Where a Name Actually Lives

$ >>> def set_total():$ ...     total = 100$ ...$ >>> set_total()$ >>> total$ NameError: name 'total' is not defined

total was assigned, and the function ran without error. It still does not exist outside the function, because a name created inside a function belongs to that function's own local scope, and that scope disappears the moment the function returns.

This is not Python being restrictive for its own sake. Every call to set_total() gets a fresh local scope, discarded when the call ends — if it did not, a function that runs a thousand times would leave a thousand stale copies of every local name behind it, and no function could ever reuse a variable name safely.

When code reads a name, Python does not search one place. It checks four scopes in order, and stops at the first one that has the name — an order usually remembered by its initials, LEGB.

$ >>> value = "global"$ >>> def outer():$ ...     value = "enclosing"$ ...     def inner():$ ...         print(value)$ ...     inner()$ ...$ >>> outer()$ enclosing

inner() has no local value of its own, so Python steps out one level to the enclosing function's value, finds it there, and stops — the module-level "global" is never even checked, because the search already succeeded at the enclosing scope.

A function can read a name defined outside it without any special syntax. The moment it tries to assign to that name, Python assumes you meant to create a new local variable instead — even if a variable with that name already exists outside.

$ >>> count = 0$ >>> def increment():$ ...     count = count + 1$ ...$ >>> increment()$ UnboundLocalError: cannot access local variable 'count'

global only ever reaches the module level. A function nested inside another function needs a different keyword to modify a name in the scope one level out — nonlocal.

$ >>> def make_counter():$ ...     count = 0$ ...     def increment():$ ...         nonlocal count$ ...         count += 1$ ...         return count$ ...     return increment$ ...$ >>> counter = make_counter()$ >>> counter()$ 1$ >>> counter()$ 2

Without nonlocal, count += 1 inside increment would hit the exact same UnboundLocalError as the previous section, for the same reason — the assignment makes Python treat count as local to increment, unless told otherwise. With it, each call to counter() reaches back into make_counter's scope and updates the same count that persists between calls.

A default value in a function signature is evaluated exactly once, when the function is defined — not fresh on every call. For a mutable default like a list, that single shared object gets reused across every call that relies on the default.

$ >>> def add_item(item, basket=[]):$ ...     basket.append(item)$ ...     return basket$ ...$ >>> add_item("apple")$ ['apple']$ >>> add_item("banana")$ ['apple', 'banana']

The second call did not start from an empty basket. Both calls share the exact same list, created once when Python read the def line. The fix is to default to None and create a fresh list inside the function body when nothing was passed.

Python passes arguments by handing the function a reference to the same object the caller has — not a copy of it. What the function does with that reference determines whether the caller ever notices.

$ >>> def add_score(scores):$ ...     scores.append(100)$ ...$ >>> my_scores = [88, 91]$ >>> add_score(my_scores)$ >>> my_scores$ [88, 91, 100]

scores inside the function and my_scores outside it are two names pointing at the exact same list. .append() mutates that list in place, so the change is visible through either name — the function never had to return anything for the caller to see it.

$ >>> def add_score(scores):$ ...     scores = scores + [100]$ ...$ >>> my_scores = [88, 91]$ >>> add_score(my_scores)$ >>> my_scores$ [88, 91]

This time nothing changes outside the function. scores + [100] builds a brand new list, and scores = ... points the local name scores at it — my_scores, outside, still points at the original. Rebinding a name only ever affects that name, never the object it used to point at.