[Tutor] Law of Demeter example request

Sean 'Shaleh' Perry shalehperry@attbi.com
Wed, 26 Dec 2001 20:33:55 -0800 (PST)

On 26-Dec-2001 Timothy Wilson wrote:
> Hi everyone,
> I'm looking over the early version of Bruce Eckel's _Thinking in Python_
> and I've got a question about the Law of Demeter, or as Bruce puts it,
> "Don't talk to strangers."
> I *think* I get it, but it would be great if someone would post a brief
> example illustrating the concept. Preferably, the example should show
> the same short program in two versions, one obeying the law and 2nd
> ignoring it.

class MoneyJar:
    def __init__(self, value):
        self.__value = value

    def addMoney(self, value):
        self.__value = self.__value + value

    def subtractMoney(self, value):
        self.__value = self.__value - value

    def getTotal(self):
        return self.__value

This class has one variable: __value.  It is accessed by 3 methods.

class MoneyJar:
    def __init__(self, value):
        self.value = value

This too has one variable.  However the only way to access it is directly.

jar = MoneyJar(10) # start with 10 units
jar.value = 5      # steal 5 units

Without a method in the middle the client can do anything to your class.

jar.value = "5 dollars"

amount = jar.value
subtotal = amount - (amount * tax)

What happens when jar.value is not a number?  Using methods lets you define how
each item is accessed.

That was the safety aspect.  However the total gains are less obvious.  What if
you need to keep a running register of each transaction.

def addMoney(self, value):
    self.register.addTransaction(addition, value)
    self.__value = self.__value + value

def subtractMoney(self, value):
    self.register.addTransaction(deletion, value)
    self.__value = self.__value - value

easily done.  Without the methods it is up to the client to remember to call
the register functions each time.  Then you add a database, then there are two
other programs trying to read the jar at the same time, ..... as you can see a
little design up front saves you in the end.  Safety is the obvious gain from
not talking to strangers, but there are many others.