A common way to represent variables on paper is to draw a box with the name of the variable on the outside. The box contains an arrow that points to the value of the variable. By convention, most variable names in Python contain only lower case letters. But there are some rules about what is or is not a legal name in Python. Names can be arbitrarily long.
Names can contain letters and numbers, but the first character has to be a letter.
Names can contain upper and lower case letters. Upper and lower case letters are different, so bruce and Bruce are different variable names. If you try to give a variable an illegal name, you will get a syntax error. It turns out that class is one of the Python keywords . Keywords are used by the language to define its rules and structure, and they can not be used as variable names.