From: Henrik Martensson <henrik.martensson@...>
Date: 2006-01-05T10:32:42+09:00
Subject: Re: Can't change the value of self

Immutable classes are less prone to bugs than mutable classes. This is
because they have a single state. They are also thread safe. 

It is good practise to favor immutability when there are no compelling
reasons for making a class mutable.

A Time object represents an instance in time. Making it mutable could
easily get confusing. Consider:

time = MutableTime.at(946702800)
rocket.launch_time = time
...
time.change(946701565) # Changes the launch time of the rocket


The launch time of the rocket can be changed without setting the
launch_time property explicitly. This is a potential source of bugs.

/Henrik


On Mon, 2006-01-02 at 02:11, Jonathan Leighton wrote:
> On Mon, 2006-01-02 at 09:47 +0900, Hal Fulton wrote:
> > James Britt wrote:
> > > Jonathan Leighton wrote:
> > > 
> > >> Can't change the value of self (SyntaxError)
> > >>     self = round_to *args
> > >>
> > >> Why doesn't it work? What can I do instead?
> > > 
> > > 
> > > "Destructive" methods must alter self using that self's existing 
> > > destructive methods.
> > 
> > To which we should add: Time has no destructive methods, since
> > Time objects are immutable.
> > 
> > But someone once created a MutableTime class -- anyone recall
> > who/when/where and whether it's complete?
> 
> Thanks for the replies. Why is Time immutable (presumably it's
> deliberately designed like that)? The MutableTime class sounds
> interesting but it's probably overkill for what I wanted to do -- I'll
> just modify variables at a higher level.
> 
> Thanks