appendix python review if all references to an object are removed--that isno variable contains reference to the object--it is automatically destroyed as part of python' garbage collection idnum avg name "john "john smithsmithaliases when one variable is assigned to another (student name)the reference contained in the variable on the righthand side is copied to the variable on the lefthand sideresulting in an alias since both variables refer to the same objectidnum avg student name "john "john smithsmithnull references variable can be set such that it does not reference any objectin which case it' called null reference in pythonthe special object none is used to set null reference for examplesuppose we execute the assignment statement idnum noneafter which idnum will be null referenceas shown hereidnum avg name student "john "john smithsmithconstants some languages support the creation of constant variables constant variableor simply constantis variable whose value cannot be changed after it has been assigned an initial value python does not support constant variablesbut it is common practice to specify constant variables with all capital lettersas illustrated in the following exampletax_rate max_size it is important to notehoweverthat there is no way to enforce the concept of constant variable and keep its value from being changed but by following the standard conventionyou provide information to yourself and others that you intend for variable in all caps to be constant throughout the program arithmetic operators python supports the common mathematical operations for both integers and realsaddition subtraction multiplication real division /*floor division modulo operator power