Name scopes, shadowing and the global keyword; UnboundLocalError

A name's scope is the part of the program where that name can be seen and used. Names assigned at the top level of a script live in the global scope and are visible everywhere in the file, including inside functions. Names created inside a function, including its parameters, are local: they exist only while that call is running and cannot be seen from outside. Using a function's local variable after the function returns raises NameError. Scopes keep functions independent: a variable called total inside one function never collides with a total inside another.