scope

Scope (LEGB)

Python resolves names using the LEGB rule: Local → Enclosing → Global → Built-in.

Files

File	Description
example.py	global, nonlocal, and nested scope demos

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Descriptive Example

Scenario

A counter factory where each instance maintains its own count using nonlocal.

def make_counter(start=0):
    count = start

    def increment(step=1):
        nonlocal count          # modify enclosing scope, not create local
        count += step
        return count

    def get_count():
        return count

    return increment, get_count

inc, get = make_counter(100)
print(inc())    # 101
print(inc())    # 102
print(get())    # 102

LEGB lookup example

x = "global"

def outer():
    x = "enclosing"
    def inner():
        print(x)        # finds "enclosing" (Enclosing scope)
    inner()

outer()

Without nonlocal, x = ... inside inner would create a new local variable, not modify the enclosing one.

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Interview Q&A

Q1: What is the LEGB rule?
A: Python searches names in order: Local (current function), Enclosing (outer functions), Global (module), Built-in (preloaded names like len, print).

Q2: What does global x do?
A: Declares that assignments to x inside the function modify the module-level variable, not create a local one.

Q3: What does nonlocal x do?
A: Declares that assignments modify the nearest enclosing (non-global) scope. Used in nested functions and closures.

Q4: What is UnboundLocalError?
A: Occurs when you assign to a name in a function (making it local) but read it before assignment. Python treats any assignment as making the name local for the entire function.

Q5: Does my_list.append(x) need global my_list?
A: No. You're mutating the object, not rebinding the name. global is only needed when you assign to the name itself: my_list = [].

Q6: What is a closure's enclosing scope?
A: The scope of the outer function where the inner function was defined — not where it was called.

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Run

python3 example.py