greeting "helloworldprint(greetingtraceback (most recent call last)file ""line in nameerrorname 'greetingis not defined if you have trouble with an example in this bookdouble-check that every character in your code--including spaces--matches the example exactly computers have no common senseso being almost correct isn' good enoughrules for valid variable names variable names can be as long or as short as you likebut there are few rules that you must follow variable names may contain uppercase and lowercase letters ( -za- )digits ( - )and underscores ( )but they cannot begin with digit for exampleeach of the following is valid python variable namestring _a list_of_names the following aren' valid variable names because they start with digit lives _balloons beornot be in addition to english letters and digitspython variable names may contain many different valid unicode characters unicode is standard for digitally representing characters used in most of the world' writing systems that means variable names can contain letters from non-english alphabetssuch as decorated letters