Programming Fundamentals › Programming Basics

Scope

The region of code where a name is visible.

Also known as: variable scope, lexical scope, local variable

Scope is the region of code where a name can be used. Outside that region, the name doesn’t exist.

def f():
    x = 10          # local to f
    print(x)

f()
print(x)            # NameError: x isn't visible here

The usual levels

• Local: inside a function (or block). Created when it runs, gone afterward.
• Enclosing: an outer function’s variables, visible to inner functions. This is what makes closures work.
• Global: top of a file or module, visible everywhere in it (global variables).
• Built-in: names the language provides, like len or console.

When you use a name, the language looks in the current scope first, then outward. The first match wins.

x = "global"
def f():
    x = "local"      # a new local x; the global is untouched
    return x

A local name with the same spelling as an outer one hides it (shadowing).

Languages differ

JavaScript’s let and const are block-scoped, but var is function-scoped. Python has no block scope: a variable made inside an if or for is visible after it (block scope).

Why it’s good

Scope keeps parts of a program from interfering. Variables you can’t see can’t be changed by accident. Keep variables in the smallest scope that works, and “name is not defined” errors usually mean you used a name outside its scope.