type myinstance sayhelloand press enter you see the message shown in figure - the change that you made to greeting has carried over to the instance of the class it' true that the use of class variable hasn' really caused problem in this examplebut you can imagine what would happen in real application if someone wanted to cause problems this is just simple example of how class variables can go wrong the two concepts you should take away from this example are as followsavoid class variables when you can because they're inherently unsafe always initialize class variables to known good value in the constructor code close the python shell window figure - the change to greeting carries over to the instance of the class creating instance variables instance variables are always defined as part of method the input arguments to method are considered instance variables because they exist only when the method exists using instance variables is usually safer than using class variables because it' easier to maintain control over them and to ensure that the caller is providing the correct input the following steps show an example of using instance variables