An attribute reference is a primary followed by a period and a name:
attributeref: primary "." identifier
The primary must evaluate to an object of a type that supports
attribute references, e.g. a module or a list. This object is then
asked to produce the attribute whose name is the identifier. If this
attribute is not available, the exception AttributeError is
raised. Otherwise, the type and value of the object produced is
determined by the object. Multiple evaluations of the same attribute
reference may yield different objects.