if that' confusing right nowdon' worry too much about itit will become clearer when we discuss these data types in later simple identifier can also be an expression we use identifiers as variables to give names to values when an identifier appears as an expressionits value is retrieved to provide result for the expression here is an interaction with the python interpreter that illustrates the use of variables as expressionsx print( print(spamtraceback (most recent call last)file ""line in nameerrorname 'spamis not first the variable is assigned the value (using the numeric literal in the second line of interactionwe are asking python to evaluate the expression in responsethe python shell prints out which is the value that was just assigned to of coursewe get the same result when we explicitly ask python to print using print statement the last interaction shows what happens when we try to use variable that has not been assigned value python cannot find valueso it reports nameerror this says that there is no value with that name the important lesson here is that variable must always be assigned value before it can be used in an expression more complex and interesting expressions can be constructed by combining simpler expressions with operators for numberspython provides the normal set of mathematical operationsadditionsubtractionmultiplicationdivisionand exponentiation the corresponding python operators are+-*/and *here are some examples of complex expressions from chaos py and convert py ( / celsius spaces are irrelevant within an expression the last expression could have been written / *celsius+ and the result would be exactly the same usually it' good idea to place some spaces in expressions to make them easier to read python' mathematical operators obey the same rules of precedence and associativity that you learned in your math classesincluding using parentheses to modify the order of evaluation you should have little trouble constructing complex expressions in your own programs do keep in mind that only the round parentheses are allowed in numeric expressionsbut you can nest them if necessary to create expressions like this (( * (spam ** by the waypython also provides operators for strings for exampleyou can "addstrings