Namespaces and scope of variables

A namespace maps names to objects, and the scope is the region of code where a name is visible. Python searches in the LEGB order: Local (inside the current function), Enclosing (outer functions), Global (module level) and Built-in (names such as len and list).

An assignment inside a function creates a local name, so x = "l" does not change a global x. To rebind a module-level variable use global x; to rebind a variable of an enclosing function use nonlocal x. Using n += 1 on a global without declaring it fails with UnboundLocalError, because the assignment makes n local before it has a value. Closures keep their own enclosing variables, so each call of the factory gets a fresh counter.

Other points: a comprehension has its own scope, so its loop variable is not visible afterwards; assigning to a built-in name such as list shadows the built-in; locals() and globals() return the current namespaces; and a default argument is evaluated once, when the function is defined.

Exam trap: reading a global inside a function is fine, but assigning to it creates a local variable unless global is declared.