An identifier occurring as an atom is a reference to a local, global
or built-in name binding. If a name is assigned to anywhere in a code
block (even in unreachable code), and is not mentioned in a
global statement in that code block, then it refers to a local
name throughout that code block. When it is not assigned to anywhere
in the block, or when it is assigned to but also explicitly listed in
a global statement, it refers to a global name if one exists,
else to a built-in name (and this binding may dynamically change).
When the name is bound to an object, evaluation of the atom yields
that object. When a name is not bound, an attempt to evaluate it
raises a NameError exception.