declared inside { } exists only within those braces and any nested blocks.

void f() {
    for (int i = 0; i < 10; i++) {
        int temp = i * 2;    // 'temp' visible only inside this loop body
    }
    // 'i' and 'temp' are both gone here
}

Parameters and arguments

• Parameter — the name in the function's definition.
• Argument — the value you pass in at the call site.

# Python — positional, keyword, default, variadic
def greet(name, greeting="Hello", *extras, **options):
    ...

greet("Ada")                        # positional
greet(name="Ada", greeting="Hi")    # keyword
greet("Ada", "Hi", "and", "bye")    # *extras captures the rest

3. What You Actually Need to Know

• Locals die when the function returns. In C++, returning a pointer to a local is undefined behavior (the stack slot is reused). In Java/Python, returning a reference to a local object is fine because the object lives on the heap — only the local name disappears.
• Shadowing silently replaces outer names. A local x inside a function hides a module-level x. Useful, but if you meant to modify the outer one, you need global x (Python) or pass it in / return it (Java/C++).
• Assignment in Python creates a local. x = 5 inside a function always declares a local x. To modify an enclosing / global one, use nonlocal or global. This surprises people coming from other languages.
• Don't use mutable default arguments in Python. def f(xs=[]) shares one list across every call that omits the argument — a classic footgun. Use xs=None and build inside the body.
• Overloading vs. defaults. Java picks overload resolution; Python uses default arguments; C++ offers both. Same outcome, different style.
• Pure functions are easier. A function that reads only its parameters and returns a value (no globals, no I/O, no mutation) is the easiest to test, reason about, and reuse. Push impurity to the edges of the program.
• Debugging clues:
  • UnboundLocalError (Python) — you assigned to a name later in a function, so the interpreter treats it as local and reading it earlier fails.
  • NameError — name isn't in any visible scope (typo or missing import).
  • Value unexpectedly persists between calls → mutable default argument or global state.
  • Stack overflow → usually runaway recursion; sometimes very deep call chains.

4. Language Differences

Python's dynamic dispatch means there's no overloading — the latest def greet(...) just overwrites the earlier one, silently.

5. Tradeoffs & Decisions

• Short functions vs. long functions. A function should do one thing at one level of abstraction. If you need a mental "section break" inside a function, that's a sign to extract another function.
• Parameters vs. globals. Passing dependencies as parameters makes functions testable and explicit. Globals make the function shorter but couple it to the rest of the program — harder to test, harder to reuse.
• By-reference vs. by-value parameters. Use by-value when the callee shouldn't affect the caller's data. Use by-reference when the data is large (avoid copy cost) or when mutation is the point. In C++, const T& is the common default for "don't copy, don't mutate."
• Returning vs. mutating. Prefer returning new values over mutating parameters — especially for data you share across owners. Mutation is fine locally; aliased mutation is the source of many bugs.
• Function length. There's no magic number, but anything over ~30–50 lines usually hides a sub-function trying to get out.

If you see a function taking 10+ parameters, it usually means the parameters want to be an object. You'd choose an explicit return over mutation when callers might share the argument.

6. Interview Cheat Sheet

• A function is a reusable block of code taking parameters and returning a value. Each call gets a new stack frame for its locals.
• Lexical scope means a name's scope is determined by where it's written, not who calls it. Python uses the LEGB rule; Java and C++ use block scope.
• Parameter is the name in the definition; argument is the value passed in.
• Mutable default arguments in Python are a footgun — the default is created once at definition time, not per call.
• Pure functions (no globals, no mutation, no I/O) are the easiest to test and reason about.
• Assignment inside a Python function creates a local unless you use global / nonlocal.

Follow-ups:

• "What's the difference between a parameter and an argument?" — Parameter is the variable in the definition; argument is the value supplied at call time.
• "Why can't I modify a global from inside a Python function?" — Assignment creates a local. Use global (or better: pass it in and return it) to modify the global binding.
• "What does returning a reference to a local mean in C++?" — Undefined behavior — the stack frame is gone and the slot will be overwritten.