Scope

Functions and modules4 min readLesson 2 of 3

Names are looked up in four places, in order: local, enclosing, global, built-in (the LEGB rule).

python

count = 0            # global

def bump():
    global count     # opt in to changing the global
    count += 1

Without global, assigning to count inside the function creates a local name instead. That is why the version without it raises UnboundLocalError . Prefer passing values in and returning results over reaching for globals. A function that only reads a global is fine; one that writes it is hard to test. nonlocal does the same trick for an enclosing function, and is mostly seen in closures:

python

def counter():
    total = 0
    def step():
        nonlocal total
        total += 1
        return total
    return step