advanced programming techniques def __init__(selfnameproductidcategorypricequantity)self name name self productid productid self category category self price price self quantity quantity the stockitem class' attributes are all validated for examplethe productid attribute can be set only to nonempty string that starts with three uppercase letters and ends with four digitsthe category attribute can be set only to nonempty string that is one of the specified valuesand the quantity attribute can be set only to number between and inclusive if we try to set an invalid value an exception is raised class decorators the validation is achieved by combining class decorators with descriptors as we noted earlierclass decorators can take only single argument--the class they are to decorate so here we have used the technique shown when we first discussed class decoratorsand have the valid_string(and valid_number(functions take whatever arguments we wantand then return decoratorwhich in turn takes the class and returns modified version of the class let' now look at the valid_string(functiondef valid_string(attr_nameempty_allowed=trueregex=noneacceptable=none)def decorator(cls)name "__attr_name def getter(self)return getattr(selfnamedef setter(selfvalue)assert isinstance(valuestr)(attr_name must be string"if not empty_allowed and not valueraise valueerror("{ may not be emptyformatattr_name)if ((acceptable is not none and value not in acceptableor (regex is not none and not regex match(value)))raise valueerror("{attr_namecannot be set to "{value}format(**locals())setattr(selfnamevaluesetattr(clsattr_namegenericdescriptor(gettersetter)return cls return decorator the function starts by creating class decorator function which takes class as its sole argument the decorator adds two attributes to the class it decoratesa private data attribute and descriptor for examplewhen the valid_string(regular expressions