Scope: global, nonlocal & LEGB

Scope is the set of rules that decides which variable a name refers to, and Python resolves it with the LEGB rule — searching Local, then Enclosing, then Global, then Built-in scopes and using the first match it finds.

Learn Scope: global, nonlocal & LEGB in our free Python course — an interactive lesson with runnable examples, a practice exercise and a quick reference.

Part of the free Python course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

Understanding scope is the difference between confidently sharing state and being baffled by a mysterious UnboundLocalError . Once you know how Python looks up names, the global and nonlocal keywords stop feeling like magic.

When Python sees a name, it searches four scopes in order and stops at the first match: L ocal → E nclosing → G lobal → B uilt-in. Here all four hold an x , and each scope sees the closest one:

This is why you can call len() anywhere — it's found in the Built-in scope, the last stop on the search.

Reading a global needs nothing special. But reassigning one from inside a function requires the global keyword — otherwise the assignment quietly creates a new local instead:

Without global counter , the line counter += 1 would try to read a local counter that doesn't exist yet — and crash. The keyword says "operate on the outer one."

nonlocal is global 's cousin for enclosing scopes. It lets an inner function reassign a variable in the function that wraps it — the foundation of closures that remember state between calls:

This one bites everyone. If a function assigns to a name anywhere in its body, Python marks that name local for the whole function — even on lines before the assignment. Reading it first then fails:

Why can you list.append() to a global list without the global keyword, but can't reassign it? Because mutating an object and rebinding a name are different operations:

Fix this counter closure so it actually increments. Replace each ___ , then run it.

✅ To touch an enclosing function's variable, use nonlocal x , not global x .

✅ Don't name variables after built-ins ( list , dict , str , id , sum ). A local name shadows the built-in for that scope.

Build a closure make_averager() that remembers every number it's given and returns the running average. Use nonlocal to track the total and count.

Lesson complete — scope holds no mystery now!

You can trace any name through the LEGB order, reassign globals with global , build stateful closures with nonlocal , diagnose the UnboundLocalError trap, and tell mutation apart from rebinding. That's a superpower for reading and debugging real code.

🚀 Up next: Custom Exceptions & Exception Chaining — design robust error handling.

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